Cabled PCI Express as a Standard
Interface for Virtual and Synthetic
Instruments
Overview
Cabled PCI Express is a cabled serial bus used for high-performance interconnect of system components. It is based on
PCI Express and so provides a scalable, high-bandwidth, low-latency bus. In measurement and automation, cabled PCI
Express is very appealing to high-performance applications using a host PC for measurement processing and analysis.
For example, cabled PCI Express is currently used to connect a host PC to a PXI (PCI eXtensions for Instrumentation)
chassis with sustained transfer rates of nearly 800 Mbytes/s. The high performance, low cost, and easy connectivity of
cabled PCI Express makes it ideal for a number of measurement applications, and therefore, its applications is expected
to expand to serve more applications in the future. This paper examines cabled PCI express technology and both its
current and future application in measurement and automation systems.
What Is Cabled PCI Express?
Cabled PCI Express is the next generation peripheral bus for servers, desktops, and laptops. Cabled PCI Express will
have the following advantages:
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Wide use in standard PCs means low implementation costs
High bandwidth – 4 Gigabytes / second
Low latency – 300-700 nanoseconds
Application software and operating systems run without changes due to no software changes from PCI model
At least 15 standardized form factors including cabled PCI Express
Only standard that is designed for chip-to-chip, board-to-board, and box-to-box applications
Cabled PCI Express is being defined by the PCI-SIG standards body as an extension of the PCI Express Base
specification. Extending PCI Express from box-to-box and over longer distances is the goal of cabled PCI Express.
Cabled PCI Express provides a simple yet high-performance bus for expanding PC and measurement I/O. Some
commercial applications of cabled PCI Express include interfaces on laptop or small form factor PC to connect
peripherals to laptops and split-system desktop PCs that can move the CPU box under the desk while keeping user I/O
easily accessible on top of it. The cabled PCI Express specification anticipates cables up to 7 meters long and still meet
the PCI Express timing requirements. It is nearing completion with the final release expected in the fall of 2006.
Products Available
National Instruments, as a leading innovator in applying commercial technology to test and measurement applications,
provides a line of remote controllers to connect VXI and PXI chassis to PCs, called MXI (Multiplatform eXtensions for
Instrumentation). The first MXI product, MXI-1, was used to connect VXI mainframes to PCs in 1991. The latest
products, MXI Express, use cabled PCI Express to connect a PXI chassis to a host PC. MXI Express remote controllers
are available now in x1 to connect to a PXI chassis and x4 configurations provide up to nearly 800 MB/s of sustained
throughput to PXI Express chassis. There is both a PCIe (see Figure 1) and ExpressCard (see Figure 2) host interface
for use with desktops and laptops, respectively. The primary purpose of using cabled PCI Express in MXI Express was
to take advantage of the higher performance, form factor options and lower costs of PCI Express.
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Cabled PCI Express as a Standard
Interface for Virtual and Synthetic
Instruments
Figure 2. NI MXI Express uses cabled PCI Express to connect a Laptop with ExpressCard to PXI
When MXI-1 first launched in 1991 it provided 10MB/s bandwidth. This was state-of-the-art and used commercial
technologies available at that time (see Figure 3). By incorporating cabled PCI Express into the MXI product line can
now sustain nearly 800 MB/s.
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Figure 3. Cabled PCI Express makes MXI Express 80X higher bandwidth than MXI-1
While bandwidth has dramatically increased, the costs of these links have decreased. Figure 4 shows how the price per
bandwidth has dropped significantly over the last 15 years.
Figure 4: From $450/MB/s to $2.25/MB/s – Cabled PCI Express Helps Decrease Price per Bandwidth
MXI Express x1 for PXI chassis cut the cost per MB/s in half from $19/MB/s to $9/MB/s. MXI Express x4 for PXI
Express chassis cut that further to $2.25/MB/s. The cost reductions show the power of adopting commercial
technologies and ensure that PXI-based virtual instrumentation will continue to grow into more price sensitive
applications.
Software-centric Synthetic and Virtual Instrumentation
Because cabled PCI Express was designed for box-to-box applications it can connect a PC to traditional instruments. As
the trend for more software-centric instrumentation increases, the need for a high-speed connection to a host processor
is even more important. The US Department of Defense, for example, has coined a term called synthetic
instrumentation, to describe these software-centric systems. The DoD has created a forum called the Synthetic
Instrument Working Group (SIWG) who’s role is to define standards for interoperability of synthetic instrument
systems. The SIWG defines synthetic instruments (SI) as:
A reconfigurable system that links a series of elemental hardware and software components with standardized
interfaces to generate signals or make measurements using numeric processing techniques.
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Cabled PCI Express as a Standard
Interface for Virtual and Synthetic
Instruments
The focus of the SIWG has been primarily on the SI concepts as applied to RF stimulus and measurement systems. Data
intensive applications, like RF testing, require a high-bandwidth, low-latency bus to connect the IF digitizers to the
processor (see Figure 5).
Figure 5. Synthetic Instrument RF Block Diagram
For example, to digitize a 50 MHz wide RF signal requires 100 Mbytes/s of bandwidth (at 2 bytes of resolution per
sample). For an input and output channel, this grows to 200 MB/s. And for increasingly common multi-channel, or
MIMO (Multi Input, Multi Output) applications, the bandwidth required can quickly scale to multiply gigabytes per
second. PCI Express, and cabled PCI Express in particular, is a leading contender for this high speed link because of its
excellent technical capabilities, wide commercial adoption, and low-cost infrastructure.
Cabled PCI Express is based on PCI Express and so provides a scalable, high-bandwidth, low-latency bus for
chip-to-chip, board-to-board, and box-to-box applications. In the past some data intensive applications couldn’t transfer
the data fast enough to the host PC and so the data was processed in the box instrument using vendor-defined software
and hardware. Cabled PCI Express opens up a new larger and faster pipe to the host PC so more applications can take
advantage of user-defined software and hardware for measurement processing and analysis. Today NI uses cabled PCI
Express to connect host PCs to PXI chassis with MXI Express. It currently sustains transfer rates of 110 MB/s for a x1
link and 798 MB/s for a x4 link. In the future, cabled PCI Express may offer a compelling bus solution for synthetic
instrumentation.
Relevant NI Products and Whitepapers
National Instruments, a leader in automated test, is committed to providing the hardware and software products
engineers need to create these next generation test systems.
Software:
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Hardware:
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Whitepapers
NI offers a Designing Next Generation Test Systems Developers Guide. This guide is collection of whitepapers
designed to help you develop test systems that lower your cost, increase your test throughput, and can scale with future
requirements. To read the entire developers guide, you can: Download the PDF (90+ page) version or view the
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