External PCI Express reaches 30 m
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 9/1/2006
PCI Express (PCIe), a serial version of the parallel PCI bus, has replaced its older cousin as the expansion bus in new PCs. Now, PCIe is coming out of the box.
External PCI Express puts from one to eight 2.5-Gbps lanes on a cable for computing-intensive applications. The cabling standard isn't finalized (it's currently at version 0.7), but it calls for a cable length of 7 m. Even so, engineers at Molex have run the bus as far as 30 m, as they demonstrated during GlobalComm 2006 (June 5-7, Chicago, IL; www.globalcomm2006.com).
To find out how they did it, I spoke with Gourgen Oganessyan, senior signal-integrity engineer, and Galen Fromm, electrical project engineer, by phone. “When you extend a cable,” said Oganessyan, “you run into problems at high data rates. Attenuation in the cable reduces differential amplitude, and reflections occur. Both cause bit errors.” To reduce bit errors, Molex partnered with Quellan, a company that makes a cable-extender module that equalizes the signal before it reaches the receivers and thus improves reception, which opens the signal eye.
For their demo, the Molex engineers used Stratix GX multigigabit transceiver blocks from Altera (standard SerDes devices) as transmitters and receivers. The devices contain programmable equalizers that can be tuned to a specific application. At the transmitters, the engineers applied pre-emphasis, amplifying higher frequencies to compensate for the low-pass-filter effect of a long cable. The SerDes devices also perform equalization at the receiver. The Quellan module has allowed Molex to optimize the features of both devices to drive the extended cable in a power-efficient way.
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| Insertion-loss measurements provided data used to equalize the transmission line. Courtesy of Molex. |
“We wanted to best utilize the equalization in the Altera device and the Quellan module,” explained Fromm. “We needed a custom module, so we provided Quellan with differential insertion-loss measurements.” From the measurements (figure), Fromm set the taps of digital filters in the Altera device's DSP core. Quellan created a module for PCIe by integrating a Molex PCIe connector into its design.
At the final stage, Oganessyan and Fromm experimented with longer and longer lengths of cable starting at 20 m. They stopped at 30 m. At the GlobalComm demo, they ran a PRBS pattern through the cable assembly
for nine hours on each day of the show, achieving a bit-error rate of less than 10–14.
