Re: DON'T CUT THOSE WIRES !!

stratton@... wrote:

> From: stratton@...
>
> > But, I think it is wise not to cut wires, and first try to see if
> > the machine can work as it is. TTY is teletype port, and yes, it
> > is a current loop, most of the time.
> >
>
> If he can find the level shifter chips on that TTY port we could
> probably figure out if it is current loop or normal RS232. For
> example, if they are 1488's and 1489's, I'm guessing they would not be
> current loop but rather normal RS232 signalling. IO ports being
> pretty robust, I would think it safe to use the continuity check
> function of a modern digital multimeter to determine what chips are
> connected to the connector (particularly pins 2 & 3) while the machine
> is powered off. Often though it is visually obvious what they drove
> the port with.

If it IS current loop, pins 2 and 3 are likely NOT the signal pins.
There's not a standard as commonly followed as RS-232 for the
pins used for current loop. Many systems don't even use DB-25
connectors for that. The standard was 20 mA for a 1 bit (same as
-12V for RS-232) and no current for a 0 bit (same as +12V in
RS-232).

Jon