RE: basic doubts in PCI (re-send)

[Andrew Ingraham <Andrew.Ingraham@digital.com>]

My reply seems to have not made it out, so I am re-sending it.


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>3) How many PCI slots we can provide in a system? Can it
>be very much large? If not, what limits the number?

People sometimes talk about 10 loads, where each expansion card slot
counts as two effective loads.  From this, you get combinations such as
two expansion cards and six motherboard loads, or four expansion cards
and two motherboard loads.

However, the PCI spec does not have this limit.  The above are
recommendations only, suggested because they make it easier to build a
system without having to analyze the "signal integrity" of the bus.

Section 4.3.3 in the PCI spec suggests that 16 slots plus motherboard
devices are possible, but maybe a worst case.

Section 4.3.5 implies that you can build larger systems (more PCI slots)
by reducing the clock frequency.

When you have many PCI slots, the bus is longer and has more "stubs".
Signal reflections, overshoot, and ringing become more difficult to
manage.  This may force you to reduce the clock frequency to allow
enough time for reflections to settle.  At the same time, the total bus
bandwidth needs to be shared among more devices, further limiting
performance.

Some PCI bus pins have pull-up resistors, see section 4.3.3.  As the
number of loads increases, Rmax decreases, while Rmin would increase
(substituting the actual number of loads instead of 16, and noting that
Iil is negative).  For 5V signaling, the two would collide at around 25
loads.  For 3.3V signaling, you might approach 32.  These are total
numbers of PCI-compliant components (chips) on the bus.  But you might
have electrical problems before reaching those numbers.

Regards,
Andy Ingraham