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Thread: Compaq Portable RAM Decoder PROM - reverse engineer help?

  1. #11
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    No, I meant in terms of function.

  2. #12
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    Quote Originally Posted by Chuck(G) View Post
    No, I meant in terms of function.
    I am not sure sorry.
    I imagine it is similar.

    If you're referring to my comment "Yes of course I can't have all 1MiB. I was meaning installing all 256Kx1 chips, and having a region of about 192K mapped above 640K for UMB. This is what is sometimes done with the IBM XT's PROM.", I was meaning the HIMEMV2 I read about, a third party replacement for the XT's PROM.

  3. #13
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    Yes, you'll note that I worked out the GAL equations for that one. I asked because that's what I was familiar with.

  4. #14
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    Quote Originally Posted by Chuck(G) View Post
    Yes, you'll note that I worked out the GAL equations for that one. I asked because that's what I was familiar with.
    Ahh right.
    Yea I'd intend to prototype and probably use a GAL in a small adapter board in the final design if I manage to figure this out.
    I have never designed a GAL, but have been experimenting.

    For starters, I am trying to recreate the original 256K PROM. Truth table seen in post #6.
    I am using the "table" method in WinCUPL. I am using some "don't care" symbols in binary notation.
    When I use the PLD file attached and program an ATF16V8B, it makes all outputs HIGH when all inputs are LOW, or only DACK0 is HIGH. Otherwise, all outputs will be LOW.
    Any idea what I'm doing wrong?
    CompaqPortable256KU35.zip

  5. #15
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    I'll have a look at it when I get a chance. I normally code GALs with equations. One thing I ran into was the lack of cells to use a 16V8 for the 5160. A 22V10 could be configured with the right logic and almost be an exact fit to the socket.

  6. #16
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    nztdm contacted me via my website, and asked for some help on this. I've took a look at it this evening, and came up with what I think is a solution. The table code in WinCUPL isn't always the most reliable, and there were some compiler errors in the '*.so' file. I simplified the equations and built an equivalent that simulates as expected. I've attached them below.

    simulation.JPG

    COMPAQPORTABLEDECODER.zip

  7. #17
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    Quote Originally Posted by Chuck(G) View Post
    I'll have a look at it when I get a chance. I normally code GALs with equations. One thing I ran into was the lack of cells to use a 16V8 for the 5160. A 22V10 could be configured with the right logic and almost be an exact fit to the socket.
    Thanks
    Just an update: someone has helped me with the GAL stuff.
    I'll now make an adapter PCB to test it in the machine.

    I've attached the WinCUPL file and .JED file for 16V8. Check the WinCUPL file for pin assignments. CompaqPortable 256K U35 GAL Replacement.zip
    The pinout may not be the most optimal for an adapter, but I didn't put too much thought into it at this stage.

    Now the task of figuring out what the truth table of the 640K version would be...
    If that is successful, maybe figuring out a UMB version, but that means desoldering the 64Kx1 ICs to replace with sockets, which I'd rather not do for fear of damaging the multilayer board.

    JD

  8. #18
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    Success!

    I've made a lil adapter, and it works perfectly in the machine.
    So if you've lost/killed your U35 RAM Decoder PROM (for 256KB on the System Board) in your Compaq Portable, it is fairly easy to recreate with a GAL and prototype board.

    20190327_014722.jpg


    I changed the pinout a little bit from the upload on post #16, to more easily make the adapter PCB.
    The GAL pretty much goes in the U35 socket backwards, with the GAL's pins bent away that'd go into the VCC and GND holes on the socket (probably unnecessary, as these pins -should- be high-Z), and wires connecting the GAL's VCC and GND pins.
    The adapter PCB is a more elegant solution.

    Pinout for the adapter PCB is in the .PLD file.
    Programmable file is the .JED, for GAL16V8B or ATF16V8B. CompaqPortable 256K U35 GAL Replacement.zip

    Now to figure out how to make the 640K version...

    JD

  9. #19
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    Question

    Have tested the following truth table, and the system works, but only counts up to 256K.

    Screenshot_20190327-033613_Excel.jpg


    http://minuszerodegrees.net/manuals/...ce%20Guide.pdf

    According to Page 117 of the Service Guide, maybe it's something to do with SW1 switch 3 and 4? In the manual, these switches are called "memory on processor board", but the switch settings aren't provided.

    However, the fact that the system still works as 256K with this PROM table (as far as counting RAM and loading DOS), means the 256K PROM would be redundant if this worked at 640K too, so it is likely not correct.
    I wonder if it's using the first half of bank 2, and none of bank 3, or if it's using 64K of each bank like usual. Might have to get the scope out.

    It is late. Will do more testing tomorrow.

  10. #20

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    Fwiw, I have a late model (rev C BIOS) Compaq Portable with 640k of RAM on board. It has 256Kx1 DRAMs in banks 2 and 3, and 64Kx1 in banks 0 and 1. There is a 74S288 PROM in socket U35, with no Compaq part number to be seen. I was able to read this chip with my GQ-4X and can confirm that the truth table in your above post is an exact match:
    Capture.jpg

    Capture2.jpg
    -Marcoguy

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