SWTPC 6800

Fred Cisin cisin at xenosoft.com
Fri Aug 5 13:57:00 CDT 2016

On Fri, 5 Aug 2016, Brad H wrote:
> 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.

OK, I don't know what your level of knowledge is on it.  Mine is NOT very 
high, so please don't be offended if I tell grandma how to suck eggs.

I would have hoped that, expecting a terminal, it would be wired as DCE 
(Data Communications Equipment), and plug straight through to a DTE (Data 
Terminal Equipment).

However, a PC is NOT expecting a terminal to be connected to it, and is 
wired DTE!  (PC is a terminal expecting to connect to a modem, not 
something to connect a terminal to.)

Therefore, ideally, the PC should connect straight through, with only
software issues of baud-rate, parity, bits, stop bits, and software 
handshaking.  That obviously is not the case, and rarely is.
Oh, wouldn't it be great if it worked that easily.

A null modem is for connecting two DTE terminals to each other.
It connects the transmit of one side to the receive of the other, and 
often numerous hand-shaking signals, which are NOT really standardized.
For example, if you want to connect a PC (DTE) to a printing terminal to 
use that as a printer, you need to cross TX and RX, and then mess with 
handshaking enough to get the PC to send characters when the printer is 
ready, and stop sending when the printer is not ready.  Sometimes, that is 
done by connecting handshake lines so that the PC ALWAYS sees the printer 
as ready, and then just send the characters slowly enough that the printer 
can keep up.  (typically 300 baud for daisy wheel, 150 or less for 
selectric).  Sometimes stop bits can help, if things are close.

They are usually wired symmetrically, but that doesn't always work.
With full handshaking, that crosses
TX and RX

With partial handshaking, that may be
TX and RX

TX and RX

It is kinda essential to get or make a breakout box.
1) to be able to try different interconnects
2) to have LEDs showing you the state of the lines.

Joe Campbell's "The RS232 Solution" is at a level that I can follow.
'course, there's always LUCK.  I had an assistant, who, the very first 
time that he saw a serial printer, he reached into one of my bins of 
random cables; and it worked!


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