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<== Date ==> <== Thread ==>

Subject: Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPUmodules)
From: Eric Norum <[email protected]>
To: Eric Bjorklund <[email protected]>
Cc: EPICS tech-talk <[email protected]>
Date: Fri, 26 Mar 2004 15:01:42 -0600
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                                 [email protected]
Advanced Photon Source                     Phone: (630) 252-4793
Argonne National Laboratory


References:
mvme5500 (was National Instruments VME-MXI-1 modules vs. modern VME CPU modules) Kate Feng
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modern VME CPU modules) Andrew Johnson
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVME CPU modules) Kate Feng
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVME CPU modules) Andrew Johnson
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPU modules) Kate Feng
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPU modules) Andrew Johnson
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPUmodules) Kate Feng
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPUmodules) Kate Feng
Re: mvme5500 (was National Instruments VME-MXI-1 modules vs. modernVMECPUmodules) Eric Bjorklund

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