RS-232 Tx / Rx monitoring LEDs?

Jim Brain brain at
Tue Aug 18 22:50:18 CDT 2015

On 8/18/2015 10:40 PM, drlegendre . wrote:
> As part of my C-64 RS-232 to M15 60mA CL project, I've put together a
> simple MAX232 based interface to connect the TTL levels on the C-64 to the
> standard RS-232 +/- levels.
> Is there any reason that I can or cannot install LEDs - on either side of
> the MAX232 converter - to give some indication of line status?
> Specifically, a pair of LEDs, one each for Tx and Rx lines, to blink / etc.
> as line status changes and data moves.
> I don't see any obvious issue - but I can conceive of a situation where it
> might screw up the line in some way I haven't considered.
> Comments?
Probably "best" to feed the TTL RX and TX lines to a 74'04 and then hook 
the output to a 330 ohm and then the cathode of a LED, with anode going 
to 5V.

But, probably good enough is to just put the diode cathode on the TX and 
RX lines, 330 ohm to 5V, and have fun.


Jim Brain
brain at

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