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Thread: Extended M847 board on pdp-8/a

  1. #1

    Default Extended M847 board on pdp-8/a

    I built an extended M847 designed by Roland and tried it on my pdp-8/a. But I couldn't get it to work. It would deposit into memory, but load address didn't work for some reason.
    So I attached my logic analyzer and read schematics and manuals. After a day of head scratching and reading manuals and schematics I recognised that something was wrong with the signal BRK DATA.

    From a sequence diagram in the schematic I got that this line has to be high when loading the address. But I saw a low signal. I made a quick change in the design to make the BRK DATA controlled directly by the micro-controller. Then I could load address as expected!

    A simple cut of the signal between U7 and U8 before it reaches pin 2 of U8 and then a wire from pin 1 of U7 to pin 16 of U1.



    Then I also had to fix the wires that was cut by the ejector holes that my board had.

    While debugging the board I made a quick and dirty tool I called PDP8CONSOLE. Unfortunately there is no support in the hardware to read back data from the memory so examine is not possible. It can probably be fixed by additional chips to gate them to the MCP23017 chip.



    I also made a small fix to the original program that Roland designed so that it would work with the patch wire change I did. Now I could load all sorts of programs!

    Here is a link to my fork of of the design by Roland: https://github.com/MattisLind/M847-e...d-version-V1.1. I will probably continue working a bit to add support for storing bigger and more programs in the onboard 1024 kbit EEPROM memory. It should be possible to have almost all pdp-8 diagnostics in there!

    Thanks Roland for this project!

  2. #2

    Default

    Cool!

    What do the "Trace" do? Is it single step?

    With Examine of memory and possibility to fetch register values this could be a supertool!

  3. #3
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    I've had out-of-the-box success with both the original (short board) and 32KMem Combo board versions of Roland's card. What am I missing here?

    Have you set up PDP8CONSOLE as one of the options on the boot board? Looks very useful!
    

  4. #4

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    Hello Jack!

    What type of CPU do you use? My machine has a M8315 board. I think that could be the reason. It works on KK8-E but not on KK8-A.

    The symtom I had when not working was that it rather than loading the CPMA it incremented the CPMA.

    No. PDP8CONSOLE is a firmware on the extended M847 board. Downloaded using the Arduino environment.
    Last edited by MattisLind; March 22nd, 2020 at 10:07 PM.

  5. #5

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    Here is the page in the M8315 schematic I was refering to:


  6. #6

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    Use the memory for the bootloader. Implement a possibility to receive RIM/BIN papertaper image over the serial port instead. Then you can load any software lightning fast (depending on the baudrate of the serial port) from the host machine.

  7. #7

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    I think I see what you mean. Probably a very good idea.

    What about this: Implement a simple binary protocol with some level of robustness that allow for commands like, deposit, load address, run etc. This tool executes in the AVR chip on the extended M847 board. Then create a command line tool to be used on the PC that reads a file, either RIM or BIN, parses it and then downloads it into the pdp8 using the binary protocol.

  8. #8

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    Quote Originally Posted by MattisLind View Post
    I think I see what you mean. Probably a very good idea.

    What about this: Implement a simple binary protocol with some level of robustness that allow for commands like, deposit, load address, run etc. This tool executes in the AVR chip on the extended M847 board. Then create a command line tool to be used on the PC that reads a file, either RIM or BIN, parses it and then downloads it into the pdp8 using the binary protocol.
    How fast is the serial port?

    I have a implementation that parses binary format (and RIM) that I used when I dumped paper tapes from PDP-8. I can send it if you're interested in it.

  9. #9
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    Quote Originally Posted by MattisLind View Post
    From a sequence diagram in the schematic I got that this line has to be high when loading the address. But I saw a low signal. I made a quick change in the design to make the BRK DATA controlled directly by the micro-controller. Then I could load address as expected!
    Is the behavior of the BRK_DATA line with your modification and firmware patch compatible with both the 8/a and 8/e?

  10. #10

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    I don't really know because I haven't been able to test yet. My guess is that it would work on the KK8-E as well.

    According to the Omnibus Spec there is a difference in how the BRK DATA signal is interpreted between the 8/e and 8/a. If this has relevance here is not that I can tell for sure.

    I checked the engineering drawings and maintenance manual for the M847 board but couldn't find anything mentioning the BRK DATA CTRL line. But the scan of the schematic is really hard to read so I might be mistaken.

    Maybe Roland has more input on the significance of the BRK DATA CTRL line in the 8/e?

    If someone would like to test, a simple test is to pull out pin 3 of U7 so that it doesn't drive the bus and see what the difference is.
    Last edited by MattisLind; March 24th, 2020 at 02:15 AM.

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