RE: PCI Signal Loading

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

I'd be cautious about this.

While most PCI output drivers may be able to support 10 loads, the REQ#
and GNT# drivers can be half-strength (see NOTE 1 below Tables 4-2 and
4-4 in sections 4.2.1.2 and 4.2.2.2, in the Spec).

Furthermore, the timing of point-to-point signals is much different than
that of bussed signals.  See Tval(ptp) and Tsu(ptp) in Table 4-6,
section 4.2.3.2.  For a 33 MHz PCI bus, that leaves only 4 ns for REQ#
interconnect delay, versus 10 ns for bussed signals.  The Spec has taken
advantage of the fact that non-bussed signals switch and settle faster.

Turning REQ# into a bussed signal has some risk.  However, adding a load
on the same card, electrically close to the source, isn't as bad as
being on two separate cards with two connectors and stubs.

(By the way, regarding the connector as a load ...  It's not the
connector itself, it's the stub of the connector and 1.5" long trace, on
bussed signals, that makes us treat add-in cards as two electrical
loads.  REQ# normally drives just one electrical load, the host
chipset.)

Regards,
Andy Ingraham