🚀 go-pugleaf

In article <MZb4w2600WB88M=4wv@andrew.cmu.edu> kg19+@andrew.cmu.edu (Kurt A. Geisel) writes:
>I am trying to convert a TTL signal into an RS-232 signal.  I was
>under the impression that this can be done simply with the MC1488...
>When the TTL input is at 0 V, the RS-232 output is at 3.25 V.  When
>the TTL input is at +5 V, the RS-232 output is at 3.94 V.  Needless to
>say, this is useless.
>Do I have a bad chip or am I missing some glue?

I'd say you have a bad chip; I breadboarded the same setup you describe
and got +4.5 at 0 and -4.5 at +5, roughly.

However, note that you are violating the rules:  the MC1488 is specced
for 9V or 12V supplies, and is not guaranteed to work properly on 5V.
Not only that, even if it does sort of work, as mine did, -4.5 and +4.5
are not RS232-conforming output voltages:  -5 and +5 are the minima.

The right way to convert TTL to RS232 is to use the Maxim MAX232 or any
of several related chips from Maxim and others.  They include both
transmitters and receivers, generate the necessary voltages themselves
from +5 (with the aid of a few capacitors), and hew rather closer to
the RS232 specs to boot.  The MC1488 is obsolete.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1989: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu