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Thread: PDP-9 at the RICM

  1. #71
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    Jul 2014
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    Yesterday we ran MAINDEC-9A D3BB-PB TC02 Basic Exerciser test #0. This moves the DECtape forward until the TC02 controller finds the End Zone, then moves the tape backwards until the End Zone, and reverses the direction again. It only uses the Timing Track and the Mark Track for this, so only a little of the controller needs to work for this test to run. This test lets you 'scope just about all of the TC02 logic to see what is working while the tape is moving.

    While the tape was moving we noticed that the pattern of lights for the Longitudinal Parity Buffer and the Read-Write buffer looked a little unusual, so we looked at the data coming from the G882 Manchester Reader-Writers. Everything looked OK except for the D0+ signal that was noisy and had a 500mV bias. We measured the resistance of the coils in the TU55 tape head, and found that the D0+ signal was open. The TU55 manual says that only DEC can change tape heads, but we ignored that and installed a head from a TU55 drive that was recently donated. Now we can see reasonable data on all 5x tape channels. We ran test #1, Search Scope Loop, and saw block numbers incrementing in the AC display on the console.

    Just as we were basking in the glory of a mostly functional TC02, and about to make a video of the pretty light pattern, the processor halted. After a little experimenting we found that the core memory would only read zeros. Oh well, maybe we can fix that today and get back to the TC02.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  2. #72

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    Well done!

    It's always funny when something starts to work and something else breaks.

  3. #73
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    The memory is working again. The B310 delay line that takes CLK and makes POST CLK\ had a non-monotonic output so the pulse amplifier that takes POST CLK\ and makes POST CLK was double triggering and making a mess of the core memory subsystem. We replaced the B310 with spare and are back to debugging the TC02 DECtape controller.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  4. #74
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    Quote Originally Posted by anders_bzn View Post
    Well done!

    It's always funny when something starts to work and something else breaks.
    Happens all the time with these ancient machines.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  5. #75

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    Quote Originally Posted by m_thompson View Post
    Happens all the time with these ancient machines.
    And not even when they were ancient - when (later) DEC machines like the PDP-11/44 and VAX 78x were still in production, many customers were familar with the "How many DEC repairmen...?" trope.

  6. #76
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    We had a very successful day working on the DEC PDP-9 and TC02 DECtape controller.

    Alex found and replaced a bad diode on the B310 delay line FlipChip that prevented the core memory from working a few weeks ago. This diode worked fine in the forward direction, but instead of preventing current flow in the reverse direction, it acted like a resistor. This caused the output signal to have two pulses instead of one, and really confused the core memory controller. A few weeks ago we replaced the B310 broken delay line with a spare. We put the original repaired B310 back in the PDP-9 because we were concerned that we had tuned the adjustable delays in the core memory with the original B310, and the replacement B310 might have had slightly different delays.

    On the TC02 DECtape controller, we found that data bit 1 from the DECtape drive was not getting latched into the Read-Write Buffer flip-flop. The S603 Pulse Amplifier that transfers this data from the Manchester Reader-Writer flipchip into the Read-Write Buffer flip-flop was dead. Mike Lill found a bad DEC D-664 diode on the Pulse Amplifier, and Emilio replaced it with a 1N4149 diode. Data bit 1 is now getting latched into the Read-Write Buffer Flip-Flop. I am getting more help fixing the PDP-9, so the repair work is going faster!

    This failed diode had the same part number and the same failure mode as the diode on the B310 module. We are speculating that the cause was a partial breakdown of the PN junction in the diode. There are thousands of these diodes in the PDP-9, so hopefully we won't have lots of these diodes fail.

    Next we found another bit in the Read-Write Buffer that would not latch data. We replaced the R201 flipchip with an untested spare and got lucky, it worked. Mike Lill found a bad transistor on the broken R201 flipchip, so we will replace it and add it to our spares.

    The Read-Write Buffer and Data Buffer in the DECtape controller are really complicated. The data from/to the tape read/write head is 3 bits, and the word size in the PDP-9 is 18 bits. The Read-Write Buffer is 6 bits, so it shifts 3 bits of tape data from one side of the Read-Write Buffer to the other and then latches 3 bits of new data from the tape head. Then the Data Buffer shifts 6 bits of data from the middle of the 18-bit Data Buffer to the front of the Data Buffer, then shifts 6 bits from the end of the Data Buffer to the middle of the Data Buffer, and then latched 6 bits of data from the Read-Write Buffer into the Data Buffer. Once 18 bits of data have been assembled in the Data Buffer it is transferred to core memory using Data-Break.

    We ran the TC02 Basic Exerciser test #1, Search Scope Loop. We can see the DECtape block numbers in the AC on the PDP-9 console display count up and down as the tape moves from end to end. This is really good progress.

    We ran the TC02 Basic Exerciser test #4, Search Find All Blocks. The printout said that there was no interrupt when it found the end of the tape. The S202 dual flip-flop flipchip that holds the Interrupt Enable bit in Status Register B would latch the data for 200ns and then turn back off. We don't have a spare S202 so we swapped an S202 from another part of the TC02 controller. Now the interrupt enable bit stays on, so that means that the problem is in the S202 flipchip.

    Mike Lill checked all of the diodes and transistors on the S202 with a DVM, but all looked OK. Next week we will make a test fixture for the S202 flipchip and do some active testing to determine what is broken, and then repair it.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  7. #77
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    Good to hear that you're making progress and even better that you're putting a team together. Are they other RICM members or are you recruiting some young folks like Dawson in Duluth?

  8. #78
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    Rhode Island
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    Jack,

    The PDP-9 helpers are two existing volunteers, and one new young volunteer. Dawson should move here in a few weeks, so can hopefully volunteer him to help keep the PDP-12 and the PDP-8/I running. With some more help we could get one of the PDP-8/S systems and the PDP-8/L out of the warehouse and into lab space.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  9. #79
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    Mar 2004
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    That's great news. How close will Dawson be to the museum? I expect he'll be pretty busy but he should be a great asset.

  10. #80

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    Sure wish I lived up that way to be able to help. I'd be glad to work on an 8i. Too bad I don't.

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