Cromemco 16FDC and IMSAI front panel - test-drb@ccmp.vtda.org

20 Sep 2001

Doug Coward &lt;mranalog(a)home.com&gt; wrote:
...
   PROT & UNPROT pins 20 & 70 - The IEEE 696
standard
    says that these pins will be at ground. Some 
    motherboards ground these lines. But grounding
    these pins on the IMSAI front panel will disable
    the front panel.  To fix this, cut the traces,
    on the front panel, right at edge connector
    pin 20 and 70. 
I'm thinking that some CPU boards expect the front panel to ground one
of these signals when the panel is putting something on the data bus
via the panel connector (and the CPU board should therefore use the
data bus on the panel connector instead of the one on the
motherboard).  Maybe I am confusing this with some other signal?

I ran into this in 1989 with a CCS Z80 board and no-name chassis
with apparent 696 motherboard.  Took me a while to figure out why
it wasn't coming up.

-Frank McConnell