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Thread: PDP-9 at the RICM

  1. #41
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  2. #42

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    Great news, now we all waiting for the TU55 and the TC02!

  3. #43
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    Quote Originally Posted by anders_bzn View Post
    Great news, now we all waiting for the TU55 and the TC02!
    That will take some time...
    Member of the Rhode Island Computer Museum
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  4. #44

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    Well, I know. I struggled with the PDP-9 tape archiving project for four years...

  5. #45
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    Well, the PDP-9 at the RICM is not running again. We had a problem where the JMP instruction would not work, but everything else did. We traced it to the PCI (Program Counter In) microcode bit not going active in microword 74. PCI does go active during microword 21, so we know the flip-flop that latches the microcode bit, and the transformer that sends the data to the flip-flop are OK. The logic that "promotes: the target microcode address from 24 to 74 in microcode word 12 is quite complicated so it took a while to prove that it was working OK. We looked at the outputs of the G210 flip-chips that ground one end of the wire on the and connect the other end wire to -15V on the G920 microcode board. You can the drivers for 7 and 4 go active to drive the wire for microword 74. We tested the voltage drop for the current steering diodes on the G920 board, and they all look OK. Since there are no pulses from the G920 for the MBO, PCI, DONE, and LI microcode bits when microword 74 is activated the issue must be in the G920 board. Maybe a bad solder connection on one of the 64 tiny wires or one of the 128 diodes has partially failed.

    Next week we will setup a 200mA current source and measure the voltage drop when the wire for microword 74 is activated. We can compare the voltage drop for other microwords to see if the resistance through the diodes is different for microword 74. Hopefully it is just a bad diode and we can get this thing working again.
    Member of the Rhode Island Computer Museum
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  6. #46
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    We finally found the problem with the PDP-9. One of the diodes on the G920 Control Memory board had a voltage drop of 1.1V where it should have been about 0.65V. This caused the current flowing through the wire for Microword 74, the JMP instruction, to be low and didn't set any of the microcode bits for that word. The Program Counter didn't get updated with the new target address, so the JMP never happened.

    We noticed that the diode adjacent to the one that we replaced looks like it is slightly cracked. We also found that most of the diodes had a voltage drop of 0.65V, but a few had a voltage drop of 0.76V. These are also candidates for replacement. We had a selection of diodes in our spares and decided that the 1N4149 was the best fit for this application.

    The system now passes Instruction Test 1, Instruction Test 2, the JMP, ISZ, and JMS tests, and the Basic Memory Checkerboard. It reports a few errors during the Memory Addressing test. We still need to do the final timing and voltage tuning for the processor and memory, so hopefully that will fix the memory problem.

    Finally we will be able to connect the TC02 DECtape controller and the TU55 DECtape drives and start debugging those parts. Eventually we will get an OS running from DECtape. We could even think about making an emulator for the RS09 disks!

    RICM_PDP-9_G920_Bad_Diode.jpg
    Member of the Rhode Island Computer Museum
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  7. #47
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    Sounds like great progress. Could you describe your testing process for the diodes in more detail? Are you able to test in circuit or are you lifting a leg before applying power from your current source?

    Diode failures seem to be noted more frequently as people move back to more "primitive" systems and having a reliable diode testing technique becomes important. Have you tried using something like the Peak Atlas in-circuit testers?

  8. #48
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    Quote Originally Posted by jackrubin View Post
    Could you describe your testing process for the diodes in more detail?
    Very simple test. On the G920 there are 64 current steering diodes on each end of the FlipChip. The drivers connect one end to ground and the other end to -15V. There are some pull-ups and a small voltage regulator on the board that we can ignore. We knew that the transformers for the microcode bits used by the JMP instruction were OK because they created a signal when other microwords were selected. For the JMP instruction it selects microword 74. None of the transformers for the microcode bits created a signal with microword 74 was selected. When microword 74 is selected the "7" pin on the G920 is connected to ground. This actually connects to 8x diodes. The "4" pin is connected to -15V. This also connects to 8x diodes. There is only one pair of diodes that is connected to both the "7" and the "4" pins through the 74 wire. That meant we only needed to test 16x diodes, 8x on each end of the board.

    My DVM can measure the voltage drop across a junction in a diode or a transistor. Since there is nothing else connected to the diodes on the G820 the DVM can measure the voltage drop without interference from other parts or needing to unsolder a pin from the PCB. For silicon parts the voltage drop is usually about 0.65V. For some of the diodes on the G920 the voltage drop was in the range of 0.76V. These diodes are candidates for later replacement. We knew where to look for the 16 possible failed diodes, so it didn't take long to find the one that had a voltage drop of 1.1V. The transformer output is from when the current stops flowing, so the turn on time of the diode and the capacitance is not critical. We picked the replacement from what we had and the one that had the highest steady-state current.
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  9. #49
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    Thanks - nice sleuth work and follow through.

  10. #50
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    Quote Originally Posted by jackrubin View Post
    Thanks - nice sleuth work and follow through.
    Stanford found that 80% of the failures in their PDP-9 were in the Control Memory. They made a replacement for the G920 module using toroids that proved to be much more reliable.

    http://www.slac.stanford.edu/cgi-wra...-tn-73-003.pdf
    Member of the Rhode Island Computer Museum
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