While searching for a "decent" USB-RS232 converter, I turned up the following two fascinating creations which have status LEDs for all the signal lines:
This concept really appeals to me from a debugging/verification point of view (although I do NOT get what the GND signal LED is expected to do… stay on all the time?). My only problem is that the blue one is $25 and the black one is $35 (despite the fact that they both appear to be identical – they're probably based on the same reference design). In my opinion, that is way too much for a device I can essentially go and grab the barebones version of off eBay for less than $3 shipped. Thus, my question is, how do these LEDs work, and would it be possible to mod the same kind of functionality into a cheaper converter (perhaps one of the FTDI breakout PCBs on eBay)?
I'd like to make the point that while I am likely to wind up with a small collection of USB converters as time goes by, this is something I see I'm only likely to need one of any time soon, so I don't mind having to put a bit of effort into building it, although I don't (yet) have the tools to work with SMT componentry; I also have no need for the end result to be compact/self-contained – [premade] PCB-based solutions are fine.
Best Answer
Most TTL-level UART signals are persistent enough that they can be viewed directly on an LED tied high or through an op-amp (to reduce loading). TX and RX, however, usually need support from the converter chip so that they don't just flicker dimly. The chips have pins that output the TX and RX states, usually open drain. You simply connect a LED and resistor from the positive supply to the pin, and the chip does all the hard work.