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Thread: 16-bit ISA IDE interface.... In GALs?

  1. #1

    Default 16-bit ISA IDE interface.... In GALs?

    Hey guys. I'm into yet another project that I may or may not ever finish.

    Here's the background, which is long and boring, so skip down if you start falling asleep. :P

    My trusty ol' PS/2 Model 25. It's the 8086 version, color. I have a PS/2 Model 30-286 on the way, whose mainboard, according to the internet (which never lies) will bolt right up to my Model 25 8086 chassis.

    Problem is, the ISA expansion riser from the 25/8086 doesn't work with either the 25-286 or the 30-286 (which also according to the internet use exactly the same mainboard).

    My first thought was to fire up the ol' bandsaw and gently cut off the 3rd ISA slot on the 30-286 riser, and then cut away part of the 25's riser mounting bracket so that the hacked M30 riser would fit. But I hate to cut things up like that. Being non-reversible and all that. D:

    Then the day after I bought the model 30, a model 30 mainboard by itself showed up, so I bought that too so I don't have to destroy the Model 30. I'll just fix it up, and I think I have everything else except the mouse (8513 monitor, model M keyboard) to put together a nice whole model 30-286 system to trade for something once I've toned out the pinout of the expansion riser and made mechanical drawings of it.

    (End of long boring exposition)

    ---

    So, since I am trying to learn kicad anyway, I got the bright idea to just design a 16-bit ISA expansion riser that would work with the Model 25/8086 mounts, in the Model 30 mainboard, without having to cut on anything.

    And of course, since my middle name is "creeping featurism", I'm trying to squeeze a 16-bit IDE interface and an XT-IDE boot ROM onto it, so as to save one of those precious precious preciousssss expansion slots.

    My google-fu being apparently nonexistent, schematics for 16-bit ISA IDE interfaces don't seem to be floating around anywhere. Based on what I know if the 8-bit XT-IDE cards, a 16 bit IDE interface ought to be far simpler, just some IO port decoding and some latches pretty much, yeah? It won't need that circular latching arrangement and delay lines to turn the 8-bit data on the 8-bit ISA bus into a 16 bit word for the IDE side, yeah?

    It may even be possible to implement it with only a couple of GALs, to save space?

    But I sure would like to examine a proper 16-bit ISA IDE schematic. Anyone have one laying around that I might peep at?

    Anyone ever try implementing an IDE interface with pretty much just GALs?

    Thanks guys. I apologize for my ignorance and apparent inability to google up a 16-bit schematic, heh.

    --------

    Edit: But in retrospect, maybe since I plan to use a CF card anyway, it might be simpler to just gin up an 8-bit XT-CF interface on it, since using a 16 bit IDE interface with an 8 bit CF card probably wouldn't be any faster anyway.
    Last edited by bladamson; November 15th, 2020 at 07:29 AM.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): Tandy 1000 EX/HX power supply, Mac IIci hard drive sled and one bottom rubber foot, Multisync VGA CRTs, Decent NuBus video card, PC-era Tandy stuff, Weird Old Unix Stuff, Aesthetic Old Serial Terminals (HP and Data General in particular)

  2. #2
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    Do not the mod 30/25 use 8086s? Another twist to the scenario. You already have the basis for a 16 bit pseudo isa bus interface.

  3. #3
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    There’s very little to a regular 16 bit IDE interface, so decoding one with a GAL shouldn’t be any problem at all. It’ll basically be about as complex as an XT-CF and the I/O portion of that fits in a 20v8 with loads to spare, unless there’s some complication related to 16 bit wait signaling I’m not thinking of.., but I *think* the IDE drive itself handles that?

    (Fitting both the I/O and the ROM decoding pushes the available pin count; as a pointless exercise I did make up equations for an XT-CF entirely in one GAL and I did make it fit, but the ROM decoding resolution was pretty low.)
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  4. #4
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    Move to a CPLD--more pins! I've got some old Xilinx PLCC 5V parts NOS, if this will be a one-off.

  5. #5

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    Quote Originally Posted by bladamson View Post
    Edit: But in retrospect, maybe since I plan to use a CF card anyway, it might be simpler to just gin up an 8-bit XT-CF interface on it, since using a 16 bit IDE interface with an 8 bit CF card probably wouldn't be any faster anyway.
    CF cards are perfectly capable of 16-bit transfers, and it would certainly be faster

    don't hamstring yourself

  6. #6

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    Can you do a 16-bit interface on an 8086-based system? I would think the motherboard doesn't understand /IOCS16 and other needed signals to do a 16-bit slot. Obviously the 8086 itself can do 16-bit transfers, so maybe hacking on a 16-bit IDE interface somehow wouldn't be too involved. I guess it depends on what signals are available where the riser plugs in.

    Adding an IDE port to the normal AT bus would probably be fine with 2x 74245s and a PLD.

  7. #7

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    I have a "promate" IDE bus interface from '89 that consists of a PAL16L8 and a bunch of 74ls24x buffers. The PAL is socketed, so reading the logic and reconstruct the schematic would be relatively easy. I can send it to you if shipping cost is not overkill.

  8. #8

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    Quote Originally Posted by bakemono View Post
    Can you do a 16-bit interface on an 8086-based system? I would think the motherboard doesn't understand /IOCS16 and other needed signals to do a 16-bit slot. Obviously the 8086 itself can do 16-bit transfers, so maybe hacking on a 16-bit IDE interface somehow wouldn't be too involved. I guess it depends on what signals are available where the riser plugs in.

    Adding an IDE port to the normal AT bus would probably be fine with 2x 74245s and a PLD.
    I actually don't know. The model 25-286 has the full AT bus, but I'm not familiar with the 8086 variant

  9. #9

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    Quote Originally Posted by maxtherabbit View Post
    I actually don't know. The model 25-286 has the full AT bus, but I'm not familiar with the 8086 variant
    Yeah, the 8086 only has an 8 bit bus, but the -286 variant mobo that I'll be installing has a full 16 bit bus. There were -286 versions of both the model 25 and model 30 (far more common on the model 30), and there was even a model 25sx that had a 386, but it is incredibly rare.

    Edit: Sorry, I wasn't clear in the original post. The extra model 30 mainboard that I have coming is also the 286 version.
    Last edited by bladamson; November 15th, 2020 at 12:41 PM.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): Tandy 1000 EX/HX power supply, Mac IIci hard drive sled and one bottom rubber foot, Multisync VGA CRTs, Decent NuBus video card, PC-era Tandy stuff, Weird Old Unix Stuff, Aesthetic Old Serial Terminals (HP and Data General in particular)

  10. #10

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    Quote Originally Posted by Chuck(G) View Post
    Move to a CPLD--more pins! I've got some old Xilinx PLCC 5V parts NOS, if this will be a one-off.
    That is very kind of you, and I really appreciate the offer, but I'd like to try to stick with stuff that's still pretty easy to get. If this works, I want to make the CAD files available so other people can make their own. And maybe if my dream of getting out of the farming racket and back into tech stuff ever comes true, I'll want to try to sell a few assembled ones on Tindie or something.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): Tandy 1000 EX/HX power supply, Mac IIci hard drive sled and one bottom rubber foot, Multisync VGA CRTs, Decent NuBus video card, PC-era Tandy stuff, Weird Old Unix Stuff, Aesthetic Old Serial Terminals (HP and Data General in particular)

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