The most common RS232 receiver was the MC1389.
It only cares about Vin >3V.

I'd like to know about any common devices
that aren't happy with just 0V. I'm sure they exist,
most likely in old telco stuff with a negative supply rail.

The only problem I've had with RS232 has been with laptop
ports that only put out 5V (Compaq Presario 12xx series)
and some Kaypro's that used CMOS (4019's I think) to drive
the lines. In both cases they had inadequate current to drive a long
cable fast (and I was driving an opto LED so I expected to
see the spec'd 30MA current at 12V, but instead had only 4.5V
to work with).

Sure, it's bending the spec's a bit, but if you're not
driving 2000ft of cable at full baud rate, it's a reasonable
compromise. I rather doubt that a 3V GPS will drive a cable
any better than the 74HC14. Parallel the unused gates for output
if you have them.

I wasn't aware the that 16F688 had an invert bit for TX.
That makes Mark Rages' one transistor solution hard to beat.
R

William Chops Westfield wrote:

> 
> On Jan 31, 2005, at 6:35 PM, Mark Rages wrote:
> 
>>> [74hc14 is] Good for short range comms since any RS232 receiver will
>>> be happy with 5V in (>3v of spec).
> 
> 
> Not all receivers will be happy with the OTHER side (0V instead of -3),
> though.  MOST of them will, but not all.  I don't think I ever took apart
> a non-working example to see what it had for a driver, but I have run into
> equipment that didn't work.  (not recently?)
> 
> BillW
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