DECstation 220 - Out of Ideas?

Ian S. King isking at uw.edu
Sat Jun 24 19:06:50 CDT 2017


On Sat, Jun 24, 2017 at 4:17 PM, Rob Jarratt via cctalk <
cctalk at classiccmp.org> wrote:

> 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
>
> I don't recall from this thread - do you have prints?  I'd try to divine
whether this is a wire-OR bus (likely).  If so, I'd suspect the P82C206.
It may not be toast itself - be sure it has sufficient power to drive the
OC transistor that's bringing down the bus, and that its connection to that
bus doesn't have excessive resistance, and that its ground (earth)
connection is also low-resistance.  A long shot: it could be something else
on that bus that's sourcing rather than sinking, but the scenario seems
unlikely.

I'd agree that it's probably not the ROMs, from the symptoms you're
describing - unless, again, one of them has an internal short/leak to rail
on its read-enable pin.  A thought: can you measure the current and its
direction on the P82C206 pin?

That's my troubleshooting thinking given that I don't have it in front of
me.  :-)  I hope this is helpful -- Ian


-- 
Ian S. King, MSIS, MSCS, Ph.D. Candidate
The Information School <http://ischool.uw.edu>
Dissertation: "Why the Conversation Mattered: Constructing a Sociotechnical
Narrative Through a Design Lens

Archivist, Voices From the Rwanda Tribunal <http://tribunalvoices.org>
Value Sensitive Design Research Lab <http://vsdesign.org>

University of Washington

There is an old Vulcan saying: "Only Nixon could go to China."


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