SWTPC 6800

Brad H vintagecomputer at bettercomputing.net
Fri Aug 5 12:04:25 CDT 2016

I don't think I have the right baud rate at all... as I mentioned, my MSI
6800 has a very well labelled serial card and I set it to whatever baud rate
I was going for, set my terminal up and it wouldn't work.  It was only by
accident that I discovered if I set the terminal to 9600 baud it worked.
Some of the other switches worked at the speeds labelled but I think 1200 or
whatever was 9600.  So I wouldn't be surprised at all if this thing was
modified beyond.  I have the MP-S, and I have the previous owner's
handwritten sticker with the baud rates for each of the 5 DIP switches.. and
so I've got it set at what should be 300, but I have tried other baud rates
up and down the line without success so far.  Going up just produces longer
strings of random characters.  Going down produces less.

I am using a 25 pin femaile to 9 pin female null modem cable.  The 9 pin
goes into my old Thinkpad 380 and the 25 pin I have jumpered.  I followed
the guide here:  http://www.swtpc.com/mholley/HiTerm/MPS_to_DB25.pdf

Now, I don't have a DB25 female connector I could solder wires to.  So what
I did was, where required to loop pin 4 to 5, I just took some jumper wire,
and went from pin 4 to 5 on the female 25 pin (at the computer end), 6 to
20, 6 to 8, and so on.  You can get two jumper wire pins into a single shaft
so the connections are all good. But maybe it doesn't work that way, I know
null modem cables are a bit different.  In any event, wired as such, the
terminal got nothing from the computer at all.

In the event, I ended up reducing to having the MP-S set up per the
instructions, and then just had the TX RX and ground connected from the card
to the female connector.  With that setup, I get a response, albeit garbled.
I'm certain cabling is an issue here.  I get a bit dyslexic with pins and
understanding how things change on a cable like a null modem.

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Fred Cisin
Sent: Friday, August 5, 2016 8:52 AM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: RE: SWTPC 6800

> but as with all problem solving, Conan Doyle had it right when he said 
> "after eliminating the impossible, whatever is left, however 
> improbable, must be the solution"

That quote is often horribly abused.  Diagnosis by elimination is inherently
faulty.  Beginners, and "experts", will often use it to justify ridiculous
premises, "because I eliminated everything else", while not having truly
eliminated some of the most basic possibilities.  "It must be the
carburetor, because everything else is new" usually meant that they had not
gapped the points properly, or had the wrong firing order.

37? years ago, it took me 3 weeks to get ink on paper with a serial printing
terminal and a TRS80, because I made stupid assumptions because I thought
that I had eliminated . . .
and didn't even realize that there was a bad connection between the serial
interface and the rest of the Expansion Interface.

Why do you think that you have the right baud rate?
It might be 9600, but if it was expecting a printing terminal, then it might
be 300, 150, 134.5, 110, 75

In spite of "standardization" (everybody has a unique one of their own),
sometimes you run into some handshaking using signals besides DSR, DTR, 
RTS, CTS.     Such as CD (8), CD2 (12)!, RI(22)
"misuse of the standard"?  absolutely.
(using db25 pinout - BTW, a DE9 is not a DB9.  A real DB9 consists of a DB
shell with 1 thru 8 and 20)

It can be frustrating.  There is at least one documented fatality from the 
frustration of serial cabling.   Guy took a printer and computer to store 
to get it cabled; after 6 weeks without success, he shot the tech.  Joe
Campbell was sure that was an urban legend, so he tracked down the case, and
mentioned it in one of his books.

I don't intend to ever do it again; I'm getting rid of my ARC Data Tek
9600 DTS-1.

Grumpy Ol' Fred     		cisin at xenosoft.com

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