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- PCI fly lead
October 13, 2008, 1:43 am
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Re: PCI fly lead
You can see imaginative examples here, of PCI Extenders.
The PCI bus is based on reflected wave switching. The
bus is not terminated. The signals bounce off the end
of the bus and back towards the source. The bus needs
time to settle, before the next clock cycle. This
translates to the PCI bus having some practical limits
to the total length. It also means, that the motherboard
design plays a part as well (uses up some length, and
can be a surprising amount).
For example, if you used a microATX motherboard, then
chances are you could use a longer extension. That
would be, because the bus length used up to route the
motherboard, could be shorter (only three PCI slots).
If the microATX board is really cheap, it may not have
a lot of PCI chips soldered to the motherboard, so less
of the length would be wasted on them.
If you had control of the PCI clock, you could slow
it down in frequency, and that would increase the
maximum length of the bus allowed.
When installing your extension, it should go in the
last slot. That maintains a simple linear physical
topology, and is less likely to yield poor or non
reproducible test results. Making a "Y" shaped bus,
well, I couldn't predict what would happen. So the
extension should go in the last PCI slot.
I'd only done an analog simulation of such a bus
once, and the number I got (subject to the
physical constraints of the project I was
working on), was 17 inches total (i.e. motherboard
plus whatever else). The number varies with the
loading on the bus (more loads, less length). If you
simply had the Southbridge (host) and an extension cable,
it could go longer than that.
The standard defines constraints on the position
of the chip on each PCI card, with respect to the
PCI edge connector. The short "stub" or copper
tracks on each PCI card, helps to make the thing
work. If you had a card which violated those
rules, such as a "combo" card that had no buffering,
then that could place some limitations on how much
additional length added to the end of the bus, would
A simpler solution, is to build up a PC which sits
open on a tabletop. By having no chassis, you have
easy access. All you need to invent in that case, is
a means of securing the card to the slot. Some of the
enthusiast web sites, have managed to build their own
versions of such things (test jigs).
Perhaps sawing up a PC case, such that you keep the
back I/O area metalwork, plus the motherboard tray part,
would be enough to help you build an "open concept"
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