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Thread: Build your own PDP 8I, Part 4..

  1. #151

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    Does this link work better? It's possible I put the first picture in a non shared area.

    https://photos.google.com/album/AF1Q...z7G2_0XvR4-Vcw

  2. #152
    Join Date
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    Hi Tradde;

    When did You move out of Colorado to Texas ??

    And, No that Link doesn't seem to work either..

    THANK YOU Marty

  3. #153

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    There, I think I found the proper sharing option. I retired in late Jan and moved to Texas in Feb. It's much cheaper
    here than it was there in Aurora.

  4. #154
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    Winnipeg, Manitoba, Canada
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    the reproduction face panels mentioned and the lights and switch board vince made

    Pegcity bits the home of praire bits and bytes a heritage lost yet not
    Heathkit h89 h77 h11 h10 with some extra bits
    Osborn 1 x2
    pdp8a with asr33 and documentation and software
    GE workmaster with series six PLC and ibm p70 (same as a ge workmaster 2)
    Commodore PC-10 II and other stuff

  5. #155
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    Been thinking about the front panel issue. Some further thoughts:

    The LD12 is based upon a PDP-8/I not a PDP-8/L. I got my letters and numbers mixed up at one point - the i and l look very similar in some typefaces...

    Marty is correct - Oscar's front panel uses a matrix arrangement of LEDs and both the PDP and LD12 don't. In order to use the LD12 with Oscar's front panel you would have to include multiplexing logic on the LD12. You would also have to deal with the keys issue (described below).

    Vince's PDP-8/i front panel recreation is getting us closer though.

    This has the PC, MB, MA, AC, LINK, DF and IF LED's (plus a few assorted status LEDs).

    The only drawback I can see with this is that there is no current limiting resistor in the bases of the driving transistors for the LEDs. On the LD12 - the logic gate outputs from the 74 series ICs drive the front panel directly, whereas the M900 card on the PDP-8/i drive the transistors via 470 Ohm current limiting resistors. You could either add the current limiting resistors to the LD12 - or use something like a 2N7000 MOSFET to replace each driver transistor on Vince's display board (assuming the pinouts are reasonably compatible).

    The keys on the other hand would require to be switched between 0V and +5V (otherwise you will blow up the 74 sieries logic on the LD12). That's fairly straightforward though. You would also need to add some form of debounce logic to the LD12 - as this is not present on Vince's board (neither is it on the PDP-8/i front panel of course).

    The next problem I can see is START, EXAMINE and DEPOSIT. There is an inherent assumption on the LD12 about the operation of these - and (at the moment) it seems to be incompatible with the operation of a real/clone PDP-8/i front panel. The basic problem is that (in a real PDP-8) the MB register is the interface between the core store and the CPU. All date to/from the core store goes via the MB register. This is not true on the LD12. Data from the memory enters the LD12 via the M bus not the MB register. There are also a few inconsistencies between the use of the PC and MA registers. I will have a further think about it - you never know it may be possible to modify the logic of the LD12 to support this way of operation.

    Whilst the internal LD12 logic was inspired by the PDP-8/I instruction set - the front panel isn't (hence the problem).

    In the meantime, I will have a look into using the LD12 front panel circuitry, but having an overlay that could be put over the indicators and keys to 'look like' a PDP-8 front panel (in terms of colour scheme etc.).

    A bit more thought required...

    Dave

  6. #156

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    Quote Originally Posted by daver2 View Post
    Vince's PDP-8/i front panel recreation is getting us closer though.

    This has the PC, MB, MA, AC, LINK, DF and IF LED's (plus a few assorted status LEDs).

    The only drawback I can see with this is that there is no current limiting resistor in the bases of the driving transistors for the LEDs. On the LD12 - the logic gate outputs from the 74 series ICs drive the front panel directly, whereas the M900 card on the PDP-8/i drive the transistors via 470 Ohm current limiting resistors. You could either add the current limiting resistors to the LD12 - or use something like a 2N7000 MOSFET to replace each driver transistor on Vince's display board (assuming the pinouts are reasonably compatible).
    Couldn't you replace the transistor with a base-collector jumper, and run the LEDs directly from the logic gates as the LD12 is expecting?

    Vince

  7. #157
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    Hi All;

    Thank You, Dave for Your Input.. I will have to re-read this a few times for me to make sense of it all..

    "" or use something like a 2N7000 MOSFET ""

    This Explains why my friend is using the 2N7000 in numerous places , I think it is for LED display purposes, on the 8080 Diode Transistor unit..

    THANK YOU Marty

  8. #158
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    >>> This Explains why my friend is using the 2N7000 in numerous places , I think it is for LED display purposes, on the 8080 Diode Transistor unit..

    And I have a design for a PDP-8 using 2N7000 transistors and resistors. No, I am not going there...

    >>> Couldn't you replace the transistor with a base-collector jumper, and run the LEDs directly from the logic gates as the LD12 is expecting?

    Thanks for the suggestion Vince.

    The LD12 outputs a 'logic 1 = LED on'. The LD12 front panel contains a 7406 open collector inverter - so the LED can be driven from +12V via a current limiting resistor.

    If you bypassed your transistor with a link, then the LED would be ON when the corresponding LD12 signal was OFF (exactly the opposite of what is required). However, you could 'turn the LED around' and change the +V supply for the LEDs to 0V/GND. The LED would then be ON when the corrseponding LD12 signal was high. The drawback with this (of course) is that you are limited to a +5V supply (driven from the LD12 TTL logic) and the VOH level from a TTL gate could be anywhere from +2.4 Volts to the VCC rail (e.g. +5.0 Volts) so the LED brightness across a number of LEDs could be different. Just a visual thing - not a functional one of course.

    Dave

  9. #159

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    Quote Originally Posted by daver2 View Post
    The LD12 outputs a 'logic 1 = LED on'. The LD12 front panel contains a 7406 open collector inverter - so the LED can be driven from +12V via a current limiting resistor.

    If you bypassed your transistor with a link, then the LED would be ON when the corresponding LD12 signal was OFF (exactly the opposite of what is required). However, you could 'turn the LED around' and change the +V supply for the LEDs to 0V/GND. The LED would then be ON when the corrseponding LD12 signal was high. The drawback with this (of course) is that you are limited to a +5V supply (driven from the LD12 TTL logic) and the VOH level from a TTL gate could be anywhere from +2.4 Volts to the VCC rail (e.g. +5.0 Volts) so the LED brightness across a number of LEDs could be different. Just a visual thing - not a functional one of course.
    Yeah, TTL doesn't do well trying to source current like that. I think you'd run into fan-out issues without a dedicated driver.

    I do, however, have stock of the M900X, if you want to be even more "authentic". That's essentially a 7404 inverter and bias resistor for the transistor, instead of the 7406. (Since you don't need the inversion, I'm sure a TTL non-inverting buffer would work nicely instead.)

    I'd suggest mounting a female header on the back of the LED board, and then a straight header instead of a right-angle header on the M900x. That way, the M900 would be daughter cards on the back of the LED board. You could go the hard way and look for connectors to go over the M900 edge connector, or you could just treat them as gigantic SMT pads and solder wires to them.

    Vince

  10. #160
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    Hi All;

    So, Dave Why are You teasing me, with this statement "" And I have a design for a PDP-8 using 2N7000 transistors and resistors. "" ??

    I will not ask about it any more, and hope when You want to, that You will share it..

    I have just gotten out the PCB of Dave's LD12, and I am thinking of working on it some..

    THANK YOU Marty

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