Re: Floating 64bit extension signals on a parked bus

[Andy Ingraham <ingraham@wrksys.ENET.dec.com>]

Tom Hotchkiss asks:

> 1. On a bus that is 64bit capable, and has been reset with 64bit
> enabled, what happens to the 64bit extension signals (AD[63:32],
> C/BE[7:4]#, PAR64) when the bus is parked on a device that is only 32bit
> capable?
>
> Our assumption is that these signals will be tri-stated, but then they
> may be floating for a long period of time, which could be a problem. 
> Another possibility is that the central resource is smart enough to know
> not to park the bus on a 32bit device on a bus that has also has 64bit
> devices, but this seems complicated.  Anybody know what happens in this
> situation?

Section 4.3.3 says: "The point-to-point and shared 32-bit signals do
not require pull-ups; bus parking ensures their stability.  The 64-bit
data path expansion signals, AD[63::32], C/BE[7::4]#, and PAR64, must
also be pulled up when they are connected."

I'm not entirely sure what "when they are connected" means, but the
rest of the sentence appears to say that the system (motherboard) must
pull up these lines.  Section 4.3 applies to motherboards.

Regards,
Andy
о