RE: PCI bus loading

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

>   I want to know what is the maximum loading for PCI bus 66Mhz/64bit ? Can
> any one help me this question?
 
PCI does not exactly define a maximum number of loads for any situation.

I think the upper (logical) limit is 32 devices, limited by the 5-bit Device
Number field during Configuration Cycles.  But approaching that limit might
require clocking the bus at a somewhat low clock speed because of the long
bus settling time, or propagation delay.

The practical limit is determined mainly by electrical (signal integrity and
timing) considerations.

The "typical" number of loads per bus segment for 33MHz PCI is 10 effective
loads, where an IC on the motherboard counts as one, and a PCI add-in card
counts as two.  Thus, four PCI slots and two motherboard devices are
typical.  But this is neither architected nor a firm limit.

The PCI spec implies that 66MHz PCI might be limited to one PCI slot, but
"more than one may be possible in certain cases" (footnote in section
7.7.7).

There is also a DC limit, determined by input leakage current and pull-up
resistor values, but usually the signal integrity/timing limit is lower.

The bottom line is that the motherboard or system designer determines what
works in his situation.  The designer may have some influence over the
factors that determine the practical upper limit.

Regards,
Andy