I take it that you've never heard of 2 pin bicolor LEDs?
Commonly used on RS232 break out boxes to do exactly what
you are asking. One LED + one current limiting resistor per signal line.
RS232 spec says up to 30mA supplied. LED lights with a few mA
so you drive the LED DIRECTLY from the RS232 signal lines.
One polarity gives you red. The other, green, and no voltage
is black.

With 4 pin bicolor you just wire the LED's back to back.
http://rocky.digikey.com/WebLib/Stanley/BRPG1211C.pdf

Why are you trying to do it the "REALLY HARD WAY' with the
Maxim device? LED don't care too about voltage. Just current.

A bunch of LM339 comparators, with bias at RS232 thresholds
(+3V, -3V, is spec) will let you drive the LEDs directly (with
current limits of course) if you really need a buffered solution.
And remember that you only need ONE current limiting resistor per
pair of LEDs since only one is on at a time.


Robert

Vitaliy wrote:

> I'm helping a co-worker design an improved RS232 breakout box (for in-house 
> use), which will have an LED for each of the lines (except ground). Each LED 
> has three states: Green (active), Red (inactive), and Off (no signal).
> 
> What is the most straightforward way to implement this?
> 
> We have a solution using a MAX248 (which is also used to convert to TTL), 
> and an inverter to drive LEDs, which takes care of the Green/Red 
> functionality. However, we're having some difficulty with finding a simple 
> and elegant solution that would turn the LED off.
> 
> Thank you in advance,
> 
> Vitaliy 
> 
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