One more try - Can you ID this S-100 Serial board?

Chris Elmquist chrise at
Thu Oct 1 18:56:36 CDT 2015

On Thursday (10/01/2015 at 06:27PM -0500), drlegendre . wrote:
> Now a question..
> Can someone give me a quick rundown on how the CPU communicates with this
> board? Does the board show up as a few bytes in the memory map, like on
> page zero? Does it connect directly to some registers in the CPU? How does
> data move from the CPU / buss into & out of the board?
> In short, how does the computer know where to "find" the board - and how do
> they converse? I'm only concerend with the serial portion, the rest is
> still a mystery - the 50 pin headers might be anything from parallel ports
> to (proprietary?) controller interfaces.

This is where I'm holding onto my theory that it is a custom design for
a specific purpose--  as I do not see the usual jumpers or DIP switches
to set I/O or memory addresses.    If it were a generic card built for
general purpose use, it would almost certainly have DIP switches to set
an I/O or memory address decode.

On S-100 systems, you have both I/O and MEMORY space.  Things like serial
port cards were almost always in I/O space and were decoded within a 256
byte block.   Different S-100 systems had their console or other I/O at
different addresses and with different chips (8251, 6850, 2661 and other
UARTs with internal register sets as well as TR1602, AY-3-1015 and similar
"dumb" parts).

Without any jumpers or switches to set the decode, you will have to
reverse the design to figure out how they did it.  It might be I/O
mapped but it might be memory mapped since it was likely purpose built
for a specific application.  Along with this, you will probably not have
operating system support that understands how to talk to this setup so
you will need to modify a CP/M BIOS or other OS I/O support to understand
how to talk to it.

The AY-3-1015 UARTs are "dumb".  Their framing format is decided by
strapping inputs to the chip and then they present a byte-wide input
register and output register and strobes to read or write those registers.
This would likely be handled by buffers with enables to gate these paths
onto the S-100 bus.

I concur with Chuck that there were and are a lot of "MCT" companies.
I just have a high confidence that the logo on Bill's board matches the
logos I am familiar with for the local MCT here in town.

Chris Elmquist

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