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Thread: PDP-9 at the RICM

  1. #61
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    We tried to control the TU55s through the controller. If we command the drive to move, it moves about 1mm, stops, and shows an END error. It thinks that it went into the end zone so it stopped the tape.

    After several hours of debugging we found that the G882 modules are oscillating when the tape is stopped. This is normal. The G882 module for the Mark track sends the decoded "data" to the Window Register, and after about 150ms the TP1 pulses show up. This starts clocking the "data" into the Window Register and eventually the code for the End Zone shows up in the Window Register. This is just random data and the TP1 pulses shouldn't be present.

    It looks like the signals from the Write Mark Track switch are not pulled down enough, so in the active state the signal is at -1.8V. It needs to be below -2.0V to be active, and this eventually lets the TP1 pulses into the Window Register before the Up To Speed signal goes active.

    Maybe I can get some time today to debug this, and compare the signals to the ones on the TC01 on the PDP-8/I.
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  2. #62
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    We replaced a defective S107 inverter FlipFlop. The control signals to the TU55 DECtape look better, but we are still getting the END error and the tape only moves about 1mm.

    Mattis punched the PDP-9 DECtape diagnostics and formatter for us.

    The PDP-9 has something like a RIM loader in microcode (HRI). The paper tapes have a bin (HRI) loader at the beginning, and the rest of the tape is in BIN format. With a little work we might be able to make a bin loader for the console port, strip off the HRI loader from the beginning of the paper tape images, and load the diags at 110 baud through the console. Getting the real paper tapes from Mattis will be a lot easier.

    I formatted a DECtape in PDP-9 format on my PDP-8/e. It was a little confusing because the word size in the formatter questions is in 12-bit words and the tape specification for the PDP-9 has 18-bit words. I think that I had it right with 384 decimal words to make 600 octal 12-bit words or 400 octal 18-bit words per block. The DECtape block size is the same on the PDP-9 and the PDP-10, but the PDP-10 uses it as 200 octal 36-bit words.
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  3. #63
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    Too many visitors yesterday to get much done on the TC02. We are still getting the END error after we send a MOVE command to the TC02 and the tape only moves about 1mm. This part of the TC02 is very complicated because there are lots of error conditions that cause the tape motion to stop. We will eventually find the problem and move on to the next issue.
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  4. #64
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    Today, after lots of false leads, we found that the signal from the processor that tells the TU55 DECtape drives to stop moving when the processor halts is always in the halt mode. No wonder the DECtape drives won't move the tape. Most likely a bad transistor in the S107 inverter flip-flop, so it should be a simple fix.
    Member of the Rhode Island Computer Museum
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  5. #65

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    Nice. I'm happy to see progress on this.

  6. #66
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    Yay, We replace a transistor on an inverter FlipChip an now the DECtapes move when we send a MOVE command to the TC02 DECtape controller. The indicator lights on the TC02 display panel flicker when the tape is moving, so it looks like it sees data from the DECtape.

    Time to run the MAINDEC diagnostics that Mattis punched for use so we can see how much of the controller is working.
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  7. #67
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    Well done!

  8. #68

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    Quote Originally Posted by m_thompson View Post
    Time to run the MAINDEC diagnostics that Mattis punched for use so we can see how much of the controller is working.
    I hope the tapes got punched allright. Those mechanical thing can be a bit finicky some times. Hard to verify the tape by the eye. Reading them in might help out. But if there is an error was it in the punching or in the reading?

  9. #69
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    Quote Originally Posted by MattisLind View Post
    I hope the tapes got punched all right.
    I tried the MAINDEC-9A D3BB-PB TC02 Basic Exerciser, and it read in OK. I haven't tried the other tapes yet, because the TC02 Basic Exerciser revealed more broken things in the TC02 DECtape controller.
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  10. #70
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    We loaded the paper tape for MAINDEC-9A D3BB-PB TC02 Basic Exerciser that Mattis Lind punched and sent us. We ran test #0, Basic Motion. This test puts the tape in Forward Motion and watches for the End Mark on the Mark Track, then reverses the motion of the tape. You can see from the indicator lights that the TC02 controller behavior is not correct, so more debugging is needed.

    Video here:

    We also tried test #11, Instruction test. This immediately halted at 03762, so the controller is generating an interrupt when it should not. I will try to fix that today.

    Thanks again to Mattis and Anders for helping with this project.
    Member of the Rhode Island Computer Museum
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