Extending PXI

Channel capacity can be a limiting factor in many test setups, but when your PXI card cage is filled to capacity, you are not out of options. A PXI Express extension system will let you add even more instruments to your system by linking an expansion chassis to the host system through a PCI Express (PCIe) cable. Chris Ni, product manager for Adlink Technology, a manufacturer of PXI chassis and modules, recently explained the benefits of using extension systems in test applications.

A: The first reason to use an extension system is to increase the number of slots available in a single computer system. Most PCs have only a few slots, and even a PXI system is limited to 20 slots. With the bus extension technology, a single PXI system can double the capacity by chaining in an extended PXI chassis. In theory, the capacity of a PXI system is unlimited. You can chain as many extended PXI chassis as needed if the allocation of system resources is not an issue.

The second reason is to separate the I/O cards from the host computer. This can help increase measurement accuracy by distancing the modules from the computer’s electronic noise. It also makes the system more modular, which makes repair easier. Separating the I/O from the host also allows installations where the instruments operate in a hazardous environment while the host is in a safer location. [Ed. note: Adlink manufactures extension systems that operate over PCIe cables as long as 7 m.]

Q: Does using an expansion chassis create timing problems in a test system?

A: Timing and triggering are handled on the PXI backplane, not the host system, so this is not an issue with chassis extension. The extension system is just a transparent bridge to the host system.

The only problem we have ever encountered with timing in an extension system occurred when a customer used its own card designs in a PCI system. The design placed the board’s PCI controller too far from the connector. It worked all right in a standard PCI system, but not in the expansion chassis. The poor placement added too much delay, and the host controller missed the card during power-up enumeration because it took too long to respond.

Q: What are the advantages to using PCIe for the extension system’s link?

A: One reason to use PCIe is that the bandwidth to the host processor is not shared, so the data rate is guaranteed. In PCI and PXI systems, the bus must be shared with all cards in the slot, so bandwidth and access are not deterministic.

Test engineers might also choose PCIe extension technology because its data throughput is high. The sustained data transfer rate of PCIe-to-PCI/PXI expansion is over 100 Mbytes/s. This allows engineers to design test systems that use a server-class computer for data processing. PCIe provides the bandwidth that allows an expansion chassis to leverage the full power of such computers.

Compared to a PCI-based expansion, PCIe also decreases latency in the interaction between host and chassis. This is because PCIe uses newer technology, including a bridge chip and an equalizer, to link the host and chassis.

Q: What options does your company offer for PXI extension systems?

A: Adlink offers both PCI-to-PXI and PXI-to-PXI extension systems using StarFabric links as well as a PCIe-to-PXI extension system. We will also be coming out with an ExpressCard expansion option for laptop users. These extension systems can use any PXI-compliant chassis from any vendor as the expansion chassis.