RE: Maximum load of PCI-BUS

["Ingraham, Andrew" <Andrew.Ingraham@compaq.com>]

> > In the PCI specification the input capasitance is specified with 10 pf.
> > Which is the maximum output load (capasitance) ?
> > 
> > In a PC hardware description book I read the maximum number of
> > participants is 10 !? (Is that true - up to date?)
> > On a ASUS motherboard CUBX are 6 pci slots and some other PCI chips
> (maybe
> > 3).
> > 
> > Can anybody tell me the rules ?
 
I believe the maximum number of devices you can have per bus segment is 32,
limited by the 5-bit Device Number field in  Configuration cycles.

You might find references to a PCI bus limit of 10 devices.  But there is no
such limit.  Ten is a recommended or typical number of equivalent loads, for
buses running at 33 MHz.  In this equivalent load count, an IC on the
motherboard counts as one load, but a plug-in card counts as two.  Thus, you
get the often-quoted suggested configurations of:
	6 motherboard devices plus 2 expansion cards, or
	2 motherboard devices plus 4 expansion cards.
Again, these are suggestions, not rules.

Elsewhere in the PCI Spec, it also suggests that 16 loads are "believed to
be a worst case;" and that system designers can have a trade-off between
number of loads and the bus clock rate.  The system designer just has to
make it work.

In addition to device (IC) input capacitance, don't forget about trace
capacitance and connector capacitance.  You might find that there is more
capacitance in the traces than in the IC pin.

Regards,
Andy