George White wrote in a message to Greg Mayman:

 RJT>> need here, I'd need something that would take a zero-based logic swing
 RJT>> and translate it to a bipolar one.  There's probably some
 RJT>> simple way to
 RJT>> do this but it's not apparent to me at the moment.

 GM> .  +5v rail------------+-----------
 GM> .                      |           |
 GM> .                 ------------    4700
 GM> .  +2.5v ref ----| -          |    |
 GM> .                |  1/4 LM339 |----+---- out
 GM> .    in ---------| +          |
 GM> .                 ------------
 GM> .                      |
 GM> .  -5v rail------------

 GW> It really needs some +ve feedback to make it into a schmitt trigger
 GW> circuit to avoid instability around the switching point on slow
 GW> signals. That needs 2 more resistors, some series input impedance
 GW> (say 10k) and one from "out" to 339 input (say 100k).

A litle snap action is always nice.    

 GM> That's the simplest way to do it with the least number of 
 GM> components. The ref source can be a voltage divider (2 x 10k)
 GM> between +5v and 0v, and the same ref can feed all four
 GM> comparators.

 GW> I'll disagree with that! :-) The simplest way is with an RS-232
 GW> line receiver like the 1489/1489A. Quad inputs, _NO_ external
 GW> components needed, only uses the logic +V rail. Spec to +- 30 V on
 GW> the input. Available to suit most system logic types (there are
 GW> CMOS ones if needed, and Schottky ones for speed). External 
 GW> components are only needed to vary the switch point.

Yeah,  but you've got it the wrong way around again.  I've got this pile of
4066 chips,  and was thinking about using them for audio switching
applications with bipolar 5v supplies,  and need to drive that logic swing
from a single-ended +5v supply.  If I were going the other way, the 1489
would be a good choice.

Interesting that you should mention that part, as I just saw it come up in
a discussion on a CNC list where somebody has to interface to some HTL
(12V-based) logic...   I wasn't sure that the threshold point would be in
the right place,  since they're designed for a bipolar input as well,  but
one respondent seemed to think that it was at around +3V or so.  I haven't
personally tried any experiments with these,  though.

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