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Thread: No serial output on PDP-11

  1. #1

    Default No serial output on PDP-11

    Hello,

    I'm poking around with my Atex PDP-11/34 and am trying to understand why there is no output to the serial port. I power on the machine and then halt it, but there is no difference no matter what I switch on the control panel.

    The TX and RX lines go to a Z80-SIO/1 Serial I/O controller, which then outputs to a serial port. I've connected it up to my logic analyzer to probe for signals. The small 8 channel 24 Mhz USB probes.

    I had connected some test leads up to: Data lines 0 through 3, (CE) Chip enable, Reset line, RSTA (Request to send), and the transmit line.

    The data lines are showing signs of data going through it, so it is working. Chip enable is used to switch the data lines from input to output, and it is jumping around, but not as much as the data lines. The reset line goes high at the start, but then sits low. RSTA goes low at start and then sits high. TX (Transmit) Goes low at start then stays high (TX goes through an inverter before going to the port).

    If necessary, I have recorded the outputs via Sigrok Pulseview and can send it.

    Thank you very much,
    Nick

  2. #2
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    What about Clear to Send? Are you looping back Request To Send to Clear To Send?

    PDP-11 stuff generally doesn't use these lines - but this uses a Z80 SIO rather than the DEC chip, so...

    I assume you have the Transmit and Receive clocks present? No clock - no data transmission.

    Dave

  3. #3

    Default

    Quote Originally Posted by Nick7928 View Post
    The TX and RX lines go to a Z80-SIO/1 Serial I/O controller, which then outputs to a serial port. I've connected it up to my logic analyzer to probe for signals. The small 8 channel 24 Mhz USB probes.
    That seems odd to me. IIRC, DEC serial ports (of the DL type) used some older UART and depended on that chip's interface design. Is there a bundle of glue logic between the CPU and the SIO? OTOH, since the 11/34 uses an external ODT ROM (248F1, IIRC) it could use a custom ODT ROM, but that would mean that any operating system would also need to have a customized console driver.

  4. #4
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    Is this system one of these:
    http://www.vcfed.org/forum/showthrea...-is-Dismantled

    If so it is not really a DEC 11/34 per se (maybe only the original box is) but is really a custom Atex J-11 based design.

  5. #5

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    Hello,

    I was told that the computer is an 11/34, but there is also a J-11 CPU in there so I would believe it to be something closer to an 11/74 or 94. I have tried setting up a session of Tera Term and seeing if that would make a difference, but it did not. I have noticed that some of the Z80 SIO data lines go to the Z80 CPU that is in the image. Also, there is a Z80 CTC so it leads me to think that there is a small Z80 computer within the J-11. Here is the image of the Z80 chips: https://imgur.com/a/6v9njbk

    Thank you,
    Nick

  6. #6
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    My guess is it started out as a bog standard DEC 11/34(A?) initially, maybe with a custom ATEX interface board and the rest being DEC standard memory and peripheral cards.
    But it looks like at some point as a mid/end of life upgrade they did a full custom replacement J-11 based CPU/memory/peripheral card for the 11/34 core.
    Unless you can track down schematics/technical documentation it is going to be a long slog.

  7. #7

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    Quote Originally Posted by Nick7928 View Post
    Hello,

    I was told that the computer is an 11/34, but there is also a J-11 CPU in there so I would believe it to be something closer to an 11/74 or 94. I have tried setting up a session of Tera Term and seeing if that would make a difference, but it did not. I have noticed that some of the Z80 SIO data lines go to the Z80 CPU that is in the image. Also, there is a Z80 CTC so it leads me to think that there is a small Z80 computer within the J-11. Here is the image of the Z80 chips: https://imgur.com/a/6v9njbk

    Thank you,
    Nick
    Well, I can't see the SIO there, but I don't think that would help much. Anyway, with Z80 systems, you commonly used a SIO in combination with the CTC. The CTC will provide the clocks needed for the SIO to work.
    So obviously, the first thing is to check if the CTC is outputting clocks to the SIO as required. Otherwise your serial ports are going nowhere.

    As far as the 11 goes, if there is a J11 in there, it is no longer an 11/34. That much is sure. Closer to an 11/84 or 11/94 then. (An 11/74 is a totally different beast that is a whole thread of its own...)
    But a J11 should be outputting something always. So you should have some action on the serial port. If nothing else, you should get to the built-in ODT of the J11.
    I wonder how functionality would be left in an 11/34 front panel with a replaced CPU. Possibly a bit limited in what actually would work. But you maybe only have a limited front panel to start with?

  8. #8
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    Quote Originally Posted by bqt View Post
    But a J11 should be outputting something always. So you should have some action on the serial port.
    J11 ODT expects to talk to a DL11 compatible UART.
    If you don't have one, you're stuck.
    Z80 peripherals aren't compatible.
    If there are no other serial interfaces, they may have glued something onto the Z80 to look like a DL11, but you
    won't know until you dig into the hardware further.
    The other thing I would do is dump any firmware in the system so you can look for clues in the code.

  9. #9

    Default

    My 11/34 expects the following to bring up the console:

    Start Console Emulator 11/34a with M9312

    Halt
    clear
    165020
    LAD
    Contr. + Boot
    Now the console should be come up over Serial line via …

  10. #10
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    Quote Originally Posted by User78 View Post
    My 11/34
    HIS "11/34" doesn't have the original processor in it.

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