DECstation 220 - Out of Ideas?
robert.jarratt at ntlworld.com
Wed Jun 28 18:00:22 CDT 2017
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> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Rob Jarratt
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> Sent: 28 June 2017 19:12
> To: 'shadoooo' <shadoooo at gmail.com>; 'General Discussion: On-Topic and Off-
> Topic Posts' <cctalk at classiccmp.org>
> Subject: RE: DECstation 220 - Out of Ideas?
> I was thinking of trying with the chip removed, so I think I will do that. I have a
> multimeter of course, which is how I have been tracing already. The scanner
> idea is a good one too, and if nothing else at least it gives me an idea of where
> to probe without too much flipping of the board.
With the 74LS125 removed I found the ROM output enable was still not going low enough to enable the ROM output. Doing a bit more tracing I reached a 74LS243 quad transceiver.
Now here I think there *is* something odd going on. I noticed that the two inputs GAB and GBA are both at about 1.95V. I am not sure, from my reading of the datasheet whether that counts as low or high, it seems to be right on the borderline if I am reading the datasheet correctly. The ROM OE signal is going to the A4 pin (pin 6) of the transceiver, so I put the scope on the B4 pin and it is a steady 1.4V. That behaviour does not seem to correspond to GAB and GBA both being high or both being low, unless what is happening is that it is considering GAB and GBA to be both high and it is trying to send the B signal to the A side, in which case two signals are sourcing to the same line and possibly not letting the ROM OE signal drop. Perhaps the next step would be to remove this transceiver (or lift A4 or B4) and see if the signal does start to drop correctly. At which point it is either the transceiver that is bad, or more likely something is telling it to do the wrong thing.
Does that seem a reasonable analysis?
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