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Thread: PDP-11/23 Bootstrap Failure?

  1. #1
    Join Date
    Oct 2008
    Kamloops, BC, Canada
    Blog Entries

    Default PDP-11/23 Bootstrap Failure?

    Okay lets try looking at this again since the last thread a while back was a spectacular disaster.

    I have a KDF11-B M8189 CPU board and a memory board M8067 that Lou gave me a number of years back. It was sent to me with two EPROMS in the bootstrap sockets that would start the machine up to a series of nice menus you could navigate from the VT100 and added support for local RX, RL and RD booting support as well as primitive diagnostics. About a year and a half ago I attempted to add a disk controller and somewhere in the process something happened. the status LED's on the CPU are now all lit which apparently means there has been a bootstrap ROM checksum error and the system halts immediately.
    As the board stands right now everything is reset back to the configuration it was at when it still worked. Here is where I am:

    -All but the CPU and ram are pulled form the BA23 chassis to aid in troubleshooting. The absolute minimum I can do.
    -I can verify that the BA23 power supply and backplane are not faulty. I have a spare KDJ11-B 11/73 CPU and M7551 memory board that when plugged in and powered I can reach the ODT.
    -I can verify the M7551 memory board is healthy enough to work with the test CPU.
    -I cannot verify if the M8067 is faulty as I do not know if the M8067 and M7551 can be interchanged.
    -I cannot verify the integrity of the bootstrap EPROMS as I do not have any image to compare with and all three of my EPROM programmers either cannot support the MCM68766C or are currently not functioning.
    -I cannot jumper anything on the KDF11-B that requires a wire wrap tool.
    -I cannot FORCE the system to drop to the ODT, even with the halt button.
    = Excellent space heater

  2. #2
    Join Date
    Nov 2014


    If you haven't already, I would check the PowerUp Mode Selection. It should be set to Mode 2, J18 to J19 installed and J18 to J17 removed - to boot from EPROM. Its also a good idea to check the other jumpers and switches for the correct setting.

    Then I would change it to Mode 1 - Console ODT and pull all the other boards including memory. This will bypass the EPROMs. The configuration is J18 to J19 removed, J18 to 17 Inserted. The small removable configuration jumpers will fit between two adjacent pins. That's what used on my board.

    If it won't go directly to ODT when power is applied, then the CPU board is the problem. If ODT works, check to read the bootstrap location (EPROM) @ 7773000 for valid data. Only then would I add memory and check for read/write, etc..

    Hope this works for you.

  3. #3
    Join Date
    Mar 2008
    Westchester County, NY


    EPROMS do occasionally suffer from bit rot. If one of the 68766 prove to be the root of your current trouble, I offer the suggestion shown in this photo : . You should be able to at least program 2764s with your eprom programmer. Ok, it's really crude, but it does work. The rightmost socket in the picture had to have another socket plugged into it. The IC's overlap each other in a stairstep fashion. Hey I'm a mechanical engineer, we rig stuff up all the time!



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