DECstation 220 - Out of Ideas?

Rob Jarratt robert.jarratt at ntlworld.com
Sat Jun 24 18:17:50 CDT 2017


As I mentioned in an earlier email, I have been trying to fix a DECstation
220. I have made what I think may be some progress, but I am out of ideas
now.

 

The problem appears to be that the 80286 CPU is shutting down and then the
Olivetti chip is RESETting it repeatedly. The reason for this seems to be
that it is failing to read the ROMs. The ROM is an ST M27512. The output
enable signal (active low) does not appear to go below about 1.8V, although
it does go up to 5V. It is as if the system is trying to read the ROM, but
somehow the signal is not going low enough to actually make the ROM read.
The signal is connected to DMAMEMR (DMA Memory Read) on a Chips P82C206
controller, and to something on the Olivetti GA099-B, it may be connected to
other components too, but I have not discovered any more so far. I have
taken the ROMs out and read them in a reader, they seem to read back OK, so
I don't think they are faulty.

 

I am not sure what could cause the signal not to go low enough, apart from a
bad P82C206 or a bad GA099-B. Does anyone have any suggestions or experience
of common causes?

 

Thanks

 

Rob



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