18 Jan
2022
18 Jan
'22
7:26 p.m.
If you include the 6875 Timer chip, follow the data sheet carefully,
it's a little tricky in its discretes (I don't remember from 40 years
ago what exactly it is).
The biggest disadvantage to the 6850 ACIA and 14411 Baud rate generator
is the baud rate is NOT software selectable.
SWTBUG was much better than MIKBUG and GMXBUG was even better.
If you want to emulate both the SWTPC and Motorola board addresses I
would strongly recommend some kind of programmable logic to handle the
very different addressing schemes.
The SS-30 I/O Bus had the address decoding on the motherboard. Each slot
was allocated 4 address (the later MC6809 version upped this to 16
address.)? Where as the Motorola system addressed differently.
On 1/18/2022 6:22 PM, Chris Elmquist via cctalk wrote:
On Tuesday (01/18/2022 at 11:35PM +0000), Jonathan
Chapman wrote:
How's about a Glitchbus board set that's
compatible? I was planning on doing it anyway.
That would be very cool. Something
along those lines was my plan B and
I even dug out a tube of 6802's for the effort. I think I could wire
up a prototype over a weekend. MC6802 is a nice "cheat" as you don't
have to mess with the two-phase clock stuff.
What would be really slick is an SBC that has everything on it to be
either an Altair 680 or an SWTPC 6800 just by changing some jumpers,
switches, etc. and putting the correct ROM monitor on the board.
If there was a PROM, a 32K SRAM, an ACIA and a bonus PIA socket, along
with a small amount of glue logic, I think we could run the ROM monitor
for either system and a bunch of legacy code in 32K of RAM-- which would
have been a big system in the day. The PIA would provide a fun GPIO
capability just for toggling bits to and from the real world.
The ACIA was the serial console device on the Altair and the later
MP-S on the SWTPC and so you would run SWTBUG on the SWTPC personality
to use that. I don't see a need for MIKBUG compatibility here, since
that requires the bit-banged console via another PIA and odd external
timer chip. Baud rate generator that can do 16x for 110, 300, 1200 and
9600 would be ideal. I'd want 110 for a real Teletype, 300 for Kansas
City Standard tapes, 1200 for a DecWriter and 9600 so that I don't fall
asleep ;-)
When can I order one! :-)
Thanks Jonathan,
Chris