IMSAI 8080 (our show so far...)

From: Dwight Elvey <elvey_at_hal.com>
Date: Tue Jun 29 19:52:04 1999

"Bill Sudbrink" <bill_at_chipware.com> wrote:
> > If I remember right it has to have addressable memory there too.
>
> Just for the address lights to light up right?

No, you don't need anything there. Here is the sequence that
is done:

1. Examine Switch
2. Place 0C3h on data cable to processor board
3. Step processor one cycle
4. Place Low address switches on cable to processor board
5. Step processor one cycle
6. Place High address switches on cable to processor board
7. Step processor one cycle
8. Processor places address on address bus and is holding
    for data from RAM to execute.

You'll notice that the key here is that the cable is most
important to be correct. The picture you had with the complete
switch to address mapping showed that the data signals from
the front panel to the processor were swapped on a byte basis,
0-7 swapped 7-0. Like I said, the reason it seem to work at
all is because swapping the bits of 0C3h is 0C3h. When you
inverted the plug and it didn't work, it was because it
was then sending something other than 0C3h to the processor.
 Like I said, it is 0-7 and 7-0 that is swapped. There
may be other issues as well but I'll have to look at the schematics
before I can say exactly what I think is the entire issue.
Dwight
Received on Tue Jun 29 1999 - 19:52:04 BST