cctalk Digest, Vol 70, Issue 3

[Brent Hilpert]

Ethan Dicks wrote:
> 
> 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.

Well, just to wade in with another 'opinion', if I have it correct:
  the transmitter must drive the lines within the range +/-5V to +/-15V,
  the receiver must respond correctly to signals in the range +/-3V to +/-25V.

The hysteresis provides some noise immunity; the wider receiver specs allows
for some line drop, noise, and common-mode bumping. There is also something
like a 1K to 7K impedance spec on a receiver.

--

If there were one thing about RS232 and (smart) modems that I wish had been
differently, it would be that the hayes modem standard had permitted some
technique to switch between data mode and control mode without having those 2
second guard periods. That was (is) so annoying waiting for dial-up software to
get the modem into control mode.

I haven't thought it through completely in relation to the RS-232 protocol
reqs., but perhaps using the control sigs in some manner, such as dropping DTR
and then sending some special character sequence; or using some
otherwise-unlikely combination of control sig states.