I have a machine with an RS-232 port. When I power ON the machine and connect the DB-9 tester all the machine's troubleshooting guide requires is to check if the RTS light is green. I see there are other LED lights like the RXD/TXD that turn red. Can anyone explain what these different LED lights indicate?

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1 Answer 1


We don't have a manual for your specific tester device, but it is safe to assume green LED means there is positive voltage and red LED means there is negative voltage.

The LEDs indicates status for each communication pin.

If a LED is not lit, then there is no voltage so nothing drives that communication pin.

So at least you can infer which wires are connected or not, which one of the devices are turned on or off, is there some extra short circuits somewhere.

The brightness can also be used as an indicator if something is not right about the voltages, like abnormally dim or bright LED indicates extra loading or short circuits with other wires.

Basically, each LED is just a simple voltmeter so you have 8 voltmeters giving you voltage readings from each of the 8 wires and can analyze signals and their changes as fast as your flicker fusion threshold allows your eyes and brain to interpret the results.

How to actually infer what works as expected and what does not is a complex subject and it depends on the context, i.e. required knowledge how the two devices work alone or together as a system, or it might be used as a tool to acquire that information and make them work together.

So there can be no specific answer what you can infer from your scenario. If the manual says RTS LED must be green and it is, well, it works up to that point then.

What each RS-232 communication wire does is a vast subject and covered in many articles about RS-232 communications elsewhere. How to debug RS-232 interface with a LED tester tool is also a vast subject requiring in-depth knowledge how RS-232 signals work and how your specific devices happen to use them.


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