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I am attempting to come up with a reasonable way of implementing a design that was intended for cheap computer applications in the 1980s and which was originally designed to use a pair of TTL PROMs (a 28S42 and a 24S10) to control a state machine and decode state to outputs. The usual recommendation I see for these devices (which are still available, but at ludicrously high price -- the cheapest I see from a distributor is in excess of £30 per chip) is to replace them with a parallel EPROM, but the fastest EPROM I can find has 45ns access time which is cutting it very fine for the 50ns cycle time I'm working to. I'd really like to find a replacement part that can match the 35ns access time of the original parts.

What kind of options do I have available? I've considered switching to an FPGA, but (1) this is a much more expensive solution than I'd ideally like and (2) FPGAs seem to only be available in surface-mount packages with narrow pin spacing, and I'm not really comfortable soldering those. I've looked at CPLDs, but I can't really tell whether they are suitable for this application: how would I encode the transition table of the state machine, for example? Are there parallel EPROMs or a similar technology out there that can match the access times of these old PROMs? Are there other possible solutions to this kind of problem that I'm missing?

Jules
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    CPLDs are probably suitable, though getting 5V compatible ones may be difficult by now. Or replace the whole state machine with a small FPGA. The engineering in any other solution is going to be worth far more than £30 though. –  Sep 23 '17 at 09:26
  • @BrianDrummond - A 28S42 is a 512 x 8 PROM. Sorry, but I can't see trying to replicate that level of complexity with a CPLD. – WhatRoughBeast Sep 23 '17 at 19:00
  • @WhatRoughBeast - that depends on how the PROM is being used. *Arbitrary* contents may not be replicable with a low-end CPLD, but since we're told it is being used as a state machine, it is quite likely that the *typical* state machine can be replicated. – Chris Stratton Sep 23 '17 at 20:27

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Beside the solutions suggested by @Brian Drummond in the comments, you could use:

  1. a 5V non-volatile SRAM (e.g. CY14E256LA-SZ25XI). Some NVSRAMs have an internal battery (which typically lasts 10 years). Other NVSRAMs (such as CY14E256LA-SZ25XI) have a built-in non volatile memory, which is copied to the memory at power up. But the recall procedure requires some time (20ms. Can you live with this?).

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  1. a 5V FRAM such as MSM5412222B-25TK-MTL.
  2. any other 3.3V non-volatile memory (if cheaper and fast enough) or 3.3V CPLD. And then use two voltage level translators.

Of course all these solutions are not pin to pin compatibile, and you'll need a PCB to make an adaptor. At the end everything might cost well more than the 30£, as Brian Drummond pointed.

But if your system uses more than one PROM, you can use only one ("large enough") NVSRAM in place of 2 or more of those PROM.

next-hack
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