Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPUmodules)

[Eric Norum <norume@aps.anl.gov>]

Eric Bjorklund wrote:
> Umm... I may be completely missing the boat here, but didn't the 
> VME-MXI-1 problem mostly involve bad data returned because of  "address 
> rot" during the READ cycles? (c.f. Eric Norum's clarification post from 
> 14 Nov. 2003).
>
> We've seen a similar problem with one of our boards here at LANSCE.  In 
> our case, the "pipelined" address was not generated by the executing 
> code.  I don't think it was from a PPC or PCI bus "pre-fetch" either, as 
> there was no read cycle generated for the new address.   So unless there 
> is some real cleverness going on between the Universe and the PCI 
> controllers, it looks like the Universe chip is just trying to 
> anticipate the next address I am going to want and putting out on the 
> bus early (the new address was the logical successor to the addresses I 
> had previously requested).  I am not aware of any cure for this  -- 
> except that in our case it is our own board, so we can fix the problem 
> at that level.
Yes,  the problem is *not* related to posted writes nor back-to-back VME 
cycles.  We see the problem even with a single VME read cycle -- the processor 
changes the address lines as soon as the processer detects that DTACK* has 
been asserted.  The non-compliant VME/MXI-1 modules do not latch the address 
and so scramble the data they're putting on to the bus.

The workaround we're using is to program the Universe II chip to latch the 
data as soon as it sees DTACK* asserted rather than waiting the short period 
(T27) required by the VME specification.  This violation of the VME timing 
requirements is acceptable as long as all modules in the crate are 
conservative in how they put data on the bus (data first, then assert DTACK*).

So far this 'fix' has worked for us.

--
Eric Norum                                 norume@aps.anl.gov
Advanced Photon Source                     Phone: (630) 252-4793
Argonne National Laboratory