Differences between Classic ROMs from AMI or MOSTEK ?


Question to the hardware hackers on this list (or maybe a retired HP
engineer): is there some particular feature in the two Clocks, SYNC
and POR which causes Classic HP Mostek ROMs to fail with a read timing that works with the like AMI parts having the same HP part number ? I have examined signals of CTCs from both Mostek and AMI on the scope and saw no feature that should make such a big diffence between go / nogo. The trouble is, the Mostek ROMs do not start up (they always give out the same dataword on IS) despite the addresses do count up. Seems some state machine within the ROM hangs, so there must be something wrong with the timing sequence, but what detail eluded me I don't know.

Any hint is much appreciated. You may use my above email address but remove everything after the first four lower case digits and the at.



Hi Bernhard,

I have an old article about the HP-35 which states in part

Production models are either all-AMI or all-Mostek because the two suppliers' chips don't really run well together. Though the parts are the same electrically, the different locations of the precharge make it impossible for the Mostek A&R to operate with the AMI ROMs.

Although they mention a different combination of chips than you do I suspect it's the same problem.

The article also mentions that the prototype HP-35 did use chips from both vendors so apparently some combinations are possible.

Hope this helps.



Hi Michael,

"different precharge location" - a very good clue, I'll reexamine both
AMI and MOSTEK CTCs, ARCs and ROMs again on the scope and watch out for
what happens around the clock pulses. Maybe it's a combined effect in
the chip set, like in the Woodstocks, where I also had some initital difficulty to operate ROMs without the ACT.

One odd observation, though: the stubborn Mostek ROM I have got is from a fully functional HP55 which has the following strange mix of ICs:

Mostek: RAMs and the 1818-0120 ROM (page 0...3)
AMI: CTC, ARC and the two other ROMs

I also have a HP45 with all-AMI chipset except for the Mostek ARC.

Both machines are 1975 vintage. Maybe the progress in process technology since 1973 has helped to relax the timing issue so HP was able to start mixing chips from different manufacturers, but a small difference was left - which is not properly implemented in my current ROM reader hardware. AMI ROMs read perfectly, though.

best regards,

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