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Thread: 8086 based IBM compatible PC schematics

  1. #1

    Post 8086 based IBM compatible PC schematics

    Greetings!

    I am looking for schematics of IBM PC compatible computers based on 8086, preferably PS/2 model 25 or model 30.
    What I am interested in exactly - is the part which is responsible for 16-bit word bus operations with 8-bit memory/IO, where bytes are shifted from higher to lower part of the bus, depending on even/odd addresses.
    I have checked the schematics for PC AT, and it gives some general idea, but still it's 286, not 8086, so I think there may be some differences.
    Also, I found the schematics for old soviet XT clone Poisk-2 with КР1810ВМ86М on board, but these are not really of good quality, and have no clear description, so I hardly understand what is going on there.
    Apparently, IBM has high quality documentation for their XTs and ATs, and I'd like to check out 8086 PS/2 models schematic as well, but it seems google knows nothing about it.

    Any help is much appreciated.

    Thank you!

  2. #2
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    You may find another Soviet 8086 clones interesting. Discussed at the end of this topic
    http://www.vcfed.org/forum/showthrea...original-PC-XT

    This "POISK-2" (don't know how to translate... may be "Pathfinder-2" haha ) is a turbo 8086 full ISA-compatible machine with onboard
    clock and EMS and has a full schematic, PCB gerbers, BIOS images e.t.c. available
    https://bitbucket.org/Haper/poisk-2-mainboard/src

    You may read an article at russian geek site habr.com via Google Translator
    https://habr.com/ru/post/407265/

    There was a person at ebay who sell PCBs.


    Other Soviet 8086 machines were not ISA-compatible and have a MULTIBUS1-alaike system bus.
    ES1840
    http://emuverse.ru/wiki/EC-184x
    ISKRA-1030
    http://emuverse.ru/wiki/%D0%98%D1%81...%80%D0%B0-1030

  3. #3
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    Search the web for the Olivetti M24 tech manual, it has schematics and is a clone based on the 8086.
    Offering a bounty for:
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  4. #4
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    If they exist, I'd love to get my hands on schematics for the Eagle 1600, an 8086 based somewhat IBM PC compatible clone. But my suspicion is it's missing exactly the circuity being discussed, as it seems to fail doing 16-bit read/writes to the 8-bit ISA bus. Hence, part of the reason it is only somewhat IBM PC compatible.

  5. #5
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    I remember a recent thread about the Amstrad PC1512 in which its schematics came up, but double checking it appears, alas, that the part the OP would be interested in is inside of a gate array chip.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  6. #6

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    A few of Tandy 1000 models have 8086 CPU. (Tandy 1000 RL)
    It is not 100% compatible with IBM PC.

  7. #7

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    Wow, thank you all for your replies!
    Now I have some good stuff to read
    From the first look it seems that 8086 PC clones use additional 74ALS245 to move a byte between hi an low portions of the bus, as well as another 74ALS245 paired with 74ALS373 on low 8 bits to latch even addressed bytes, while waiting for odd byte to be transferred to higher 8 bits. But on 286 PC AT, they used 74ALS646, replacing 74ALS245 / 74ALS373 pair with one single chip. Is there any significant difference between these two approaches?
    I'll also need to figure out where CE and DIR signals to control those transceivers / latches are coming from, but that's another part of the story.

  8. #8
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    Quote Originally Posted by lazy View Post
    I'll also need to figure out where CE and DIR signals to control those transceivers / latches are coming from, but that's another part of the story.
    Those control signals need to be generated by the motherboard - that’s probably the number one reason why 8086 motherboards are more complex than 8088 ones. A good high-level description of what needs to happen is in Chapter 8 of “ISA System Architecture” (third edition) by T. Shanley and D. Anderson - you can find a cheap used copy on Amazon.

  9. #9
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    Doesn't the M24 also use a few PALs on its motherboard?

  10. #10
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    Quote Originally Posted by Chuck(G) View Post
    Doesn't the M24 also use a few PALs on its motherboard?
    Yes, but for the bus steering logic they used an 82S123, a 32 x 8 bipolar PROM. The schematic is at page 275 and the PROM contents & description of its operation are at page 113 of the Theory of Operation manual. I would have suggested to look at this as an example but it’s well know that the implementation is “wrong”, ie the way the M24 splits 16bit transfers into two 8-bit cycles is not compatible with IBM’s. As you would know as you helped design a replacement for the bus correction kit in this very forum

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