cctalk Digest, Vol 70, Issue 3

[Tony Duell]

> 
> On Wed, Jun 3, 2009 at 11:37 AM, Dave McGuire <mcguire at neurotica.com> wrote=
> :
> > On Jun 3, 2009, at 12:06 AM, Jay West wrote:
> >>
> >> These devices had a lot of trouble with RS423, where the voltage was muc=
> h
> >> less than RS232. I don't recall the exact numbers but RS232 was somethin=
> g
> >> about 12V swings
> >
> > =A0+/- 15V
> 
> Formally, yes, that's the max signalling voltage, but in practice,

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. Anything between -3V and +3V is illegal.

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.

This explains a little circuit that I've sene used in small RS232 
peripherals that run off their own wall-wart. Namely to use a 7905 (-5V) 
regulator to power the logic. The common +ve raill (wall wart +ve, 
regulator common pin) is the +5V line, the regulator output is the logic 
ground (so there's the normal 5V supply, right way round, for the logic). 
The RS232 driver is a discrete transsitor circuit running between the +5V 
line and the (unregulated) input to the regulator -- the -ve side of the 
wall-wart. Provided the latter is large enough, this does meet the RS232 
spec.

-tony