4 Jun
2009
4 Jun
'09
10:27 a.m.
On Wed, Jun 3, 2009 at 2:41 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Of course. Most devices didn't bother to detect an illegal state on an
input line and moan about it. They just had an input stage with a
threshold somewhere between -3V and +3V. Any valid RS232 signal would be
handled correctly,
IIRC the well-known 1489 RS232 receiver chip has a threshold a little
about 0V. You can drive the input (RS232 side) of such a chip with a TTL
level signal and expect it to work -- I've done it myself many times for
testing/quick hacks. If I know the RS232 device I am tryign to drive uses
a 1489, I might well feed TTL signals into it. But I wouldn't do that in
anything I ecpected others to use.
Incidentally, I am suprised that any older RS232 device required 15V
signals and wouldn't work on 12V signals. I've got some older HP RS232
interfaces (11205, 11206) that use 741 op-amps to drive the RS232
outputs. They run off a +/-9V supply The slightly later HP11284 uses 1488
drivers, again running off +/-9V. AFAIK that is totally within the RS232
spec.
I thoght that one state was a positive voltage
between +3V and +25V wrt
signal ground and the other state was a negative voltage between -3V and
-25V wrt signal ground.
You might be on to something there... I checked and the 1488/1489 pair
is rated up to +/-30VDC.
Anything between -3V and +3V is illegal.
Illegal, yes, but as others have pointed out, 0V might work with some
modern equipment. There's no requiremnt, AFAIK, for the 2
votlages to be of the same
magnitude. A signal which swings between +12V and -5V is perfectly valid.
True, and I've seen +12V/-5V designs when the manufacturer didn't want
to spring for -12V but had -5V lying around. I couldn't tell you
where right now, but it was in something from the 1970s.