🚀 go-pugleaf

Kurt Geisel writes:

>Thanks for all the help.  The consensus seems to be: it should work,
>but you really should be using +/- 12v.  Well, this is very temporary
>and very hardware specific, so I think I'll stick with it for now.
>The port I'm trying to drive seems to accept stuff well under the 232
>spec.

In fact, the 1489 (RS-232 receiver, the most likely chip you're talking
to), is usually biased so that the threshold voltages are about 1V and
3V (it's specs are rather loose). You have to add a negative bias in order
to force the lower threshold below 0V, and most designs don't bother.
The upshot of this is that *in most cases* you don't need a special driver
at all - just use a CMOS inverter. I like to wire two in parallel for
lower impedance. TTL thresholds are a little close to the 1489's for
comfort.

BTW, use another CMOS inverter for the receiver: RS-232 signal goes through
100k or so to limit current right into the inverter, and put a couple of
diodes from GND to input and from input to VCC, both cathodes pointing up,
to clamp the input for safety. For best noise immunity, use a Schmitt
inverter. Not commercial quality, I know, but I've been doing this for
years, and have yet to have a malfunction or part failure.


Isaac
isw@cup.portal.com