Any idea what PROM is intended in this diagram is ?PROM

It is from the rational R1000 computer.

Edit: Its not my project, I was trying to help the Danish data museum. They are making an emulator for this computer. And I realized I might have misunderstood their original question, but I will let this question stand, and accept the best answer.

  • 5
    I have no idea but you have schematics and specs for the R1000? I'm terribly interested in the machine language it ran (that was designed for running their Ada superset) - do you have any information on that? (Former Rational employee, but strictly software, not hardware, and on the cross-compilers at that. So even at the time I had no information on the R1000 architecture or internals itself - but I'd like to know!)
    – davidbak
    Commented Feb 26, 2022 at 22:14
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    Its not my project, I was trying to help these guys getting information: datamuseum.dk/wiki/Rational/R1000s400 . They are making an emulator.
    – Stefan
    Commented Feb 27, 2022 at 9:26
  • 1
    @davidbak perhabs you help the people out, if you have knowledge that they lack. Contact [email protected] or twitter (at)bsdphk
    – Stefan
    Commented Feb 27, 2022 at 17:45

2 Answers 2


Most likely a standard 4 KiBit 512 by 8 Bipolar PROM.

Going from pinout and assumed DIP 20 it's either an

  • AMD 27S28 (OC output), or
  • AMD 27S29 (Tristate output)

From the circuit shown I'd assume it being a 27S29. Timing wise both are the same.

Compatible types for AM27S29 (AM27S28) are

  • Harris HM7649 (HM7648)
  • Monolitics Memories MM53/6349 (MM53/6348)
  • National DM74S472 (74S473)
  • Signetics S82S147 (S82S146)
  • Texas Instruments SN74S472 (SN74S473)

I wouldn't get confused by different (inverse) logical address/data numbering, as that works fine as long as the programming file is coded accordingly.


it is almost certainly a 512x8 bipolar prom.

It is probably being used as a decoder.

They were common in the late 70's and 80's before fast CMOS ones were available. They are all obsolete now although some are still available in the surplus market.

Apple used them on the Apple II disk controller and printer cards.

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