cctalk Digest, Vol 70, Issue 3

[Ethan Dicks]

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.

> 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.

-ethan