Are 9 LEDs on a USB–RS232 adapter really necessary?
Every inexpensive USB–RS232 adapter ships with two LEDs, and together
they answer exactly one question: is data moving. This looks at what the
other seven signal lines actually do, why a link most often fails on one of
them rather than on the data pair, and what to measure when it does. It
closes with the script that exercises all nine channels after unboxing.
DeStatiX A9
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RS232 signalling