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I used 3.3v microcontroller. I want to add TX LED and RX LED communication on RS232, RS485, TTL/CMOS.

My question:

Led communication must be mounted as SINKING or SOURCING on I/O microcontroller for avoid interfere in communications?

Picture in here: Datasheet

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  • \$\begingroup\$ Whats your question? \$\endgroup\$
    – Kris Bahnsen
    Commented Aug 13, 2013 at 23:32
  • \$\begingroup\$ Led communication must be mounted as SINK(pull down) or SOURCE(pull up) on I/O microcontroller for avoid interfere in communications? \$\endgroup\$
    – new bie
    Commented Aug 13, 2013 at 23:38
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    \$\begingroup\$ What about your set-up would make it interfere either way? \$\endgroup\$
    – The Photon
    Commented Aug 14, 2013 at 0:29
  • \$\begingroup\$ Are you asking which, sink or source? Or, sink/source, versus something else? What exactly do you mean by "SINK" or "SOURCE"? How about a schematic? \$\endgroup\$
    – Phil Frost
    Commented Aug 14, 2013 at 1:10
  • \$\begingroup\$ The question is not pointeless; it's entirely possible that a line driver drives "high" or "low" better than in the other direction - however, changing the arrangement will change the "sense" of when the LED is on, which may not be desired. \$\endgroup\$
    – Chris Stratton
    Commented Sep 10, 2013 at 21:06

1 Answer 1

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RX and TX lines at TTL signals** between the μC and the line driver IC*** is a good place to connect the LED indicators. There's no good reason to connect the LEDs to RS-232 lines. RS-485 lines are bidirectional, so the LEDs alone wouldn't tell if the data is TX or RX on RS485.

** 0 to 3.3V serial
*** RS-232 or RS-485 line driver, and such

Back to TTL signals. You can connect the LED between signals and GND **, or between signal and Vcc ***. Either would work as long as the LED current is low enough (limited by a series resistor). LEDs add some capacitance, which might be a problem at high speeds above, say, 1M bits per second.

** source in the O.P. terminology
*** sink

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