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GPIB-COM
User Manual
June 1990 Edition
Part Number 320197-01
© Copyright 1989, 1991 National Instruments Corporation.
All Rights Reserved.
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Limited Warranty
The GPIB-COM is warranted against defects in materials and workmanship for a period of two
years from date of shipment. National Instruments will repair or replace equipment which proves
to be defective during the warranty period. This warranty includes parts and labor. A Return
Material Authorization (RMA) number must be obtained from National Instruments before any
equipment is returned for repair. Faults caused by misuse are not covered under the warranty.
During the warranty period, the owner may return failed parts to National Instruments for repair.
National Instruments will pay the shipping costs of returning the part to the owner. All items
returned to National Instruments for repair must be clearly marked on the outside of the package
with an RMA number.
No other warranty is expressed or implied. National Instruments shall not be liable for
consequential damages. Contact National Instruments for more information.
Important Notice
The material in this manual is subject to change without notice. National Instruments assumes no
responsibility for errors which may appear in this manual. National Instruments makes no
commitment to update, nor to keep current, the information contained in this document.
Copyright
Under the copyright laws, this manual may not be copied, photocopied, reproduced, translated, in
whole or in part, without the prior written consent of National Instruments Corporation.
Trademark
IBM is a registered trademark of International Business Machines Corporation.
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FCC/DOC Radio Frequency Interference Compliance
This equipment generates and uses radio frequency energy and, if not installed and used in strict
accordance with the instructions in this manual, may cause interference to radio and television
reception. This equipment has been tested and found to comply with (1) the limits for a Class B
computing device, in accordance with the specifications in Part 15 of U.S. Federal
Communications Commission (FCC) Rules, and (2) the limits for radio noise emissions from
digital apparatus set out in the Radio Interference Regulations of the Canadian Department of
Communications (DOC). These regulations are designed to provide reasonable protection
against interference from the equipment to radio and television reception in residential areas.
There is no guarantee that interference will not occur in a particular installation. However, the
chances of interference are much less if the equipment is used according to this instruction
manual.
If the equipment does cause interference to radio or television reception, which can be
determined by turning the equipment on and off, one or more of the following suggestions may
reduce or eliminate the problem.
•
•
•
•
•
Operate the equipment and the receiver on different branches of your AC electrical system.
Move the equipment away from the receiver with which it is interfering.
Relocate the equipment with respect to the receiver.
Reorient the receiver's antenna.
Be sure that the equipment is plugged into a grounded outlet and that the grounding has not
been defeated with a cheater plug.
If necessary, consult National Instruments or an experienced radio/television technician for
additional suggestions. The following booklet prepared by the FCC may also be helpful: How to
Identify and Resolve Radio-TV Interference Problems. This booklet is available from the U.S.
Government Printing Office, Washington, DC 20402, Stock Number 004-000-00345-4.
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Preface
Introduction to the GPIB-COM
The GPIB-COM is a high-performance talk/listen interface board that makes communication
possible between IEEE-488 devices and IBM personal computers and compatibles (hereafter
referred to as PCs) equipped with software that uses the serial ports.
Organization of This Manual
This manual is divided into the following sections:
Section One, Introduction, contains a brief description of the GPIB-COM including a listing of its
features, accessories, and components.
Section Two, Configuration and Installation, describes how to configure and install the GPIB-
COM into your system.
Section Three, Function Description, shows a block diagram of the GPIB-COM and describes the
functional components of the GPIB-COM.
Section Four, Running Diagnostic Tests, describes how to run the diagnostic tests that are shipped
with the GPIB-COM.
Section Five, Programming the GPIB-COM, presents a description of the GPIB-COM Serial Port
Emulator registers and information on programming the IBM serial adapter.
Appendix A, Specifications, lists the specifications of the GPIB-COM board.
Appendix B, Multiline Interface Command Messages, contains an ASCII chart and a list of the
corresponding GPIB messages.
Appendix C, Operation of the GPIB, describes GPIB terminology and protocol for users
unfamiliar with the GPIB.
Related Documents
The following documents contain information that may be helpful as you read this manual:
•
ANSI/IEEE Std 488-1978, IEEE Standard Digital Interface for Programmable
Instrumentation
•
•
•
•
IBM Options and Adapters Manual
IBM PC Technical Reference Manual
IBM Disk Operating System Manual
INS8250A Data Sheet, National Semiconductor
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Preface
Abbreviations Used in This Manual
The following abbreviations are used in the text of this manual.
≤
is less than or equal to
is greater than or equal to
plus or minus
ampere
≥
±
A
C
hex
in.
Celsius
hexadecimal
inch
I/O
kbyte
m
Mbyte
MHz
msec
r
input/output
1000 bytes
meter
megabyte
megahertz
millisecond
read
r/w
sec
V
VDC
w
read/write
second
volt
volts direct current
write
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Contents
Section One
Introduction..........................................................................................................................1-1
GPIB-COM Characteristics............................................................................................1-1
Section Two
Configuration and Installation......................................................................................2-1
Configuration..................................................................................................................2-1
Switch and Jumper Settings................................................................................2-3
Section Three
Function Description........................................................................................................3-1
The GPIB-COM Interface ..............................................................................................3-1
GPIB-COM Components...............................................................................................3-3
Address Decoding ..............................................................................................3-3
Configuration Jumpers .......................................................................................3-3
GPIB Acceptor and Source Handshaking...........................................................3-3
Mode Control Logic............................................................................................3-3
Interrupt Control Logic .......................................................................................3-3
Direction Buffers ................................................................................................3-4
Section Four
Running Diagnostic Tests...............................................................................................4-1
The GPIB-COM Test Commands..................................................................................4-1
-1 printer test.......................................................................................................4-1
-c change COM configuration.............................................................................4-3
Section Five
Programming the GPIB-COM......................................................................................5-1
The GPIB-COM Registers .............................................................................................5-1
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Contents
Line Control Register..........................................................................................5-8
Programming the Serial Adapter ....................................................................................5-16
Interrupt-Driven Method.....................................................................................5-16
Appendix A
Specifications.......................................................................................................................A-1
Appendix B
Multiline Interface Command Messages..................................................................B-1
Appendix C
Operation of the GPIB......................................................................................................C-1
Types of Messages .........................................................................................................C-1
Talkers, Listeners, and Controllers..................................................................................C-1
GPIB Signals..................................................................................................................C-2
Data Lines.......................................................................................................................C-3
NDAC (not data accepted)..................................................................................C-3
DAV (data valid) ................................................................................................C-4
Interface Management Lines...........................................................................................C-4
ATN (attention)...................................................................................................C-4
REN (remote enable)..........................................................................................C-4
SRQ (service request).........................................................................................C-4
EOI (end or identify) ..........................................................................................C-4
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Contents
Figure 1-1. GPIB-COM Board ................................................................................................1-2
Figure 2-1. GPIB-COM Parts Locator Diagram......................................................................2-1
Figure 2-2. Possible Settings for GPIB-COM Jumpers...........................................................2-4
Figure 2-3. Jumper W1 Settings ..............................................................................................2-4
Figure 2-5. Listen Address Setting for Listen Address 10 Hex................................................2-6
Figure 3-1. Block Diagram of GPIB-COM .............................................................................3-2
Figure 4-1. Plotter Output.........................................................................................................4-2
Figure C-1. GPIB Cable Connector ..........................................................................................C-3
Figure C-2. Linear Configuration of the GPIB Devices............................................................C-5
Figure C-3. Star Configuration of GPIB Devices .....................................................................C-6
Tables
Table 2-1. IBM PC Serial Port Adapters ................................................................................2-3
Table 2-2. Factory Default Settings and Optional Configurations...........................................2-3
Table 5-1. GPIB-COM Registers ...........................................................................................5-1
Table 5-2. Interrupt Types and Priorities.................................................................................5-7
Table 5-3. Word Length Select Bits........................................................................................5-9
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Section One
Introduction
This section contains a brief description of the GPIB-COM interface and a list of its characteristics
and components.
GPIB-COM Characteristics
The National Instruments GPIB-COM is a high-performance talk/listen interface that converts data
between a standard serial port format and IEEE-488 General Purpose Interface Bus (GPIB)
format for use with IEEE-488 printers and plotters. It can be used with any serial port software
on the PC. The GPIB-COM looks like a standard serial port to the IBM PC operating system and
software. It can be used to interface IEEE-488 devices to any PC software that uses the serial
ports.
The GPIB-COM has the following hardware features:
•
•
•
•
•
•
•
•
emulates the standard PC serial adapter interface so that no additional software is needed
can be configured as either COM1, COM2, COM3, or COM4
compatible with any software using the serial ports on the IBM PC and its compatibles
transparent addressing of unaddressed Talkers and Listeners
choice of talk-only mode for use with listen-only printers and plotters
choice of REN mode, which operates an instrument in the remote mode
choice of SRQ enable, which allows monitoring of asynchronous service requests
choice of IFC mode, which sends an interface clear when the computer is powered on
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Introduction
Section One
Figure 1-1 shows the GPIB-COM interface board.
Figure 1-1. GPIB-COM Board
What Your Kit Should Contain
Your kit should contain the following components:
Item
Part Number
GPIB-COM interface board
GPIB-COM User Manual
180750-01
320197-01
420212-45
GPIB-COM Diagnostic Test Diskette
Note: The GPIB-COM Diagnostic Test Diskette contains the com.exediagnostic test
that is described in Section Four, Running Diagnostic Tests.
Make sure each of these items is in your kit. If any item is missing, contact National Instruments.
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Section One
Introduction
Optional Equipment
Item
Part Number
Double-Shielded Cables:
GPIB Type X2 Cable - 1 m
GPIB Type X2 Cable - 2 m
GPIB Type X2 Cable - 4 m
763061-01
763061-02
763061-03
* In order to meet FCC emission limits for a Class B device, you must use a shielded
GPIB cable. Operating this equipment with a non-shielded cable may cause interference
to radio and television reception in residential areas.
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Section Two
Configuration and Installation
This section contains information on how to configure and install the GPIB-COM into your
system.
Configuration
Figure 2-1 shows the locations of the GPIB-COM configuration jumpers and switches.
Figure 2-1. GPIB-COM Parts Locator Diagram
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Configuration and Installation
Section Two
When installing the GPIB-COM you must determine which serial port the GPIB-COM board will
respond to and select the appropriate base address and interrupt level. The GPIB-COM can be
configured to one of four base addresses:
•
•
•
•
3F8 for Serial Port 1
2F8 for Serial Port 2
3E8 for Serial Port 3
2E8 for Serial Port 4
Note: DOS and BIOS only recognize base addresses 3F8 and 2F8.
The GPIB-COM is shipped from the factory set to 3F8. It must be reconfigured to another base
address if another device (such as a printer adapter card or a built-in serial port) is already at that
address.
DOS has reserved device names for the serial ports it finds in the system: COM1 and COM2.
The serial port adapters are named in the order in which they are found. When the computer is
powered on, DOS searches the serial port addresses for installed adapters. It first searches 3F8,
then 2F8. If only one device is installed, that device is named COM1 regardless of what its I/O
address is. If there is more than one serial device installed, the first one found is assigned COM1
and the second one found is assigned COM2.
Some communications software also recognizes COM3 (base address 3E8) and COM4 (base
address 2E8). If you wish to use COM3 or COM4, set the jumpers to the desired base address as
shown in Figure 2-2. DOS and BIOS do not recognize COM3 or COM4. Consult your software
documentation to see which interrupt level, if any, is required. If interrupts are not used, the
jumper can be removed or stored with only one side on a pin.
The GPIB-COM may need to be reconfigured if its interrupt level conflicts with another device. If
the base I/O address of the GPIB-COM does not conflict with any other device in your computer
and the GPIB-COM still does not work with your software package, reconfigure the GPIB-COM
to a different interrupt level.
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Section Two
Configuration and Installation
Table 2-1 shows the standard base I/O address and interrupt level for each serial port.
Table 2-1. IBM PC Serial Port Adapters
Name of Port
Serial Port 1
Serial Port 2
Serial Port 3
Serial Port 4
Base I/O Address (hex)
Interrupt Level
3F8
2F8
3E8
2E8
4
3
Not Used
Not Used
Switch and Jumper Settings
Table 2-2 shows the factory settings and optional configurations for the switches and jumpers on
the GPIB-COM.
Table 2-2. Factory Default Settings and Optional Configurations
GPIB-COM
Base I/O Address
Interrupt Level
Default
3F8
Optional
2F8, 3E8, 2E8
3
4
If you need to change the factory settings, continue on. If you do not need to change the factory
settings, skip to Installation later in this section.
Base I/O Address and Interrupt Selection
The base I/O address and interrupt line used by the GPIB-COM are determined by the jumpers
located at positions W2 and W5. The jumpers are set at the factory for base I/O address 3F8 hex
and interrupt level 4.
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Configuration and Installation
Section Two
Figure 2-2 shows the four possible combinations of jumper settings.
W5
W5
XF8
XE8
2X8
XF8
XE8
2X8
•
•
•
•
3X8
3X8
•
•
IRQ3
IRQ3
IRQ4
IRQ4
a. COM1: Jumpers Set to Base I/O Address
3F8 hex and Interrupt Level 4 (Default)
b. COM2: Jumpers Set to Base I/O Address
2F8 hex and Interrupt Level 3
W5
W5
XF8
3X8
XE8
2X8
XF8
3X8
XE8
2X8
•
•
•
•
•
IRQ3
IRQ3
IRQ4
IRQ4
•
•
•
c. COM3: Jumpers Set to Base I/O Address
3E8 hex (Interrupts Not Used)
d. COM4: Jumpers Set to Base I/O Address
2E8 hex (Interrupts Not Used)
Figure 2-2. Possible Settings for GPIB-COM Jumpers
Talk/Listen Modes
If you are using your GPIB-COM with a listen-only printer or plotter, you can configure the
GPIB-COM for a talk-only mode. In this mode, the GPIB-COM is always a Talker and never
sends any interface commands. To set the talk-only mode, change jumper W1 from T/L to T as
shown in Figure 2-3:
W1
W1
•
•
a. Talk/Listen
(Default)
b. Talk Only
Figure 2-3. Jumper W1 Settings
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Section Two
Configuration and Installation
Talk/Listen Address and Special Function Selection
The GPIB-COM interface board has eight onboard DIP switches you can use to configure the
GPIB controller responsibilities. In the talk/listen mode, the first five switches set the talk or listen
address of the external device(s) that will be attached to the GPIB-COM. In the talk-only mode,
the GPIB-COM does not send a talk or listen address.
The three remaining switches (REN, IFC, and SRQ) have the following special functions:
Note: An asterisk (*) after a signal name indicates that the signal is inverted (negative logic).
REN*
Asserts/unasserts the Remote Enable (REN) line that places an instrument in remote
mode. The ON position enables REN. The OFF position disables REN.
IFC*
Enables/disables the Interface Clear (IFC) option on the GPIB-COM. If enabled, the
GPIB-COM issues an IFC when it is initialized. The ON position enables IFC on
initialization. The OFF position holds IFC unasserted.
SRQ*
Enables/disables the Service Request (SRQ) option. If enabled, the GPIB-COM
monitors the SRQ line through the Parity Error Bit (PE) in the Line Status Register.
When the switch is ON, the PE bit reflects the status of the SRQ line. When the
switch is OFF, the PE bit is always clear.
The GPIB-COM factory default switch configuration is for device listen address 5, REN* not
asserted, IFC* enabled, and SRQ* enabled. Figure 2-4 shows the factory default switch settings
for the GPIB-COM.
Key
= side you must press down for default setting; Off = 1; On =
This side down for logic 1
This side down for logic 0
U13
1
2
4
8
16
REN
IFC
SRQ
Figure 2-4. Switch Setting for REN* OFF, IFC* ON, SRQ* ON, and Listen Address 5
(Default Setting)
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Configuration and Installation
Section Two
Figure 2-5 shows the switch configuration needed when using a device with listen address 10 hex.
The listen address varies with each device so check the listen address of your device and set these
five bits accordingly.
Key
= side you must press down for Listen Address 10;
Off = 1; On = 0
= used for setting REN* ON, IFC* ON, and SRQ* ON
This side down for logic 1
This side down for logic 0
1
U13
2
4
8
16
REN
IFC
SRQ
Figure 2-5. Listen Address Setting for Listen Address 10 Hex
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Section Two
Configuration and Installation
Figure 2-6 shows the switch configuration for REN* asserted, IFC* disabled, and SRQ* disabled.
Key
= side you must press down for REN* ON, IFC* OFF, and
SRQ* OFF; Off = 1; On = 0.
= used for setting Listen Address
This side down for logic 1
This side down for logic 0
U13
1
2
4
8
16
REN
IFC
SRQ
Figure 2-6. Switch Setting for REN* ON, IFC* OFF, and SRQ* OFF
Installation
Once you have changed, verified, and recorded the switches and jumper settings, you are ready to
install the GPIB-COM.
The following steps are general installation instructions. Consult the user manual or technical
reference manual of your personal computer for specific instructions and warnings.
1. Turn off your computer.
2. Remove the plug from its power source.
3. Remove the top cover or access port to the I/O channel.
4. Remove the expansion slot cover on the back panel of your computer.
5. Insert the GPIB-COM in an unused full-length slot with the IEEE-488 receptacle sticking out
of the opening of the back panel. It may be a tight fit, but do not force the board into place.
6. Screw the right angle mounting bracket of the GPIB-COM to the back panel rail of your
computer.
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Configuration and Installation
Section Two
7. Check the installation.
8. Replace the expansion slot cover of your computer.
9. Connect the GPIB cable to the GPIB-COM.
Special Considerations When Using the GPIB-COM
The GPIB-COM does not support XON/XOFF protocols. If your application software uses
XON/XOFF protocols, send your printer and plotter output to a file on disk. From DOS, send
your file to the GPIB-COM by using the DOS command:
copy filename COM1:
copy filename COM2:
The DOS MODE command is used to set the RS-232 parameters that will be used when COM1
and COM2 are accessed through DOS. Sometimes a timeout error may occur when writing
through the GPIB-COM to a slow device such as a printer or a plotter. If this happens, use the
MODE command to disable the timeout function by typing MODE COM1:12,,,,P. Some
software applications use the COM port baud rate setting to control the data transmission speed.
To obtain the fastest possible transfer rates, set the baud rate setting to 9600 baud. The Pat the end
disables the timeout. For more information on the MODE command, refer to the IBM Disk
Operating System Manual. This command can also be used in an autoexec.batfile to disable
timeouts when you start up the computer.
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Section Three
Function Description
This section contains a block diagram of the GPIB-COM, followed by a description of each of its
functional components.
The GPIB-COM Interface
The GPIB-COM is a completely transparent interface between GPIB devices and any IBM PC
software that uses serial ports. A set of I/O registers identical to those on the standard IBM serial
adapter is used. On the GPIB side, the GPIB-COM includes a limited subset of interface
functions that address a GPIB device to listen when the computer sends data to the serial port and
to talk when the computer is waiting to receive data from the serial port. In order to be able to
respond immediately to commands and data sent to the serial port, the GPIB-COM must be the
GPIB System Controller and cannot work with other Controllers present.
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Function Description
Section Three
Figure 3-1 shows a block diagram of the GPIB-COM.
IRQ3
Config.
Jumpers
Interrupt
Control
IRQ4
Config.
Jumpers
Address Bus
AEN
INS8250A
Compatible
Registers
Address
Decoding
IOW*
IOR*
Direction
Buffer
Data Bus
Mode Control
Logic
Direction
Buffer
Acceptor
Handshake
Source
Handshake
GPIB
Transceivers
GPIB
Figure 3-1. Block Diagram of GPIB-COM
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Section Three
Function Description
GPIB-COM Components
The interface consists of the following functional components:
•
•
•
•
•
•
•
•
Address Decoding
Configuration Jumpers
INS8250A Compatible Registers
GPIB Acceptor and Source Handshaking
Mode Control Logic
Interrupt Control Logic
Direction Buffers
GPIB Transceivers
Address Decoding
Address decoding monitors the PC address bus to recognize when a GPIB-COM address is
present and enables a read and/or write to its registers.
Configuration Jumpers
These onboard jumpers select the base address, GPIB controller mode, and interrupt request line
used by the GPIB-COM.
INS8250A Compatible Registers
The main component of an IBM PC compatible serial adapter is an INS8250A Asynchronous
Communications chip or its equivalent, which handles RS-232 communication. The INS8250A
compatible registers on the GPIB-COM emulate the serial adapter registers so that the GPIB-
COM will operate transparently with any serial port driver.
GPIB Acceptor and Source Handshaking
This circuitry converts PC serial port handshaking into GPIB handshaking. This process is
completely software-transparent.
Mode Control Logic
This circuitry determines whether the GPIB-COM should be a Talker (and the target device a
Listener) or a Listener (and the target device a Talker).
Interrupt Control Logic
This circuitry generates and masks interrupts on GPIB conditions which correspond to the
conditions that would cause the INS8250A circuit to generate an interrupt.
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Function Description
Section Three
Direction Buffers
These buffers control the direction in which data information travels.
GPIB Transceivers
The GPIB-COM is interfaced to the IEEE-488 bus by National Semiconductor 75160A and
75162A transceivers. These integrated circuits are specifically designed to provide power-
up/power-down bus protection (glitch-free). The GPIB-COM counts as one IEEE-488 bus load,
thereby allowing an additional 14 devices to be connected before exceeding the bus loading
restrictions imposed by the IEEE-488.
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Section Four
Running Diagnostic Tests
The GPIB-COM Test Commands
The GPIB-COM Diagnostic Tests can be used to verify the configuration of the GPIB-COM and
its connection to a GPIB printer or plotter.
To run a GPIB-COM diagnostic test, connect a printer or plotter to the GPIB and run the program
com.exefrom the GPIB-COM diagnostic test diskette. The following message will appear on the
screen:
NATIONAL INSTRUMENTS GPIB-COM DIAGNOSTIC TESTS
This program assumes the GPIB-COM is configured at base address
3F8 (COM1).
If you reconfigured your GPIB-COM you must change the address
that this program uses. To do this, enter command c from the
menu.
Enter one of the following commands:
-1 printer test
-2 plotter test
-c change COM configuration
-q quit
?
A short explanation of each command is given in the following paragraphs.
-1 printer test
Select this test to verify the configuration of the GPIB-COM when connecting it to a GPIB ASCII
printer. This test will send characters to the printer.
Entering this command returns the prompt:
Starting printer test.
Press any key to stop printing.
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Running Diagnostic Tests
Section Four
The printer should begin printing a string of characters. To stop the test, press any key. The
following message then appears:
Printer test finished.
Please check output to printer with user manual instructions.
The output on the printer should read:
NATIONAL INSTRUMENTS GPIB-COM DIAGNOSTIC TEST!"#$%&'()*+,-.
/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]_'abcdefghijkl
mnopqrstuvwxyz{|}~ !"#$%&'()*+,./0123456789:;<=>?@ABCDEFGHI
JKLMNOPQRSTUVWXY Z[\]_'abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'
()*+,./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]_'abcdefghi
jklmnopqrstuvwxyz{|}~ !"#$%&'()*+,./0123456789:;<=>?@ABCDEFG
HIJKLMNOPQRSTUVWXYZ[\]_'abcdefghijklmnopqrstuvwxyz{|}~
If the printer does not respond, an error message appears on the screen along with a message
telling you to recheck all of your connections and jumper settings. If the test still fails after
everything has been checked, write down the error message, if any, and call National Instruments.
-2 plotter test
Select this test to verify the configuration of the GPIB-COM when connecting it to a GPIB HP-GL
plotter. This test will send output to the plotter.
When this command is selected, the computer prints the following message:
Starting plotter test.
As illustrated in Figure 4-1, the plotter begins to draw four ovals.
Figure 4-1. Plotter Output
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Section Four
Running Diagnostic Tests
When it is finished, the following message appears on the screen:
Plotter test finished.
Please check output to plotter with user manual instructions.
If the plotter does not respond or the output on the plotter does not match Figure 4-1, check again
to be sure that all of the connections are tight and the jumpers are all set correctly. Try the test
again. If it still does not work, write down any error messages that appear and call National
Instruments.
-c change COM configuration
This command is used to change the base address of the GPIB-COM to match the actual board
settings. If you have changed the jumper settings to something different than the default settings,
you must run this command.
When this command is selected, the following menu appears:
This program assumes the GPIB-COM is configured at base address
3F8 (COM1).
Enter the number of the correct address.
1) base address 3F8 (COM1)
2) base address 2F8 (COM2)
Enter 1, 2, or <return> for no change?
Type in the number of the address that you have configured the board to. The computer prints out
your choice and returns to the main menu.
-q quit
This command ends the test program and returns to DOS.
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Section Five
Programming the GPIB-COM
This section presents a description of the GPIB-COM Serial Port Emulator registers and
information on programming the GPIB-COM. You need to use this section only if you are
writing your own serial port device driver.
The GPIB-COM Registers
IBM's serial adapter is a plug-in card for the PC that handles RS-232 communication. The main
component of the adapter is an INS8250 Asynchronous Communications chip or its equivalent,
which is controlled by programming a set of registers on the I/O channel. The names of these
registers and their I/O addresses are given in Table 5-1. Each register is located at a certain offset
from the base address of the board. The addresses are given with an X for the first digit and a Y
for the second digit, where XY=3F for COM1 (base address 3F8), XY=2F for COM2 (base
address 2F8), XY=3E for COM3 (base address 3E8), and XY=2E for COM4 (base address
2E8). Notice that sometimes two registers share the same I/O address. The DLAB bit (bit 7 of
the Line Control Register) determines which register will respond when these addresses are
accessed.
Note: Throughout this section, the term set will be used to mean that a bit is a logical 1, and clear
will mean that the bit is a logical 0.
Table 5-1. GPIB-COM Registers
I/O Address
Offset from
Base Address
Register
Type
XY=3F for COM1
XY=2F for COM2
XY=3E for COM3
XY=2E for COM4
Transmitter Holding Register
Receive Buffer Register
Divisor Latch LSB Register
Divisor Latch MSB Register
Interrupt Enable Register
write
read
read/write
read/write
read/write
XY8 (DLAB=0)
XY8 (DLAB=0)
XY8 (DLAB=1)
XY9 (DLAB=1)
XY9 (DLAB=0)
XYA
XYB
XYC
XYD
XYE
0
0
0
1
1
2
3
4
5
6
Interrupt Identification Register read
Line Control Register
Modem Control Register
Line Status Register
read/write
read/write
read
Modem Status Register
read
The following pages describe the functions of each register on the serial adapter and on the GPIB-
COM. For a more complete description of the serial adapter, refer to the IBM Options and
Adapters Manual.
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Programming the GPIB-COM
Section Five
Transmitter Holding Register
Offset from Base I/O Address = 0
Register Address = XF8
DLAB bit in Line Control Register = 0
7
6
5
4
3
2
1
0
D7
D6
D5
D4
D2
D1
D0
D3
W
The Transmitter Holding Register contains the character to be sent to the serial output, with bit 0
being the least significant and bit 7 the most significant. It functions identically on the GPIB-COM
and the INS8250.
Bit
Mnemonic
Description
7-0w
D[7-0]
Data Bits 7 through 0
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Section Five
Programming the GPIB-COM
Receive Buffer Register
Offset from Base I/O Address = 0
Register Address = XF8
DLAB bit in Line Control Register = 0
7
6
5
4
3
2
1
0
R
D7
D6
D5
D4
D2
D1
D0
D3
The Receive Buffer Register contains the character received from the serial input, with bit 0 being
the least significant and bit 7 the most significant. It functions identically on the GPIB-COM and
the INS8250.
Bit
Mnemonic
Description
7-0r
D[7-0]
Data Bits 7 through 0
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Programming the GPIB-COM
Section Five
Divisor Latch Least Significant Byte (LSB) Register
Offset from Base I/O Address = 0
Register Address = XF8
DLAB bit in Line Control Register = 1
7
6
5
4
3
2
1
0
DL7
DL6
DL5
DL4
DL2
DL1
DL0
R/W
DL3
Bit
7-0r/w
Mnemonic
Description
DL[7-0]
Data Bits 7 through 0
Divisor Latch Most Significant Byte (MSB) Register
Offset from Base I/O Address = 1
Register Address = XF9
DLAB bit in Line Control Register = 1
7
6
5
4
3
2
1
0
DL15
DL14
DL13
DL12
DL10
DL9
DL8
R/W
DL11
The value stored in these two registers is used to determine the baud rate for serial communica-
tions. The 16-bit number formed by the Divisor Latch LSB and MSB is divided into a 1.8432
MHz clock to produce the baud rate.
The Divisor Latch Registers are ignored by the GPIB-COM circuitry; however, they are available
on the GPIB-COM and can be written to and read by software.
Bit
Mnemonic
Description
7-0r/w
DL[15-8]
Data Bits 15 through 8
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Section Five
Programming the GPIB-COM
Interrupt Enable Register
Offset from Base I/O Address = 1
Register Address = XF9
DLAB bit in Line Control Register = 0
7
0
6
0
5
0
4
0
3
2
1
0
R/W
RLS
THR
RDA
MS
The Interrupt Enable Register is a read/write register that allows the programmer to selectively
enable or disable each of the four possible types of interrupts generated by the INS8250. The high
four bits are not used and are permanently cleared. The lower four bits each enable one type of
interrupt as listed below.
Bit
Mnemonic
Description
7-4r/w
0
Reserved Bits 7 through 4
These bits always read as 0.
Modem Status Interrupt Enable Bit
3r/w
MS
This bit enables a modem status interrupt when set to logical 1.
The modem status interrupt occurs when any of bits 0 through 3
of the Modem Status Register are set to logical 1.
This bit is identical on the GPIB-COM and the INS8250, but
bits 0 through 3 of the Modem Status Register are set for
different conditions as noted under the Modem Status Register
description.
2r/w
RLS
Receive Line Status Interrupt Enable Bit
This bit enables a line status interrupt when set. The line status
interrupt occurs when any of bits 1 through 4 of the Line Status
Register become set.
This bit is identical on the GPIB-COM and the INS8250, but
bits 1 through 4 of the Line Status Register are set for different
conditions as noted under the Line Status Register description.
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Programming the GPIB-COM
Section Five
Bit
Mnemonic
Description
1r/w
THR
Transmitter Holding Register Interrupt Enable Bit
This bit enables a transmitter holding register empty interrupt
when set. The transmitter holding register empty interrupt
occurs when the INS8250 becomes ready to send another
character.
This bit functions identically on the GPIB-COM and the
INS8250.
0r/w
RDA
Received Data Available Interrupt Enable Bit
This bit enables a received data available interrupt when set. The
received data available interrupt occurs when a character is
received from the serial input and stored in the Receive Buffer
Register.
This bit functions identically on the GPIB-COM and the
INS8250.
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Section Five
Programming the GPIB-COM
Interrupt Identification Register
Offset from Base I/O Address = 2
Register Address = XFA
7
0
6
0
5
0
4
0
3
0
2
1
0
R
ID1
ID0
INT
The Interrupt Identification Register is a read-only register which tells you when an interrupt is
pending and if so, what kind of interrupt it is. This register functions identically on the GPIB-
COM and the INS8250.
Bit
Mnemonic
Description
7-3r
0
Reserved Bits 7 through 3
These bits always read as 0.
Identify Interrupt Register Bits
2-1r
ID[1-0]
These two bits identify the interrupt that is pending. If more
than one interrupt is pending, only the one with the highest
priority is identified. The types and priorities are given in Table
5-2.
Table 5-2. Interrupt Types and Priorities
Bit 2
Bit 1
Type of Interrupt
Priority
0
0
1
0
1
0
Receiver Line Status
Received Data Available
Transmitter Holding
Register Empty
1st
2nd
3rd
4th
1
1
Modem Status
0r
INT
Interrupt Pending Bit
This bit is clear if an interrupt is pending. If set, no interrupt is
pending.
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Programming the GPIB-COM
Section Five
Line Control Register
Offset from Base I/O Address = 3
Register Address = XFB
7
6
5
4
3
2
1
0
DLAB
SBRK
STP
EVEN
STB
WL1
WL0
R/W
PEN
The Line Control Register is a read/write register that allows the programmer to set the RS-232
parameters for the INS8250. The function of each bit in this register is explained below.
Bit
Mnemonic
Description
7r/w
DLAB
Divisor Latch Access Bit
This bit controls which registers will respond to I/O addresses
XF8 and XF9. This bit is set to access the Divisor Latch LSB
and MSB, and cleared to access the Transmitter Holding
Register, the Receive Buffer Register, and the Interrupt Enable
Register.
This bit functions identically on the GPIB-COM and the
INS8250.
6r/w
5r/w
SBRK
STP
Set Break Control Bit
When set, this bit causes the SOUT line of the serial cable to be
forced to a logical 0 state.
This bit is ignored by the GPIB-COM.
Stick Parity Bit
When this bit is set, the parity bit transmitted and detected will
be the inverse of the EVEN bit (bit 4), regardless of the number
of 1's in the data word.
This bit is ignored by the GPIB-COM.
Even Parity Select Bit
4r/w
EVEN
If this bit is set, even parity will be generated and checked. If
this bit is clear, odd parity will be generated and checked.
This bit is ignored by the GPIB-COM.
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Section Five
Programming the GPIB-COM
Bit
Mnemonic
Description
3r/w
PEN
Parity Enable Bit
If this bit is set, the INS8250 will generate and check parity
according to the values of the STP and EVEN bits (bits 4 and
5). If this bit is clear, the INS8250 will not generate or check
parity bits.
This bit is ignored by the GPIB-COM.
Stop Bit Control Bit
2r/w
STB
This bit controls the number of stop bits sent and verified by the
INS8250. If this bit is set, two stop bits will be used. If this bit
is clear, one stop bit will be used.
This bit is ignored by the GPIB-COM.
Word Length Select Bits
1-0r/w
WL[1-0]
These two bits set the length of the words sent or received
according to Table 5-3.
Table 5-3. Word Length Select Bits
Bit 1
Bit 0
Word Length
0
0
1
1
0
1
0
1
5 bits
6 bits
7 bits
8 bits
These bits have no meaning on the GPIB and are ignored by the
GPIB-COM circuitry.
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Programming the GPIB-COM
Section Five
Modem Control Register
Offset from Base I/O Address = 4
Register Address = XFC
7
0
6
0
5
0
4
3
2
1
0
R/W
LOOP
OUT1
RTS
DTR
OUT2
The Modem Control Register is a read/write register that controls RS-232 output lines for
communication with a modem or modem emulator. The function of each bit in this register is
explained below.
Bit
Mnemonic
Description
7-5r/w
0
Reserved Bits 7 through 5
These bits always read as 0.
Loopback Diagnostic Test Bit
4r/w
3r/w
LOOP
OUT2
When set, this bit internally connects the outputs of the INS8250
back to the inputs to allow the chip to be tested by writing data to
itself and reading it back for verification.
This bit is ignored by the GPIB-COM circuitry.
Auxiliary Output 2 Bit
The OUT2 serial port output is the inverse of this bit. All
interrupts from the serial adapter are disabled when this bit is
cleared.
When this bit is cleared, interrupts are disabled on the GPIB-
COM.
2r/w
1r/w
OUT1
RTS
Auxiliary Output 1 Bit
The OUT1 serial port output is the inverse of this bit.
This bit is ignored by the GPIB-COM circuitry.
Request to Send Bit
The RTS serial port output is the inverse of this bit. When this
bit is set, it indicates that the Controller is ready to send data.
On the GPIB-COM, this bit is used along with the DTR bit to
control the CTS bit of the Modem Status Register.
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Programming the GPIB-COM
Bit
Mnemonic
Description
0r/w
DTR
Data Terminal Ready Bit
The DTR serial port output is the inverse of this bit. This bit is
set when the Controller is ready to communicate.
On the GPIB-COM, this bit is used along with the RTS bit to
control the CTS bit of the Modem Status Register.
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Programming the GPIB-COM
Section Five
Line Status Register
Offset from Base I/O Address = 5
Register Address = XFD
7
0
6
5
4
3
2
1
0
R
TEMT
THRE
BI
PE
OE
DR
FE
The Line Status Register provides information about the status of the data transfer. On the GPIB-
COM this register is implemented as a read-only register. Writing to the line status register will
not change its contents. The function of each bit in this register is explained below.
Bit
Mnemonic
Description
7r
0
Reserved Bit
This bit always reads as 0.
Transmitter Shift Register Empty Bit
6r
5r
4r
TEMT
THRE
BI
This bit is cleared when a character is transferred from the
Transmitter Holding Register to the Transmitter Shift Register
and set when the character has been shifted out of the Shift
Register onto the serial output line.
On the GPIB-COM, there is no shift register so this bit behaves
exactly the same as the THRE bit.
Transmitter Holding Register Empty Bit
This bit is cleared when the processor writes a character into the
Transmitter Holding Register and set when the character has
been transmitted and the INS8250 is ready to send another
character.
This bit functions identically on the GPIB-COM and the
INS8250.
Break Interrupt Bit
This bit is set to logical 1 when the serial data input remains in
the logical 0 state for longer than one full word transmission
time.
On the GPIB-COM, this bit is always clear.
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Programming the GPIB-COM
Bit
Mnemonic
Description
3r
FE
Framing Error Bit
This bit is set when the received character does not have a valid
stop bit.
On the GPIB-COM, this bit is always clear.
Parity Error Bit
2r
PE
This bit is set when the received character does not have the
correct parity. It is cleared when the processor reads the Line
Status Register.
On the GPIB-COM, this bit is set when the SRQ* line of the
GPIB is asserted. When the special function selection DIP
switch has been set to disable the SRQ feature, this bit is always
clear.
1r
OE
Overrun Error Bit
This bit is set when a new character is received and stored in the
Receive Buffer Register before the processor reads the previous
character, thus overwriting and destroying the previous
character. It is cleared when the processor reads the Line Status
Register.
On the GPIB-COM, this bit is always clear.
Data Ready Bit
0r
DR
This bit is set when a character has been received and stored in
the Receive Buffer Register. It is cleared when the processor
reads the Receive Buffer Register.
This bit functions identically on the GPIB-COM and the
INS8250.
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Programming the GPIB-COM
Section Five
Modem Status Register
Offset from Base I/O Address = 6
Register Address = XFE
7
6
5
4
3
2
1
0
R
DCD
RI
DSR
CTS
DDCD
TERI
DDSR
DCTS
The Modem Status Register gives the state of the modem control lines and tells whether any of
these lines have changed state since the register was last read. This register is read only on the
GPIB-COM. Writing to this register will not change its contents. The function of each bit in this
register is explained below.
Bit
Mnemonic
Description
7r
DCD
Data Carrier Detect Bit
This bit reflects the inverse of the serial port DCD input signal.
On the GPIB-COM, this bit is always clear.
Ring Indicator Bit
6r
5r
RI
This bit reflects the inverse of the serial port RI input signal.
On the GPIB-COM, this bit is always clear.
Data Set Ready Bit
DSR
This bit reflects the inverse of the serial port DSR input signal.
On the GPIB-COM, this bit is always set to indicate that the
interface is ready to transfer data.
4r
3r
CTS
Clear to Send Bit
This bit reflects the inverse of the serial port CTS input signal.
On the GPIB-COM, this bit is the logical AND of the DTR and
RTS bits of the Modem Control Register.
DDCD
Delta Data Carrier Detect Bit
This bit is set when the serial port DCD input signal changes
state and cleared when the processor reads the Modem Status
Register.
On the GPIB-COM, this bit is always clear.
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Section Five
Programming the GPIB-COM
Bit
Mnemonic
Description
2r
TERI
Trailing Edge Ring Indicator Bit
This bit is set when the serial port RI input signal changes from
a logical 1 to a logical 0 and cleared when the processor reads
the Modem Status Register.
On the GPIB-COM, this bit is always clear.
Delta Set Ready Bit
1r
0r
DDSR
DCTS
This bit is set when the serial port DSR input signal changes
state and cleared when the processor reads the Modem Status
Register.
On the GPIB-COM, this bit is always clear.
Delta Clear to Send Bit
This bit is set when the CTS input changes state and cleared
when the processor reads the Modem Status Register.
On the GPIB-COM, this bit is the logical AND of the DTR and
RTS bits of the Modem Control Register.
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Programming the GPIB-COM
Section Five
Programming the Serial Adapter
The operation of the serial adapter is controlled by software, either IBM BIOS or an application.
To transfer data to and from the serial port, the communications parameters must first be set up in
the following manner:
1. Load the baud rate divisor into the divisor latch.
2. Store the RS-232 parameters in the Line Control Register.
3. Send the modem control signals by writing to the Modem Control Register.
4. Enable interrupts, if used, by writing to the appropriate bits in the Interrupt Enable Register.
Once the control registers have been programmed correctly, the software can transmit and receive
serial data by polling, interrupts, or both.
Polling Method
In the polled method of operation, the Line Status Register is continuously read to check if the Data
Ready Bit is set. When this bit becomes set, the Receive Buffer Register is read to get the
character that just came from the serial port, and the process continues. To transmit data, the Line
Status Register is polled until the Transmitter Holding Register Empty Bit is set. A byte can then
be written to the Transmitter Holding Register, and polling continues.
Interrupt-Driven Method
In the interrupt-driven mode of operation, the desired interrupts are enabled through the OUT2 bit
and the Interrupt Enable Register. An interrupt handler is installed to respond when an interrupt
occurs. Software processes can perform other tasks until an interrupt is received. When an
interrupt occurs, the handler reads the Interrupt Identification Register to determine the type of
interrupt and takes appropriate action, such as reading the Receive Buffer Register in the case of a
received data available interrupt, writing to the Transmitter Holding Register if it was a THRE
interrupt, or printing an error message in the case of a line status interrupt.
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Section Five
Programming the GPIB-COM
The GPIB-COM Controller Function
Because most serial port applications involve communication with only one serial line, the GPIB-
COM imposes some restrictions on the GPIB in order to be compatible with existing serial port
software. The GPIB-COM must be System Controller of the GPIB and will not work with other
Controllers. The behavior of the GPIB-COM depends on the setting of the talk-only jumper.
If the jumper is set to talk-only, the GPIB-COM assumes that there is a listen-only device
connected to the GPIB. When a byte is written into the Transmitter Holding Register, the GPIB-
COM sets the THRE bit of the Line Status Register to 0, puts the contents of that register on the
GPIB data lines, and begins the source handshake sequence. When the Listener releases NDAC*,
the THRE bit is set back to 1 and the GPIB-COM waits for another byte to be written into the
Transmitter Holding Register.
If the jumper is set to talk/listen, the GPIB-COM assumes that the device connected to the GPIB
can be addressed to listen or talk. When the computer is powered on, the GPIB-COM sends the
talk address that is set on the DIP switches, addresses itself as a Listener, and unasserts NRFD*.
If the GPIB device sends a byte, the GPIB-COM accepts it, stores it in the Receive Buffer
Register, and sets the DR bit of the Line Status Register. At the end of the GPIB handshake, it
keeps NRFD* asserted until the computer reads the byte from the Receive Buffer Register.
If the computer writes a byte to the Transmitter Holding Register, the GPIB-COM sends the GPIB
device its listen address from the DIP switches and addresses itself to talk. Then it sends the byte
to the GPIB device, which is now addressed as a Listener. When the GPIB-COM begins a source
handshake, it starts a 200-msec timer. If the source handshake is completed before the 200 msec
is expired, the GPIB-COM remains addressed to talk until the timer runs out. If the computer
writes another byte before the timer runs out, the GPIB-COM restarts the timer and sends the byte.
When 200 msec has passed since the last byte was written, the GPIB-COM sends the talk address
and addresses itself to listen again. If a byte was waiting in the Receive Buffer Register when the
computer wrote to the Transmitter Holding Register, the GPIB-COM remembers this and when it
becomes a Listener again, it asserts NRFD* until the computer reads the byte from the Receive
Buffer Register.
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Appendix A
Specifications
This appendix lists the specifications of the GPIB-COM board.
Power Requirement
+5 VDC (± 5%)
0.75 A typical
Physical
Dimensions
4.2 in. by 8.75 in.
I/O Connector
IEEE-488 Standard 24-pin
Operating Environment
Component Temperature
Relative Humidity
Emissions
0° to 70° C
5% to 90%, noncondensing
FCC Class B
Storage Environment
Temperature
-55° to 125° C
Relative Humidity
5% to 90%, noncondensing
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Multiline Interface Command Messages
Appendix B
Multiline Interface Messages
Hex Oct Dec ASCII Msg
Hex Oct Dec ASCII Msg
00
01
02
03
04
05
06
07
000
001
002
003
004
005
006
007
0
1
2
3
4
5
6
7
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL
20
21
22
23
24
25
26
27
040
041
042
043
044
045
046
047
32
33
34
35
36
37
38
39
SP
!
"
#
$
%
&
'
MLA0
MLA1
MLA2
MLA3
MLA4
MLA5
MLA6
MLA7
GTL
SDC
PPC
08
09
010
011
012
013
014
015
016
017
8
9
BS
HT
LF
VT
FF
CR
SO
SI
GET
TCT
28
29
050
051
052
053
054
055
056
057
40
41
42
43
44
45
46
47
(
)
*
+
,
-
.
MLA8
MLA9
0A
0B
0C
0D
0E
0F
10
11
12
13
14
15
2A
2B
2C
2D
2E
2F
MLA10
MLA11
MLA12
MLA13
MLA14
MLA15
/
10
11
12
13
14
15
16
17
020
021
022
023
024
025
026
027
16
17
18
19
20
21
22
23
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
30
31
32
33
34
35
36
37
060
061
062
063
064
065
066
067
48
49
50
51
52
53
54
55
0
1
2
3
4
5
6
7
MLA16
MLA17
MLA18
MLA19
MLA20
MLA21
MLA22
MLA23
LLO
DCL
PPU
18
19
030
031
032
033
034
035
036
037
24
25
26
27
28
29
30
31
CAN
EM
SUB
ESC
FS
GS
RS
SPE
SPD
38
39
070
071
072
073
074
075
076
077
56
57
58
59
60
61
62
63
8
9
:
MLA24
MLA25
MLA26
MLA27
MLA28
MLA29
MLA30
UNL
1A
1B
1C
1D
1E
1F
3A
3B
3C
3D
3E
3F
;
<
=
>
?
US
Message Definitions
DCL
GET
GTL
LLO
Device Clear
Group Execute Trigger
Go To Local
MSA My Secondary Address
MTA My Talk Address
PPC
PPD
Parallel Poll Configure
Parallel Poll Disable
Local Lockout
MLA My Listen Address
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Appendix B
Multiline Interface Command Messages
Multiline Interface Messages
Hex Oct Dec ASCII Msg
Hex Oct Dec ASCII Msg
40
41
42
43
44
45
46
47
100
101
102
103
104
105
106
107
64
65
66
67
68
69
70
71
@
A
B
C
D
E
MTA0
MTA1
MTA2
MTA3
MTA4
MTA5
MTA6
MTA7
60
61
62
63
64
65
66
67
140
141
142
143
144
145
146
147
96
97
98
`
MSA0,PPE
MSA1,PPE
MSA2,PPE
MSA3,PPE
MSA4,PPE
MSA5,PPE
MSA6,PPE
MSA7,PPE
a
b
c
d
e
f
99
100
101
102
103
F
G
g
48
49
110
111
112
113
114
115
116
117
72
73
74
75
76
77
78
79
H
I
J
K
L
M
N
O
MTA8
MTA9
68
69
150
151
152
153
154
155
156
157
104
105
106
107
108
109
110
111
h
i
j
k
l
m
n
o
MSA8,PPE
MSA9,PPE
MSA10,PPE
MSA11,PPE
MSA12,PPE
MSA13,PPE
MSA14,PPE
MSA15,PPE
4A
4B
4C
4D
4E
4F
MTA10
MTA11
MTA12
MTA13
MTA14
MTA15
6A
6B
6C
6D
6E
6F
50
51
52
53
54
55
56
57
120
121
122
123
124
125
126
127
80
81
82
83
84
85
86
87
P
MTA16
MTA17
MTA18
MTA19
MTA20
MTA21
MTA22
MTA23
70
71
72
73
74
75
76
77
160
161
162
163
164
165
166
167
112
113
114
115
116
117
118
119
p
q
r
s
t
u
v
w
MSA16,PPD
MSA17,PPD
MSA18,PPD
MSA19,PPD
MSA20,PPD
MSA21,PPD
MSA22,PPD
MSA23,PPD
Q
R
S
T
U
V
W
58
59
130
131
132
133
134
135
136
137
88
89
90
91
92
93
94
95
X
Y
Z
[
\
]
MTA24
MTA25
MTA26
MTA27
MTA28
MTA29
MTA30
UNT
78
79
170
171
172
173
174
175
176
177
120
121
122
123
124
125
126
127
x
y
z
{
|
}
MSA24,PPD
MSA25,PPD
MSA26,PPD
MSA27,PPD
MSA28,PPD
MSA29,PPD
MSA30,PPD
5A
5B
5C
5D
5E
5F
7A
7B
7C
7D
7E
7F
^
_
~
DEL
PPE
PPU
SDC
SPD
Parallel Poll Enable
Parallel Poll Unconfigure
Selected Device Clear
Serial Poll Disable
SPE
Serial Poll Enable
Take Control
Unlisten
TCT
UNL
UNT
Untalk
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Appendix C
Operation of the GPIB
History of the GPIB
The GPIB is a link, bus, or interface system through which interconnected electronic devices
communicate. Hewlett-Packard invented the GPIB, which they call the HP-IB, to connect and
control programmable instruments manufactured by them. Because of its high system data rate
ceilings of from 250 kbytes/sec to 1 Mbyte/sec, the GPIB quickly became popular in other
applications such as intercomputer communication and peripheral control. It was later accepted as
the industry standard IEEE-488. The versatility of the system prompted the name General
Purpose Interface Bus.
Types of Messages
Devices on the GPIB communicate by passing messages through the interface system. There are
two types of messages:
•
•
Device-dependent messages, often called data or data messages, contain device-specific
information such as programming instructions, measurement results, machine status, and data
files.
Interface messages manage the bus itself. They are usually called commands or command
messages. Interface messages perform such functions as initializing the bus, addressing and
unaddressing devices, and setting devices for remote or local programming.
Note: The term command as used here should not be confused with some device instructions
which are also called commands. Such device-specific instructions are actually data
messages.
Talkers, Listeners, and Controllers
There are three types of GPIB communicators. A Talker sends data messages to one or more
Listeners. The Controller manages the flow of information on the GPIB by sending commands to
all devices.
Devices can be Talkers, Listeners, and/or Controllers. A digital multimeter, for example, is a
Talker and may also be a Listener. A printer or plotter is usually only a Listener. A computer on
the GPIB often combines all three roles to manage the bus and communicate with other devices.
The GPIB is a bus like a typical computer bus except that the computer has its circuit cards
interconnected via a backplane bus whereas the GPIB has standalone devices interconnected via a
cable bus.
The role of the GPIB Controller can also be compared to the role of the computer's CPU, but a
better analogy is to the switching center of a city telephone system.
The switching center (Controller) monitors the communications network (GPIB). When the
center (Controller) notices that a party (device) wants to make a call (send a data message), it
connects the caller (Talker) to the receiver (Listener).
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Operation of the GPIB
Appendix C
The Controller usually addresses a Talker and a Listener before the Talker can send its message to
the Listener. After the message is transmitted, the Controller usually unaddresses both devices.
Some bus configurations do not require a Controller. For example, one device may only be a
Talker (called a talk-only device) and there may be one or more listen-only devices.
A Controller is necessary when the active or addressed Talker or Listener must be changed. The
Controller function is usually handled by a computer.
System Controller and Active Controller
Although there can be multiple Controllers on the GPIB, only one Controller at a time is Active
Controller or Controller-in-Charge. Active control can be passed from the current Active
Controller to an idle Controller. Only one device on the bus, the System Controller, can make
itself the Active Controller.
GPIB Signals
The interface bus consists of 16 signal lines and 8 ground return or shield drain lines. The 16
signal lines are divided into three groups:
•
•
•
eight data lines
three handshake lines
five interface management lines
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Appendix C
Operation of the GPIB
Figure C-1 shows the arrangement of these signals on the GPIB cable connector.
1
2
3
4
5
6
7
8
9
13
14
15
16
17
18
19
20
21
DIO1*
DIO2*
DIO3*
DIO4*
EOI*
DAV*
NRFD*
NDAC*
IFC*
DIO5*
DIO6*
DIO7*
DIO8*
REN*
GND (TW PAIR W/DAV*)
GND (TW PAIR W/NRFD*)
GND (TW PAIR W/NDAC*)
GND (TW PAIR W/IFC*)
GND (TW PAIR W/SRQ*)
GND (TW PAIR W/ATN*)
SIGNAL GROUND
10 22
11 23
12 24
SRQ*
ATN*
SHIELD
Figure C-1. GPIB Cable Connector
Data Lines
The eight data lines, DIO1 through DIO8, carry both data and command messages. All
commands and most data use the 7-bit ASCII or ISO code set, in which case the eighth bit, DIO8,
is unused or used for parity.
Handshake Lines
Three lines asynchronously control the transfer of message bytes among devices. The process is
called a three-wire interlocked handshake and it guarantees that message bytes on the data lines are
sent and received without transmission error.
NRFD (not ready for data)
NRFD indicates when a device is ready or not ready to receive a message byte. The line is driven
by all devices when receiving commands and by Listeners when receiving data messages.
NDAC (not data accepted)
NDAC indicates when a device has or has not accepted a message byte. The line is driven by all
devices when receiving commands and by Listeners when receiving data messages.
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Operation of the GPIB
Appendix C
DAV (data valid)
DAV tells when the signals on the data lines are stable (valid) and can be accepted safely by
devices. The Controller drives DAV when sending commands, and the Talker drives it when
sending data messages.
The way in which NRFD and NDAC are used by the receiving device is called the Acceptor
Handshake. Likewise, the sending device uses DAV in the Source Handshake.
Interface Management Lines
Five lines are used to manage the flow of information across the interface.
ATN (attention)
The Controller drives ATN true when it uses the data lines to send commands and false when it
allows a Talker to send data messages.
IFC (interface clear)
The System Controller drives the IFC line to initialize the bus to become Controller-In-Charge.
REN (remote enable)
The System Controller drives the REN line, which is used to place devices in remote or local
program mode.
SRQ (service request)
Any device can drive the SRQ line to asynchronously request service from the Active Controller.
EOI (end or identify)
The EOI line has two purposes. The Talker uses the EOI line to mark the end of a message string.
The Active Controller uses the EOI line to tell devices to identify their responses in a parallel poll.
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Appendix C
Operation of the GPIB
Physical and Electrical Characteristics
Devices are usually connected with a cable assembly consisting of a shielded 24-conductor cable
with both a plug and receptacle at each end. This design enables devices to be connected in either a
linear or a star configuration, or a combination of the two. See Figures C-2 and C-3.
Figure C-2. Linear Configuration of the GPIB Devices
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Operation of the GPIB
Appendix C
Figure C-3. Star Configuration of GPIB Devices
The standard connector is the Amphenol or Cinch Series 57 MICRORIBBON or AMP CHAMP
type. An adapter cable using non-standard cable and/or connector is used for special interconnect
applications.
The GPIB uses negative logic with standard TTL logic levels. When DAV is true, for example, it
is a TTL low level (≤ 0.8 V), and when DAV is false, it is a TTL high level (≥ 2.0 V).
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Appendix C
Operation of the GPIB
Configuration Restrictions
To achieve the high data transfer rate that the GPIB is designed for, the physical distance between
devices and the number of devices on the bus is limited.
The following restrictions are typical:
•
A maximum separation of 4 m between any two devices and an average separation of 2 m
over the entire bus.
•
•
A maximum total cable length of 20 m.
No more than 15 devices connected to each bus, with at least two-thirds powered-on.
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Title: GPIB-COM User Manual
Edition Date
Part Number:
June 1990
320197-01
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If you find errors in the manual, please record the page numbers and describe the errors.
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|