PCI Express goes cabled
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2007 2:00:00 AM
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Just when you thought you’ve seen enough computer buses, along comes cabled PCI Express (PCIe) and its companion, cabled PXI Express (PXIe). Engineers developing synthetic-instrument-based test systems for military applications have shown interest in cabled PCIe because it has enough bandwidth for applications where digital signal processing (DSP) takes place in a separate box from measurement or stimulus systems.
“Cabled PCI Express is still a few years away from making it into everyday PCs,” said Richard McDonell, PXI and instrument control product group manager at National Instruments. “It requires less processing than parallel PCI, uses a thin cable, and can transfer about 250 Mbytes/s per lane. A four-lane PCIe system can transfer 1 Gbyte/s, although effective throughput is about 800 Mbytes/s.”
Today, you can get that speed between a PC and a PXI chassis, but some applications demand more speed. Wade Lowdermilk, engineering fellow at BAE Systems in San Diego, found that neither USB nor Ethernet had the high bandwidth and low latency he needed to move data from a digitizer to a processor. His system consists of two 2-channel PCI digitizers (14 bit, 100 Msamples/s, and 8 bit, 1 Gsample/s) mounted in a VXI system through adapter cards.
The VXI bus doesn’t have the bandwidth to move data from the digitizers to processor cards quickly enough. Lowdermilk tried PCI-to-USB and PCI-to-Ethernet interface cards to circumvent the VXI bus, but found that neither had the bandwidth he needs. “The link was not real time,” he lamented. His only option is cabled PCIe. “I like cabled PCI Express because it supports multiple platforms,” he said. “I can use it to transfer data to an outside processor or FPGA at speeds that let me view processed data in real time.”
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A PXIe based synthetic-instrument system needs the high bandwidth that only cabled PCIe/PXIe can deliver. |
Currently, the best cabled PCIe interface from National Instruments can handle four lanes, each at 2.5 Gbps per lane, or about 800-Mbytes/s data throughput. Lowdermilk needs eight lanes, but he is able to use a four-lane, PCIe generation 2 link, which runs at 5 Gbps per lane.
Lowdermilk had another application that requires cabled PCIe with higher speeds than currently available. He’s working with National Instruments and Phase Matrix on a PXIe-based synthetic-instrument system that uses two 18-slot PXIe chassis, one for measurement and the other for stimulus (see figure). His application requires two 8-bit digitizer channels that run at 2 Gsamples/s. To process and view the signals in real time, Lowdermilk needs a 16-lane PXIe link or an 8-lane PXIe generation 2 link. He currently has preproduction PXIe interface cards that he’s evaluating for his system. He’ll either build his own interface cards or work with National Instruments to develop the cards
Cabled PCIe shows promise for applications that process data separately from digitizers. Should it become available in standard PCs, cabled PCIe could find use in video systems. It has already attracted the interest of test engineers.
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