74123... What's it do?

From: Dwight Elvey <elvey_at_hal.com>
Date: Mon Jul 12 12:08:07 1999

"Bill Sudbrink" <bill_at_chipware.com> wrote:
> I know that it is a "one-shot". What does that
> mean? The pinouts in the Chip Directory are
> no help. Several seem to be involved in the
> DEPOSIT logic on the IMSAI front panel.
> Also, I can't quite figure out what the logic
> table for the 74107 is trying to say.
>
> Thanks,
> Bill

Hi
 There was a bunch of talk about what the chips
do but not a lot about what they do in the IMSAI.
First you have to understand how the front panel
interfaces with the system. The front panel causes
the processor to do the actual work by single stepping
and forcing instructions on the data bus during
the instruction phase of the processors execution.
 Different switch operations require different
numbers of single steps or numbers of clock cycles.
An example would be the examine switch. When you use
this switch, the processor single steps through
one instruction and 3 memory fetches. It executes
the JMP instruction with the address switches as the
address to jump to. The 3 fetches are all controlled
by a sequence of one shots, configured in a cascade.
When the sequence is done, it is the processor that generates
the address that points into the S-100, and not the front
panel switches. This use of the processor made it
possible to reduce the amount of hardware needed
for the front panel. You'll note that there is no
address counter on the front panel. It uses the natural
sequencer in the processor for things like examine next.
Dwight
Received on Mon Jul 12 1999 - 12:08:07 BST