RS-232 Tx / Rx monitoring LEDs?

tony duell ard at
Thu Aug 20 11:13:36 CDT 2015

> A consideration with RS232 is that the signals swing to either side of 0, so if you use an LED
>  referenced to 0, it either needs a series diode or a sufficiently high reverse voltage rating.  

A diode in inverse parallel with the LED Is more normal when you want to run an LED off effectively
an AC supply. In the case of RS232 signals it is common to use a red and a green LED in inverse 
parallel, either separate LEDs or one of the 2-wire bicolour LEDs which are precisely a pair of LEDs
in inverse parallel. 

Most of the little RS232 terters are just that, I think with 3k series resistors for the LED pairs.


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