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:
•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-SIGstandards 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.
