MITS 8800B CPU Board - test-drb@ccmp.vtda.org

14 Jun 2005

...
 I have an extensive inventory and can source the parts
and get this poor, battered
board back to its old self again. However, looking through the doco I noticed that
the 8800B requires a display/control and interface boards. A question for those of
you that own a MITS8800 - does it need those boards for a turnkey system? Can I
plug the CPU board into the backplane and run it without the d/c and i/f cards,
assuming I set up the ROM and RAM cards for the system? If not, (since I do not
have the d/c and i/f cards) I'll have to try and build them from the cct diagrams
I have obtained. This is do-able (but a lot of work) if I can get my hands on some
blank S100 w/wrap boards, but I noticed there's a 1702 EPROM chip and I'd need to
get the memory map for this chip to burn my own. 
The two main things the front panel provides to the running systems (not counting
debug functions) are:

  - The front panel generates the MEMORY WRITE signal to the bus. If you do not
    have a front panel, then you will need to generate this signal with a little
    "glue logic".

  - The front panel also generates a POWER ON JUMP, allowing the processor to
    launch at an address other than 0000 - this is necessary if you want to run
    an operating system such as CP/M which requires RAM at 0000.

    If you don't need RAM at 0000, then just put your bootstrap ROM at 0000 and
    you should be good to go.

    If you do need RAM at 0000, then  you will need to add a bit of circutry to
    allow your rom to "MOVE". One simple way, assuming you want to put your ROM
    at an address >= 8000 is to upon RESET, double-map your ROM to it's correct
    address (>= 8000) AND at 0000. While doing this, you need to pull the
    phantom signal to disable any RAM at 0000 - Begin your bootstrap code with
    a JMP to it's correct address (+3) ... As soon as you see an instruction
    fetch with the high bit set, release the double-map at 0000 and phantom,
    and your system will be operating normally from the ROM at the higher
    address.

Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
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