Quote:
Originally Posted by SiriusHardware
In today's news, Colin's PROMs arrived today and have now been read.
Results:
UD6 - matches "basic-2-c000.901465-01.bin", checksum = 3838
UD8 - matches "edit-2-n.901447-24.bin", checksum = FDBF *
UD9 - matches "kernal-2.901465-03.bin", checksum = 7C98
* although an 8K device this IC only contains 4K of code, the checksum is of the lower half (0000 - 07FF). Upper half contains all FF.
UD7, marked 901465-02 - is dud. Contains almost all 0x80, the first two lines contain Strings of 'BE', 'A2', 'A6' and '82'. It's not a well chip, as we had already feared from Colin's measurements on it when compared to its sister UD6.
For replacements, there are a couple of direct-fit options that I can see.
One is to use an 8K / 24 pin MCM68764 EPROM with the 4K of code programmed into the upper half. The only pinout difference between the original PROM and the MCM68764 is pin 21: On the original PROM it is an active-high chip select: On the MCM68764 it is A12, the highest address pin, which, when plugged into the socket in the PET, is held permanently high, selecting the upper half of the IC.
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Those are my thoughts for possible substitutes - can anyone see any snags with those or suggest anything else which might be drop-in compatible?
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From a recent thread, here:
https://groups.io/g/DataioEPROM/message/5787 they suggested that the Chip Select / Enables are the 'standard' active-low polarity - On C64 KERNAL at least, but might have done it differently on the PET.
And you can also use the very-similar MCM68766 - Although both MCM6876x ones are now quite difficult / expensive (>=$20) to buy.
I also found some links to these handy PCB's:
http://store.go4retro.com/2364-adapter/ that let's you use a standard 2764 to replace a 2364
Or with links to set CS polarity:
http://store.go4retro.com/23xx-adapter/
- The blank PCB's are quite cheap, at $1.50, but no-doubt postage from USA could be quite a bit more than if there's any UK suppliers / getting them from China.
They also do one for the obscure MOS 6540 ROM's, on some PET's:
http://store.go4retro.com/6540-adapter/