SWTPC 6800

Brad H vintagecomputer at bettercomputing.net
Fri Aug 5 16:15:43 CDT 2016

I think I will have to figure out how to do that.  Additionally I have one of those PC based oscilloscopes on the way.  I don't know how to use them 100% but I'm about to learn I guess. :)

I have one more question for you guys -- I have a few CT-1024 terminals and would really like this system to work with one of those.  However, all of the CTs are quite delicate and are set I think for 7, E, 2 @ 110 baud via soldered jumpers.  I'm a bit reluctant to try pulling them apart to get in there and fix that.  Is there a way to change the parity, etc settings on the SWTPC to match the terminal?  Is it necessary?

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Wayne Sudol
Sent: Friday, August 5, 2016 10:35 AM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: SWTPC 6800

Hi Brad. Troubleshooting baud rate and parity issues can be very frustrating. Might i suggest using an simple rs-232c breakout box to see what signals are being produced on the port?  Check if the txd and rxd blink when you hit the carriage return on your pc? Then you'll know if the garbage seen is really a baud issue or just some noise that's put on the line when you power up the swtpc.

On Fri, Aug 5, 2016 at 10:04 AM, Brad H <vintagecomputer at bettercomputing.net
> wrote:

> I don't think I have the right baud rate at all... as I mentioned, my 
> 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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