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Thread: Seeking memory card for InBoard/386

  1. #31

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    Update: I have it working stably with eight 80ns 256K Simms and 74F logic chips. I'll post more details soon.

  2. #32

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    Quote Originally Posted by harrison.frazier View Post
    It uses 2 74* chips for the Row and Column strobe and two hex inverters for the address lines. That does beg the question- what is the 5th 74* chip doing on the original design? I don't have one, this is the only pic I could find online. It looks to be another hex inverter.
    I think that the additional chip was for the address lines as well. Im not sure anymore but the address lines were split into 2 "busses" for the banks. Since there are originally 9 address line (18bit=256k) the 3 hex-inverters would very nicely split the lines to have individual address lines per bank. Its pretty cool to see that my original layout was not wasted and you have brought it back to life ... you must be very close to getting the module to work. Maybe there are timing issues since you get the parity errors later on?

    - Could your inboard have a problem with parity? I own 5 Inboard 386 of which only 2 work anymore and i had to replace a parity chip on one of them.
    - Voltages may drop or be unstable. Do you use the original PSU or a new one?
    - Do you have another inboard to check the memory card?
    - Do you have another memory card?

    Maybe you can give some PCBs to other users to cross-check their results?
    I will try to order a bare PCB based on your provided layout.
    I will try some PCB manufacturers tomorrow, since PCBway has problems with the layout.

    -Jonas
    Last edited by stynx; October 23rd, 2020 at 09:58 AM.

  3. #33

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    Harrison has sent me the following as PM:
    The expansion board is completely stable with 74F logic chips and eight 256K 80ns simm modules! Once I swap to 70ns modules I get parity errors again.

    Very nice

  4. #34
    Join Date
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    Quote Originally Posted by stynx View Post
    Harrison has sent me the following as PM:
    The expansion board is completely stable with 74F logic chips and eight 256K 80ns simm modules! Once I swap to 70ns modules I get parity errors again.

    Very nice
    sounds like setup/hold time problems with faster ram

  5. #35

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    Quote Originally Posted by Al Kossow View Post
    sounds like setup/hold time problems with faster ram
    Maybe. But it could have to do with the way the ram chips react as well. There are a lot of things that can be different between ram chips... we have to test several chip types before we can have answers. Harrison will send me 2 of his 5 prototype PCBs for testing. We will try to get some more results since we now know that the design is mostly working.

    Maybe we should open a new thread for the further research

    I have opened a new thread to have a central point to post updates:
    Inboard-386-PC-2mb-expansion-CLONE

    -Jonas
    Last edited by stynx; October 23rd, 2020 at 11:49 PM.

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