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Thread: PDP8/A - Quick look and Teardown

  1. #1
    Join Date
    Jan 2019
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    Default PDP8/A - Quick look and Teardown

    Even though none of them are yet operational, I've been messing about with PDP-11s for long enough now.
    It's time to have a quick look at my PDP-8s, starting with the PDP-8/A.

    https://www.youtube.com/watch?v=aEq2K7ylXjc

  2. #2

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    Thanks for the video - fascinating!

  3. #3
    Join Date
    Jul 2014
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    Looks like a nice configuration. I just fixed two 8/a front panels and need to fix a third. The RK8/E and the RK05 can be challenging to get working, but are very reliable once they are working.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  4. #4
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    Jan 2019
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    Quote Originally Posted by m_thompson View Post
    The RK8/E and the RK05 can be challenging to get working...
    That's a worry, but is as I imagined.
    I'm not sure if I'll ever get around to going that far down the rabbit hole.

  5. #5

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    Thanks for the video. That is a clean machine. I have three 8/a's in my collection one of which is in operating condition although at the moment the left floppy drive of the RX01 takes about 15 minutes to warm up enough to boot. It looks like it is something in the head amplifier circuitry.

    You appear to be missing the M8317 (option 2) board listed on the label on the back of the machine. The M8317 contains 4 subsystems:
    1. Memory extension
    2. Timeshare.
    3. Power fail and auto restart.
    4. Boot loader.


    The machine will operate without the M8317 but you will have access to only the first 4k of memory. The missing boot loader functionality will mean that you will have to toggle in the boot instructions through the console. The power fail and auto restart function was not used by the OS/8 operating system because OS/8 does not use interrupts. Upon detection of a power failure an interrupt would occur and the interrupt service routine would save the state of the machine and halt. When power came back the CPU would proceed and the state of the machine could be restored and operation could continue. The machine could continue to operate for a few milliseconds after power fail was detected. Because of the core memory a few milliseconds was more than enough time to save the Link, AC, and MQ registers and the interrupt service routine would already have save the PC. This functionality was almost never used due to lack of support in the OS. The timeshare option was used by the TSS/8 operating system. Pretty much all it did was cause an interrupt when an IOT instruction was detected while running in User mode. The operating system would then look at the instruction and provide some service in place of that IOT. The console IOT's were emulated to provide a way to send and receive data to that user's console.

    If you cannot locate an M8317 you can use the boards from the 8/e. The M837 contains the memory extension and the timeshare logic in the 8/e and will work just fine in the 8/a if the M8317 is not present.

    The bootstrap functionality on the 8/e came from the M847 card.

    The power fail option on the 8/e was provided by the M848 card.

    The M8317 replaced those three boards.

    Best wishes!
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  6. #6
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    Jan 2019
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    Quote Originally Posted by DougIngraham View Post
    The machine will operate without the M8317 but you will have access to only the first 4k of memory.
    The missing boot loader functionality will mean that you will have to toggle in the boot instructions through the console.
    Thanks Doug.

    I didn't know about the memory limitation without the M8317 (I didn't see any reference to that in the manual I was reading)
    Since the M8317 wasn't in the machine or the box of accessories, I almost certainly don't have it, though maybe it's stashed somewhere that I've missed.

    As for getting any Operating Systems running on this gear, that seems so far off to me right now.
    I will be happy just to have them working correctly from the front panel.

    Cheers

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