SWTPC 6800

Chris Elmquist chrise at pobox.com
Fri Aug 5 10:55:58 CDT 2016

On Thursday (08/04/2016 at 11:46PM -0700), Brad H wrote:
> Yes.. I tried 7 bits.. different parity settings, speeds etc.  Couldn't quite nail it down.  In every tutorial online for the 6800 being used with PC terminal, they go 8 N 1.. nobody mentions specifically if you are supposed to use hardware or xo/off or nothing though.  So that's another thing.  I'm also confused because some docs mention baud rate settings for the cpu board?
> I'm also not sure if bad RAM or bad TTL etc could be contributing to just throwing out random junk too.

On SS-50, the baudrate clocks are sourced from the CPU board onto approx
5 pins on the backplane.  Originally, the top speed was 1200 baud and the
MP-S and other cards would be jumpered to whichever clock (and therefore
pin on the backplane) was the desired rate for that port.

Over time, people wanted the ports to run at higher speeds and so would
make a change on the CPU board (cut and jump, flip a switch, depending
on design) to repurpose a backplane clock line from its original less
desireable rate to a faster one.  For example, 150 baud was seldom used
and might get repurposed to a 9600 baud clock.

So, you need to know something about the CPU board's design in order to
know what clock rates get routed onto which backplane baud clock signals
and then you need to know which backplane baud clock signal the MP-S
card is connected to for its baud clock.

These were almost always 16X clocks too, so if you scope the backplane
signals, the rates you see should be 16 times what the baud rate is.


> -------- Original message --------
> From: Dave Wade <dave.g4ugm at gmail.com> 
> Date: 2016-08-04  11:33 PM  (GMT-08:00) 
> To: "'General Discussion: On-Topic and Off-Topic Posts'" <cctalk at classiccmp.org> 
> Subject: RE: SWTPC 6800 
> "Random Stuff" on serial ports can be a speed, data-bits, or even parity
> mis-match. 
> I assume you have tried tweaking these?
> Dave
> > -----Original Message-----
> > From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Brad H
> > Sent: 05 August 2016 06:58
> > To: 'General Discussion: On-Topic and Off-Topic Posts'
> > <cctalk at classiccmp.org>
> > Subject: RE: SWTPC 6800
> > 
> > I think this 6800 is live but I am being dogged by my inabilities when it
> comes
> > to cabling.  I'm going to purchase a 'proper' db25 female connector and
> the
> > molex pins for the MP-S connector and solder it up.  I tried sort of using
> > jumper wires to make it go.  When I turn the 6800 on, the PC terminal I'm
> > using reacts by producing single or strings of random characters.
> > According to the SWTPC 'system checkout' stuff, if I get anything at all
> > showing up on the terminal it usually means the 6800 is alive.  Based on
> the
> > notes written on the MP-S, I'm confident I have the correct baud rate and
> bit
> > (8) and parity (1) settings, but alas, doesn't seem to work.  When I ran
> into
> > this sort of thing with MSI 6800 it turned out the baud rate switch on the
> > serial card wasn't correct, despite factory labelling.  That might be the
> case
> > here but not really sure how to sort it.
> > 
> > -----Original Message-----
> > From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
> Birkel
> > Sent: Thursday, August 4, 2016 12:29 PM
> > To: 'General Discussion: On-Topic and Off-Topic Posts'
> > <cctalk at classiccmp.org>
> > Subject: RE: SWTPC 6800
> > 

Chris Elmquist

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