Number of devices (was RE: shailaja)

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

> Can somebody tell me the maximum number of pci devices that can be
> connected to a bus?
 
For conventional PCI, I think the maximum number of devices per bus
segment is 32.  There is a 5-bit Device Number field in configuration
cycles which defines this limit.

Because most (but not all) systems derive the IDSEL signals from the
AD[31:11] bits, no more than 21 devices could be addressed this way.

However, almost every conventional PCI system will reach electrical and
signal integrity limits well before either of these logical limits is
approached.

The PCI architects wrote in the PCI Spec that 16 devices are "believed
to be a worst case."  However, achieving even that many devices on one
bus segment, almost definitely requires running the PCI clock
significantly lower than 33 MHz.  The recommended pull-up resistor
values in the PCI Spec are based on up to 16 devices.

10 "loads" is a more practical upper limit at 33 MHz, where each device
on the motherboard or system board counts as one "load", but each PCI
plug-in card (with one device per card) counts as two effective
electrical "loads."  This gives rise to the typical four slot
configuration (four PCI connectors) found on many motherboards.  Four
slots = eight loads, plus the host bridge, plus perhaps one more device
on the motherboard.  With careful design and layout, backed up by lots
of simulations, one can push it a little further.

At 66 MHz, one or two cards would be the more practical limit.

CompactPCI achieves a larger number of loads by using different
connectors, and a slightly different "termination" scheme.

Using non-compliant PCI adapter cards, or passive bus extenders, are
likely to reduce the maximum number of devices even further.

Note that I am talking about the number of loads per bus *segment*.  By
using PCI-PCI bridges, it is possible to have subordinate bus segments
that extend the total number of devices that can branch off a bus.

Regards,
Andy