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Thread: What did I do to my PDP-8 today.

  1. #71

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    Hey Vince. If you use a 74HCT238 you don't need to invert the outputs thus avoiding the sacrilege of using a 3 input nand as an inverter.
    Doug Ingraham
    2nd owner of Straight 8 SN1173
    5 other PDP-8's including an 8/i and a DECSet 8000
    SOL-20

  2. #72

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    Quote Originally Posted by DougIngraham View Post
    Hey Vince. If you use a 74HCT238 you don't need to invert the outputs thus avoiding the sacrilege of using a 3 input nand as an inverter.
    Probably true, but then I'd have wasted the 7410 gates, and would probably end up tying off the inputs anyway. There's also a pair of decoders breaking out the IOT operations later, and some trivial rework with a couple more 74138 in place of those eliminates the extra inversions.

    More troubling to me was the departure from "re-implementing the PC8E with modern parts", moving toward "designing a PC8E work-alike". Feels like a slippery slope that leads back to the design where all the logic is just a single ATF1508. (I've got scripts to do the that, and more reliably than I can design stuff, already.)

    Vince

  3. #73

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    Quote Originally Posted by MattisLind View Post
    I have spent some time with my lab pdp-8/a lately.



    By using switching PSUs it is much lighter to carry around in the lab which is nice. But since it is an 8A100 chassis it lacks the E-connector to support hex wide core memories. I did put a 20V PSU and a -5V PSU from the very beginning so it has now become time to create that little board that should be sitting where there are two holes to the right in the chassis.

    I spent some time yesterday working on the CAD layout for the board that would hold five ECS connectors. I also included the four bank select lines to allow for 128 k memory.
    That looks like a very nice replacement for the DEC G8018 PSU. Can you share any additional details (sources, part #'s, wiring diagram, etc..)?

  4. #74

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    Well, once I gave up on the routing problem (used the autorouter), making a "modern" PC8E board in the spirit of the old didn't turn out to be so bad.

    Mainly I used 74HCT30 gates instead of the 314 gates, which meant I didn't need so many inverters. The other changes are more or less what Roland had suggested. I did switch to LS138 decoders, as those are much easier to find. Here's a PDF schematic if anyone cares :
    http://svn.so-much-stuff.com/svn/tru...M840/M840X.pdf

    It may even be similar enough to the original design to "just work".

    Vince

  5. #75

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    Quote Originally Posted by ftcnet View Post
    That looks like a very nice replacement for the DEC G8018 PSU. Can you share any additional details (sources, part #'s, wiring diagram, etc..)?
    Don't really have a very good list. Just the screen shot of the Aliexpress purchase. The PSUs are cheapo chinese made PSUs. But it do seems to work just fine.


    The 24V supply was modified to supply 20V by changing a resistor. I had to create my own PWR OK circuit since the one on the small board in the back was removed to fit all the supplies. The M8316 board was not happy without a proper PWR OK signal generated after 5V got OK.

  6. #76

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    Plugging along on PDP-8 projects...

    Today (well, yesterday) I finished drawing the M8300 Major Registers board:
    http://svn.so-much-stuff.com/svn/tru...EC/Mxxx/M8300/

    The schematic is a bit of a departure from the DEC drawing, as I don't have the flexibility to draw implementation detail for the chips, etc. So mine is organized to do all 12 bits of a particular function on the same sheet. (I actually think that is also a little easier to follow.) The board drawing is meant to follow the E schematic implemented on the B etch quite closely.

    I also did a quick CAD drawing of the metal bit of the 8/I Bezel:
    http://svn.so-much-stuff.com/svn/trunk/3D/8IPanel/

    Last week with Bill and Rick's help I repaired my archive of decus-8-889 , which is an early implementation of ADVENTURE. Latest ADVENT source and binary can of course be found on Rick's site. Bill is working to reconstruct the historical timeline:
    http://svn.so-much-stuff.com/svn/tru...c/decus/8-889/
    https://www.rickmurphy.net/advent/
    https://poetnerd.com/pdp8-alive/advent/timeline

    Also cleaning up my inventory of parts and whatnot. Got labels on the the "chips in tubes", got some of the other parts moved to new parts bins that fit on a shelf where I can reach them, etc.

    Vince

  7. #77

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    I made my Omnibus hardware debugging station ready. The programmers console mounted above the real chassis to allow for accessing the boards on extenders. The slot E connectors in place.



    First object to reapir was 8k memory stack. When measuring on the boards I found all memory signals quite weird. So I had to start debug the CPU board. A 74S74 in the memory timing system on the CPU had gone bad.

    But the memory boards didn't start working just because of that. Now it was failing more consistently returning just 0 what ever I read out. Previously I got random data.

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