cctalk Digest, Vol 70, Issue 3

[Tony Duell]

> 
> On Wed, Jun 3, 2009 at 2:41 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
> > 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.

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.

> 
> > 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.

I am not suprised. There's no reason not to do this, it meets the standard.

-tony