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Old 13th Apr 2018, 6:37 pm   #2
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,482
Default Re: MK14 schematic revisions

Do you have a link to the late version of the diagram? I'm only familiar with the older version.

From what you say it sounds as though the mod partly addresses an earlier oversight, in so far as the PROMs should never have been activated solely when A11 was in one state or another, it should always have involved at least the RDS signal, just as you say.

It is slightly baffling that the design allows a write to PROM space which ends up activating the PROM outputs. Maybe the thinking was just that nobody would ever try to write to a PROM address, as that would be an illogical (!) thing to do.

I've read somewhere recently that the late(st?) issue MK14 board included some kind of improvement to the address decoding - On early through middle issues there are multiple images of the RAM, the PROMs, etc throughout the address range because the address decoding is only partial. In particular, the PROMs appear no less than 4 times in the first part of the address range.

So maybe this change - adding WDS and RDS to the enable signal for the PROM block - is used in conjunction with a change somewhere else which stops the PROMs from appearing as multiple images throughout the first 2K of the address range and frees up some of that range so that more RAM can be mapped into it - in which case it does make sense to have the WDS signal involved as well, as the chip select for RAM will obviously need to be activated by a write signal as well as a read signal.

Last edited by SiriusHardware; 13th Apr 2018 at 6:48 pm.
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