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Thread: PDP-9 at the RICM

  1. #51
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    Sometimes the maintenance manuals are difficult to understand...

    Last week we when working on the PDP-9, we followed the System Adjustment Procedure and adjusted the Margin Voltage to the core memory sense amplifiers +7V and -7V relative to the normal +10 volts when running a memory diagnostic program. This margin voltage change is supposed to provoke memory errors. We then adjust the sense amplifier slice voltage so the voltage increase to provoke a memory error is the same as the voltage decrease. When we adjusted the margin voltage it didn't provoke any errors. We decided that this was a good thing, and that the core memory was working perfectly.

    After lots of discussion during the week, and Alex studying the schematics for the sense amplifier, we found that when the sense amplifier margin voltage is run through the +/-7V range it makes just a teeny-tiny change to slice reference voltage in the sense amplifier. The manual really said that there should be no memory errors when the margin voltage is in the +/-7V range, but that we should have run the margin voltage through the whole +/-10V range.

    We reran the core memory diagnostic and ran the margin voltage through the whole +/-10V range. At just a little over +10V we saw a few memory errors, and at -10V there were no errors. We decided to leave the sense amplifier slice voltage adjustment where it was, because we might disturb the "perfectly" working core memory.

    As another project, we are working on an LT Spice simulation of the G008 (Master Slice Control) and G009 (Sense Amplifiers) flipchips. We were able to capture the STROBE SAL, and the Field 0 Sense wire differential signals on our Rigol 'scope, and save the traces as a CSV file. LT Spice and import the CSV file and use the signals as inputs to the simulation. Now we can see how the simulation reacts to real signals.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  2. #52
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    Yesterday we wired the DC power to the TC02 DECtape controller (thanks Charles Lasner) and the two TU55 DECtape drives that we recently installed in the PDP-9 I/O cabinet. We were not sure that the power supply for the TC59 that was already installed in the cabinet had enough output to also power the TC02 DECtape controller, and two TU55 DECtape drives. When we first powered on the cabinet the +10V and the -15V from the power supply would not go to up to the normal voltage range. I was quite close to the wiring while checking the voltages and then this electrolytic capacitor blew with a big bang! The voltages then went to and acceptable range, so we knew where all the power was going. The bridge rectifier on the power supply is getting quite warm, so we will replace the power supply with a larger one.

    RICM_TC02_capacitor.jpg

    We still need to make ribbon cables to connect the indicator panel to the TC02 controller. Without the cables installed all of the indicators should be on. It looks like we have some bad connections, and possibly bad transistors on the indicator panel to debug and repair.

    RICM_TC02_Indicators.jpg

    Anders sent us replacement S series FlipChips for the ones that were missing in the TC02 controller, so after we get the wiring completed we can see if the TC02 will talk to the PDP-9. The TC02 is very similar to the TC01 DECtape controller that is in the PDP-8/I, so we are familiar with how it works and how to debug and repair it.

    Once we get the TC02 controller and one of the TU55s drives working we will need to make a PDP-9 Advanced Software System Keyboard Monitor System DECtape and see if it will boot.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  3. #53

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    I'm happy to see that you are getting closer!

    I have a separate power supply for my TC02. Are pictures of any interest?

  4. #54
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    Quote Originally Posted by anders_bzn View Post
    I have a separate power supply for my TC02. Are pictures of any interest?
    Yes, it would be interesting to see how yours is wired.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  5. #55
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    The original power supply in the I/O cabinet for the TC59 Magnetic Tape controller didn't have enough power to also feed the TC02 DECtape controller. We added another +10V/-15V power supply just for the TC02 and TU55 DECtape drives. Now the supply voltages for everything in the I/O cabinet are OK.

    We replaced all of the defective bulbs in the indicator panel, and now they all light up. The only thing left to do before debugging the controller is wire the 115VAC power for the TU55 motors, and make 3x indicator panel cables for the TC02.

    Maybe next Saturday we can wire up the I/O cables, load the TC02 Basic Exerciser and see if the controller is alive.

    RICM_PDP-9_TC02_Display-Panel.jpg
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  6. #56
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    What bulbs does the panel require? Were you able to find present-day replacements or are you using old spares? I've got a couple of TC02 panels to deal with and should probably start with the "easy" stuff.

  7. #57
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    Quote Originally Posted by jackrubin View Post
    What bulbs does the panel require? Were you able to find present-day replacements or are you using old spares? I've got a couple of TC02 panels to deal with and should probably start with the "easy" stuff.
    I could only find Dialight 507-397-1475-600 replacements. The originals are flush with the front panel. The replacements stick out about 1/8".
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  8. #58
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    We connected up the I/O cables from the PDP-9 processor to the TC02 and borrowed the indicator display cables from the TC59 controller. When executing TC02 I/O instructions there is no response from the TC02. Looks like we have some debugging to do.
    Member of the Rhode Island Computer Museum
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  9. #59
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    Looks like the S107 module in the TC02 inverts two of the device address bits from the processor. One of the inverted bits is inactive, so the next project is to check on the S107. We may have a slower R107 module so we can substitute to continue debugging.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  10. #60
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    Several of the Margin Power switches were on, and we didn't have the Margin Power Supply cable installed, so there were unpowered sections of the TC02. With the Margin Power switches in the correct positions the indicator light patterns look much more reasonable.

    After a few more hours of debugging we found an R202 Dual-Flip module in the slot where an R002 Diode Cluster module should have been. This prevented all of the Status Register A related IOT instructions from working. Now we can select a DECtape drive and tell it to go forward and backward. There is still an issue with selecting DECtape drive 1 that we need to debug.

    The Status Register B instructions still have issues. If we read Status Register B the contents don't stay in the AC for long. Turning on bit-14 in the IOT instruction clears the AC before ORing the contents of the TC02 register. It is possible that this isn't being decoded properly and clearing the AC while it is ORing the TC02 register contents. This issue should be fun to debug.

    There are two undocumented maintenance instructions that let you read/write the Data Buffer in the TC02. Writing to the Data Buffer ORs the AC with the Data Buffer. My guess is that the Data Buffer is not being cleared first. I can't tell from the schematic yet how the Data Buffer gets cleared.

    The TC59 Magnetic Tape controller has an IOT instruction to force a data-break. This lets you read/write the Data Buffer register and then force a data-break to see if the transfer actually works. There is one unused decoded IOT instruction in the TC02, so I may see if I can implement this test feature.

    We are making progress, but are not read to run diagnostics yet.

    None of the paper tape punches at the RICM are currently working.
    Is there anyone in this group that could punch the TC02 diagnostics on fan-fold paper tape?
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

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