>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.

Unfortunately such an arrangement also tends to load a lot of RS232 drivers 
so that they no longer produce the required voltage. Having been through 
that loop myself I certainly would not recommend it.

A nicer way is to use an NPN and PNP transistor pair to drive the LEDs which 
are powered by a 9V battery. 

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