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Thread: Model 4 128K U72 PAL?

  1. #1

    Default Model 4 128K U72 PAL?

    Well, I still haven't found a Model II at a price I want to pay, but I have a Model 4 on the way to fix up. There was a non-working non-gate-array version with the 128k expansion that the bidding went crazy on at the last minute, so I bought what appears to be a gate-array version with the same non-working issue instead. I hope the lack of discrete logic doesn't make it too hard to fix, haha. But the green CRT will be nicer I think, anyway.

    So yeah anyway, so since this one only has 64k, I'm looking to figure out how the U72 PAL works, that is installed in place of the shunt when the model 4 is upgraded to 128k. The technical manual explains the different available memory maps, but it doesn't seem (at least not that I've found) to explain the conditions under which the map is selected, nor how the banks are selected, nor any truth tables or equations for the U72 PAL.

    There were also some 3rd party expansions that plugged into the CPU socket ( http://www.trs-80.org/supermem/ ), that could go up to 1024k in the Model 4. I'm curious as to how the banking on those worked, too. This seems like something that would be super easy to reimplement with SRAM, maybe even easier than reverse-engineering the 128k PAL, if I can find info on how the banking works.

    If you guys can point me in the right direction to find this information, I'd really appreciate it. If there's no PAL data available, I'd probably be willing to pay a modest security deposit for someone to loan me theirs so I can read out a truth table for it and reimplement it in a GAL. I would of course post all the info here once I'd sussed it all out, so that everyone that wanted to could burn their own.

    Thanks, guys!
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): TRS-80 Model II,12,16,6000, Mac IIci hard drive sled and one bottom rubber foot, Hercules card + mono monitor (preferably IBM 5151), Multisync VGA CRTs, 040 or 601 card for Mac IIci, Decent NuBus video card, Commodore PC(286+), PC-era Tandy stuff, Aesthetic Old Serial Terminals, Amiga 2000 or 3000UX

  2. #2
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    Quote Originally Posted by bladamson View Post
    I hope the lack of discrete logic doesn't make it too hard to fix, haha. But the green CRT will be nicer I think, anyway
    Just for the record, you're *sure* it's a Gate Array? I ask because the color of the screen isn't necessarily a 100% giveway, there are tweeners out there. My Model 4 has the non-GA motherboard (Identifiable by the serial port orientation) but its got a green CRT.
    Last edited by Eudimorphodon; May 17th, 2020 at 03:58 PM.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

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    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  4. #4
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    If you did indeed get a gate array version ( RS-232 out the back) then you don't need the PAL U72 to do the 128K upgrade; it's just a jumper change.
    --
    Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

  5. #5

    Default

    Cool, thanks guys.

    I should have searched a little deeper for that PAL source I guess. :P

    Yea, it may be non-GA. I'm just guessing based on the color of the screen and the arrow key layout (it has the inverse-T). I guess we'll see when it gets here.

    It has a no-video issue, which seems to be pretty common on the selection I've seen go through ebay. I'm assuming capacitor failure somewhere, but I guess I'll know more once it gets here and I can poke around with my scope. But the HV section seems to be working, as there's a raster. It's just not "right", though.

    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): TRS-80 Model II,12,16,6000, Mac IIci hard drive sled and one bottom rubber foot, Hercules card + mono monitor (preferably IBM 5151), Multisync VGA CRTs, 040 or 601 card for Mac IIci, Decent NuBus video card, Commodore PC(286+), PC-era Tandy stuff, Aesthetic Old Serial Terminals, Amiga 2000 or 3000UX

  6. #6

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    Ah, i recognize that ebay

  7. #7
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    I wonder if the inverted-T cursor keys are a more reliable tell for a GA than the green screen. My non-GA green does have the older keyboard.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  8. #8
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    Computer News 80 did some refurbished Model 4's with both a green screen and clustered PC-style arrow keys, and yet they were non-gate-array models (I have one). The ONLY easily visible external difference that is definitive is the exit location of the RS-232 port. If it's out the back, it's a gate-array; out the bottom or not at all it's a non-GA. For the 4P it's harder yet; you have to look at the chips on the motherboard to tell, as a non-GA motherboard will fit into the plastic-latch case that's normally for the GA. For the desktop model 4 if it's a 26-1067A, 1068A, 1069A, or 1070 (the 4D) then it's supposed to be a gate-array board; as the case is different between the GA and non-GA this is pretty clear, but harder to quickly verify than the RS-232 port exit location.
    Last edited by lowen; May 18th, 2020 at 01:26 PM.
    --
    Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

  9. #9

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    Quote Originally Posted by Eudimorphodon View Post
    I wonder if the inverted-T cursor keys are a more reliable tell for a GA than the green screen. My non-GA green does have the older keyboard.
    I read somewhere that people complained about the T cursor keys, so they went back to the old style later, too. But I've seen 4Ds with the inverted T, so I dunno if that's true!
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): TRS-80 Model II,12,16,6000, Mac IIci hard drive sled and one bottom rubber foot, Hercules card + mono monitor (preferably IBM 5151), Multisync VGA CRTs, 040 or 601 card for Mac IIci, Decent NuBus video card, Commodore PC(286+), PC-era Tandy stuff, Aesthetic Old Serial Terminals, Amiga 2000 or 3000UX

  10. #10

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    Quote Originally Posted by bladamson View Post
    I read somewhere that people complained about the T cursor keys, so they went back to the old style later, too. But I've seen 4Ds with the inverted T, so I dunno if that's true!
    All of the 4GAs and 4Ds shipped with the inverted-T keyboards. All of the 4GA keyboards were membrane keyboards as well as the first 4D keyboards. At a later point the 4Ds switched to Alps keyboards.

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