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Thread: Gt40

  1. #1
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    Default Gt40

    The RICM just received a GT40 graphics terminal as a donation. It has been in storage for decades, so it will need a little restoration. Fortunately no mouse nests were found so the restoration should not be too difficult. The processor is a normal 9-slot PDP-11/05 with the addition of a VT11 graphics display processor. The VT11 also includes boot ROMs that will allow you to download programs from a host computer.

    The H-740 power supply in the 11/05 chassis is the same as in the PDP-8/F. PDP-8/M, and DW8E-PC, so we know it well. We even made an LT-Spice simulation of this power supply so we could figure out how the +5V regulation works. The donor gave us a NOS spare power supply, so getting working power should be easy.

    We didn't get a 375 light pen or LK40 keyboard with the GT40. The donor said that the keyboard didn't come with the GT40 when he got it. It would be nice to get these devices, but since they are so rare we might have to make replacements. We wold be able to play Moonlander if we had a light pen. The DL11 serial port is not in the chassis, but I believe that we have several in the warehouse.

    The VR17 display is very similar to the VR14 display in the PDP-12, so we can use the video output from the PDP-12 to test the monitor before we get the VT11 working.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  2. #2

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    I think I can help you with the light pen. I have
    four but no GT40 unfortunately.

  3. #3
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    Default

    Quote Originally Posted by MattisLind View Post
    I think I can help you with the light pen. I have
    four but no GT40 unfortunately.
    That would be very nice. I also have to check to make sure that the G840 flipchip is installed to support the lightpen, and not just the G838. We could probably make a G840 if necessary.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  4. #4
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    Default

    Nice! My introduction to DEC came in 1973 at the console of a GT44, the big brother to this system. Lots of late night Spacewar and Lunar Lander. Even got some work done during the day.

  5. #5
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    New South Wales, Australia
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    Default

    that's cool! there aren't many GT systems floating around. I am currently restoring its big brother the GT44. Though sadly I don't have the VR17 to go with it. They are an interesting line of systems.

  6. #6
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    We also received an 11/24 with a VS11 and monitor from the same donor.
    We haven't touched this system yet.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  7. #7
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    Default

    We reformed the capacitors in the power supply and connected a DC power supply to the input capacitors. The +15, +5, and -15V regulators all worked OK.
    We put the power supply back in the chassis, heard a loud hum when we turned it on, and the circuit breaker popped.
    The AC diode bridge had a shorted diode, so we borrowed one from another broken power supply.
    Now some of the LEDs on the console light up, but the switches don't do anything.
    Time to check the console lock microswitch and wiring.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  8. #8
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    The console on the 11/05 is unresponsive, so we have some debugging to do.

    I was thinking of buying one of the KM11 replicas to aid in debugging. I noticed that a sticker on the backplane says that it is a GT40 backplane, not an 11/05 backplane.
    Is the KM11 slot still in the GT40 backplane?
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  9. #9

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    Did you try it with or without UNIBUS terminators? The other day I forgot both terminators and the machine was pretty much dead. Installing them and everything started working as I was used to.

    If it is not a terminator issue, my guess it is a Control board problem. I had one 11/05 that only reacted slightly when pressing START. The other keys did nothing. One microcode ROM was bad. The frontpanel is mostly microprogram controlled, but there are some extra handling of the START switch which do some extra non-microprogram controlled actions.

    It looks like the GT40 is quite fully stuffed with things. Maybe there is no slot for the KM11 there?

  10. #10
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    We would need to remove the M7013 VT11 Display Control & Character Generator to reveal the KM11 slot, if it has not been rewired for use with the VT11.

    I don't think that it has terminators installed now.
    They go in slots AB5 and AB3.
    Slot AB3 has M7014 VT11 Bus Control & Bootstraps in it.
    So only one terminator in slot AB5?

    There is no M7800 DL11 installed, so maybe we need a grant jumper in that slot.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

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