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Thread: Rainbow 100A post error MAIN BOARD - Z80 CRC - ERROR 18

  1. #1

    Default Rainbow 100A post error MAIN BOARD - Z80 CRC - ERROR 18

    My Rainbow 100A is showing a MAIN BOARD - Z80 CRC - ERROR 18

    It has been in storage for around 20 years, and had a PSU rebuild and general check over before it was powered up again.

    I have checked the roms and they are readable, but could have bit rot.
    Unfortunately the rom images for a 100A seem to be rare, so watching this thread to see if they become less so.


    Any other pointers on a likely cause of the error would be gratefully received.


  2. #2


    You seem to have read the ROMs of your Rainbow 100A - could you please share the dumps despite your suspicion of bitrot - the more places we start form the better.


  3. #3
    Join Date
    Jun 2012
    UK - Worcester


    Hi Mark,

    Welcome to VCFED.

    The other side of the coin is that the machine has tried to reset the Z80 (or perform some other test on the Z80) and is reporting an error of some kind rather than a fault in the EPROM.

    I have found the source code for the 100B BIOS on Github (OK, not exactly the same as yours) but that (plus the technical manual and schematics) may help pinpoint the error a bit more.

    As per the previous post - it would be good to image and post your ROMs just in case they are good and this will help either yourself in the future or someone else.

    GITHUB link:


  4. #4

    Default Rainbow 100A roms

    I have attached the full 100A rom set from mine.

    I did parse the source from the 100B rom, but it didn't make much sense to me.
    Assembly was never my thing, which is why I stuck to hardware
    Attached Files Attached Files

  5. #5


    1. check the ROMs. ROMs for 100-A aren't meant to be used on a 100-B (and vice versa) - as there are subtle hardware differences. The 100-A uses 3 x 8 K EPROMs instead of 2 x 8 K, for example.

    2. replace the Z80. Newer ones are CMOS instead of NMOS and are usually rated for higher frequencies. It shouldn't matter, as long as they are pin compatible.

    3. check the diskette controller board and the disk drive. It is likely the RX50 needs to be replaced after very long storage (because of resin / dust buildup).

    As a last resort, you could also replace the CPU or the WD1793 floppy controller chip, as both are socketed. Just be careful not to touch or bend any pins.


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