Unix-pc tty000 Problem Solved

[Craig Johnson]

Back in January I posted a request for help with a problem I was having
with my tty000 serial port.  I reported that the port seemed to have
broken over Christmas vacation, and that it now hung when put into raw
mode.  Output was hung and input acted like it was unblocked, ie.
returning the last character received, over and over.  In this
condition it was impossible to login with a passworded account over the
serial port.  I received no (as in zero) replies to my request :-(.
But, I've now solved the problem and am ready to share my experience
with you.

It seems that my problems started when I switched from having a printer
on the serial port to a new one on the parallel port.  I just didn't
realize that caused the problem since I did the switch weeks before
Christmas and it wasn't until I fired up the system after Christmas
that I noticed a problem.  By then I had had the printer configuration
changed long enough that it didn't occur to me that it was related.

Now, for the technical explanation.  Up until last night I had only a
3-wire connection between the 7300 and the terminal.  When the serial
port is used by the printer it is set up in a mode which ignores modem
control (no, this isn't explained anywhere).  When I had connected a
terminal in place of the printer, I had not turned off the printer
configuration, so the port responded nicely to the terminal without
caring about modem control.  In this mode, a 3-line serial connection
worked just fine.

After I changed over to a printer on the parallel port, the serial port
was no longer configured to ignore modem control.  It sort of worked,
but would hang after a bit.  This may have been due to CD, DTR, or
CTS.  I didn't pursue determining precisely which lines were involved.

Last night it finally occurred to me that this might be my problem so I
checked out the wiring diagram in the manual and modified the plug
wiring at the 7300 end of the serial connection.  The manual requires
that pins 4, 5, and 6 be tied together and pins 8 and 20 be tied
together.  Once these connections were made, everything worked fine.
Mystery solved!

The moral of the story:  Assume the worst about modem control lines
on the Unix-pc and don't try to get by on a 3-wire connection without
jumpering other lines.

	Craig V. Johnson		...!fluke!vince
	John Fluke Mfg. Co.
	Everett, WA