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Old 17th Sep 2020, 1:23 am   #39
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: I found it !. A very sorry looking MK14

The PROMs are bipolar fusible-link PROMs and there are anecdotes to the effect that that particular type of PROM can fail due to the micro fine wire fuses re-growing and making contact again.

I certainly have come across a few cases where bipolar PROMs have died. However, there's no reason to assume they are dud without due cause.

If you have a conventional EPROM programmer you can lash up an adaptor to read a pair of 4-bit wide PROMs as though they are a common EPROM. This sketch (attached) illustrates just such an adaptor to allow a 32 by 8 bit PROM to be read by an eprom programmer which thinks it is reading a 27256.

Just make a version of this which places the MK14 PROMs side by side so that the low nibble and high nibble are read out as one byte, common up the address lines A0-A8 and limit the address range being read by the eprom programmer software to 0000 - 01FF (512 bytes).

The attached drawing shows only A0 to A4 connected because the device in the example was a 32-byte device, you will obviously need to connect A5 through A8 as well, in order to read 512 bytes of memory.

My MK14 is also issue II and I've always just taken SOC's word for it that NADs was not introduced on the edge connector until issue III. I'll have to check that when I next have my original machine down for any reason.
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Last edited by SiriusHardware; 17th Sep 2020 at 1:39 am.
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