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Thread: Possible replacements for a malfunctioning M7264-EB CPU?

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  1. #1

    Default Possible replacements for a malfunctioning M7264-EB CPU?

    Hello everyone!

    I'm writing this as my parents' company is having some issues with their hardware.
    hey have been directed to this forum for help and they don't know much English to begin with.
    Please excuse any mistakes in translation, as I'm not especially familiar with vintage computers.

    We have a CNC machine that is being controlled by an LSI 11 (PDP 11) microcomputer with a 16bit Q bus.
    The CPU in question is M7264-EB with 8Kb of memory, and we also have two more boards with 24Kb of memory each.

    The CNC machine itself is fully operational, but our CPU seems to be malfunctioning.

    Is there a way to replace the existing CPU and memory with something like M7554-01, or to somehow modernize the system?
    We have the schematics for the whole system that we can provide if needed - CPU, memory modules, as well as the UART modules.

    Thanks for reading, and thank you in advance for your help!

  2. #2
    Join Date
    Nov 2014


    Do you have some diagnostic information that points to the processor being bad? Given the age of the machine, the problems could be intermittant connections, bad power supplies etc..

    If you have ruled out other problems, then I would suggest getting another M7264-<x>B rather than switching to a different processor. <x> here is refers to different memory chip vendors.
    Alternately a M7270 and a few memory board updates or reconfiguration may also work.

    Beyond that there are all sorts of complications you would have to investigate before using a different processor such as:

    • Floating point FIS instruction requirements.
    • Memory board refresh requirements.
    • The LSI 11-03/02 will run with odd program addressing defects, while the later cpu's trap and/or halt on them.
    • Does the program have any timing dependency on the LSI 11/03.
    • Backplane addressing and other differences.

    Some of this can be determined from the inventory details. The timing or odd addressing problems typically won't reveal itself until you run the program on the replacement processor.

    See for some detail on the LSI-11 CPU variants.
    More DEC fans can be found over at
    Good luck sorting it out,

  3. #3


    Hey Jerry,

    Thank you very much for the detailed write-up!
    I've translated and forwarded it to my parents' company, and hopefully that'll have been enough information to solve the issue.
    If any further issues arise, hope it's alright with you if I get in touch again

    Have a wonderful day, week, and holidays!



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