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I have a device that sends data (telemetry) over an RS232 serial port. The RX line is not used, this device only sends one string every second.

I need to read this data from 3 different devices, each of which has an RS232 port. The question is: can I split the TX line of the RS232 and read the data simultaneously from 3 devices, i.e. make this connection:

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Could there be problems? Are buffers or anything like that necessary?

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  • \$\begingroup\$ Shouldn't be a problem - almost certainly the sending device will be able to drive multiple receivers. \$\endgroup\$
    – Kevin White
    Commented Jun 4 at 21:17
  • \$\begingroup\$ @KevinWhite Not necessarily. RS-232 is a point-to-point connection. For example a TI MAX3232 guarantees it can drive worst case load of 3k presented by one receiver with the bare minimum +/- 5.0V voltage. While it could work, it does not have to. \$\endgroup\$
    – Justme
    Commented Jun 4 at 21:30
  • \$\begingroup\$ "Are buffers ... necessary?" Maybe, maybe not. You need to check the data sheets of your TX and RX devices to see if the loadings are compatible. You also need to run a ground line between all four devices so the 0 V rails on each device need to either be at the same potential or at least be floating. \$\endgroup\$
    – Graham Nye
    Commented Jun 4 at 21:53
  • \$\begingroup\$ The Driver is a TI MAX232, and the 3 receiver is a MOXA UPORT 1250I \$\endgroup\$
    – Federico Massimi
    Commented Jun 5 at 5:10
  • \$\begingroup\$ @Justme - hence the almost certainly. I agree it is beyond spec but in the MAX3232 case for example typically the driver can do more than 10mA and still be above the receiver input thresholds. \$\endgroup\$
    – Kevin White
    Commented Jun 5 at 15:09

1 Answer 1

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RS-232 is a point-to-point connection.

A single transmitter is only required to drive a load presented by a single receiver. And the cable capacitance of specified amount.

If you don't know the electrical properties of the TX output or the RX inputs, you can't be sure if it will work or not if you simply connect TX output to three RX inputs.

Feel free to try it, but there may be problems. It is unlikely to damage anything.

For production use, I would advice against connecting three receivers to one transmitter. For lab prototyping, do try. I've monitored serial comms by connecting a PC to sense traffic between two devices, which means each device was driving two receivers with one transmitter.

For any real world scenario, you might want to build an active 3-way RS-232 repeater/splitter/buffer or whatever you want to call it.

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