RS-232 Tx / Rx monitoring LEDs?

dwight dkelvey at
Wed Aug 19 22:40:50 CDT 2015

I think the main issue is that TTL is usually a weak pullup and a hard
RS 232 levels are equal up and down.
Putting it on the RS232 lines could even improve signal quality because
the signal is usually poorly terminated, causing reflections.
The recommended lengths and Baud rates are intended to account for this.
Usually the line is lower impedance than the input termination,
requiring the source to absorb the reflected signal.
This is why 422/485 came to be. The impedance of the loads better
match that of the termination.

> Subject: Re: RS-232 Tx / Rx monitoring LEDs?
> From: nf6x at
> Date: Tue, 18 Aug 2015 21:27:19 -0700
> To: cctalk at
> Those little RS232 testers with LEDs built into a double DB25 connector box are usually just made of LEDs and resistors connected to each signal line. They can load signals enough to cause problems at high speeds, with weak drivers, or with long cables, but usually they don't cause problems. If you're concerned, you could always include jumpers or switches to disconnect the LEDs when they're not needed.
> Of course, buffering the TTL signals eliminate any such problems. But on the other hand, using two LEDs connected with opposite polarities on each RS232 level signal lets your discriminate between driven positive, driven negative, and open. That can come in handy when debugging things where the other end may or may not be driving properly, or may be mis-wired.
> For an example, feel free to take a look at this little modular jack RS232 tester that I made:
> The red/green LED pairs show whether each line is high, low or open. Resistor values may vary depending on the LEDs that you choose.
> -- 
> Mark J. Blair, NF6X <nf6x at>

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