RS-232 Tx / Rx monitoring LEDs?

Paul Koning paulkoning at
Thu Aug 20 08:27:26 CDT 2015

> On Aug 19, 2015, at 11:40 PM, dwight <dkelvey at> wrote:
> I think the main issue is that TTL is usually a weak pullup and a hard
> pulldown.
> RS 232 levels are equal up and down.

That depends.  Open collector outputs, yes, but those are the less common case.  Totem pole outputs have comparable drive strength in both directions, that's precisely their purpose (to provide symmetric rise/fall times when driving capacitive loads).

> Putting it on the RS232 lines could even improve signal quality because
> the signal is usually poorly terminated, causing reflections.

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.  And the driver voltage range spec for RS232 is quite loose; it might be just a couple of volts but it can be as high as 15 volts or so.


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