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Thread: Nascom2 IC70 Pin11 grounded

  1. #1

    Default Nascom2 IC70 Pin11 grounded


    I'm trying to get my old NASCOM 2 board running as a standard system after issues with the CPM disc drives and controller board.

    I have a ROM and memory chips for the board, bit at present it is not starting up, and I was looking at the modifications to the board and IC70 has pin 11 ( the direction control ) unplugged from the socket and attached to ground.

    Does anyone know why that was done, I expect it was to do with the CPM modifications but not sure.

    Also the buffer chip IC45 is currently missing - which I suspect is part of the same modification,

    Thanks for any help.


  2. #2
    Join Date
    Jun 2012
    UK - Worcester


    This looks like a 'non-standard' CPM modification to me.

    IC45 will be required as it interfaces the NAS-SYS and BASIC ROMS to the CPU along with any of the local on-board RAM/EPROM devices. Some one has removed this buffer to prevent any of these devices interacting with RAM that I guess has been located at a base of $0000.

    IC70 pin 11 being grounded has something to do with the VIDEO RAM bus buffer (it looks to be permanently in one direction). I could work out which way of I looked up the data sheet.

    In my opinion, the first thing to do is to post the switch settings for your N2 so I can see how someone has configured it.

    My guess would be that there would be a CPM BIOS ROM somewhere - and the power-on start will have been set to there rather than $0000 to execute NAS-SYS on start-up.

    Yes, have a look around for any further modifications and post those.

    By the way, welcome to VCFED as this is your first post. We have a dedicated NASCOM board as well if you are not aware. I can provide a link to it if you are interested via PM.


  3. #3


    Hi Dave

    Thanks for the response, I had thought that it must be a "non-standard" mod as it did not match any of the documentation I had found about changing the NASCOM 2 for CPM. The system was last used in the late 1980's - my CPM 2.2 receipt is dated 1984, and I cannot remember what mods I made at the time and my documentation does not seem to cover them.

    I agree that IC45 seems to isolate the Nascom2 Rom and Ram sockets, and I think IC70 pin 11 to prevent the Video ram being visible, but why that chip was not removed I don't know.

    The full CPM system had a VFC and MAP80 Ram card, and the VFC card has a rom chip that loads the operating system from the floppy. The system worked when I first powered it up a month ago but floppy drive B never worked and drive A packed up soon afterwards soon after I had got CPM to load.

    I'll take a note of the switch settings and post them once I get my mind around them all.

    No I did not know about the dedicated NASCOM board, I'll send you a PM.

    Thanks again


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