Signals & Noise: January 18, 1996
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Your article "PCI: real versus ideal" (EDN, Oct 26, 1995, pg 59) provides a great service to the design community. The article has substance, as opposed to articles in other trade magazines that just repeat the sales brochures.
My company, Emulex, has designed a PCI-to-Fibre-Channel host-adapter board, and I was the project leader for the PCI-interface portion. In building and designing the interface, our team ran into issues pertaining to interpretation of the PCI specification and in guessing which PCI systems would work in most systems. Your article validates a lot of our decisions and finally convinces our management that we did the right things.
However, I do have a question about the PCI-bus commands that you used in your tests. In Figure 2 (pg 61), I noticed that the burst-read was started by command "0210," which is "Memory Read." None of the systems in our lab can sustain a burst of more than eight words. We attributed this lack of endurance to the inability of our PCI interface to use the cache commands ("Memory Read Line" and "Memory Read Multiple").
Did you try and use those cache commands? If so, how did the results differ from using the noncache commands?
Vi Chau
Principal Engineer
ASIC Development
Emulex Corp
Costa Mesa, CA
Editor’s note : Thanks for the nice words about the PCI story. Your question is a good one, and it illustrates why we had so much HP equipment on hand. Processor-based performance testing is destined to underperform a full-speed PCI bus because the processor has other things to do besides run the bus. All our tests used the PCI bus exerciser, which has dedicated hardware that lets the bus run at full speed for as long as you want.
Brian Koga
Concept Development Inc
briank@cdvinc.com
On EDN’s Sept 1, 1995 cover, "Transmitting voice and vata: all you need is one phone line" should read "Transmitting voice and data..." Many readers caught the typo, but the most clever response came from John Owens: [I]won’t senv anything but v’s. Vut, i still reav your mag.
John Owens
Vir, Telecommunications
Large Government Agency
St Louis, MO
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