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RS485 on rPi

[last updated: 2025-10-06]
RS485 home page
rPi serial communication
see also GMC GUI project
rPi home page
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  • Hardware:
    • Connections:
      There are five connections required between the rPi and the MAX module.
      • +5v and gnd to power the MAX485
            (strictly speaking the MAX could be powered with a separate supply instead of from the rPi,
            but the gnd connection between them is still required.)
      • 3 signal connections:
          rPi digital output -to- MAX-DE/RE: allow the rPi to control the MAX's enable line (High to Tx, Low to Rx)
            you can use your choice of rPi DO for this signal.

          rPi-Tx -to- MAX-DI: serial data sent from the rPi to the MAX module (Max-DI = digital input)

            rPi serial port (UART0) uses GPIO14 for the Tx line

          MAX-RO -to- rPi-Rx: serial data received by the MAX485 on its RS485 bus, and sent to the rPi Rx receive pin

            rPi serial port (UART0) uses GPIO15 for the Rx line.

          (Max-RO - received output)

        RS485
      • Two of these signals, both signals From the rPi To the MAX module, specifically the enable control and the rPi-Tx signals, can be directly connected to the MAX485,
        because the 3.3v output from the rPi will still be read as digital high on the 5v MAX485.
      • However the 5v RO signal sent From the MAX485 To the rPi would damage the 3.3v inputs on the rPi if it were directly connected,
        so a level shift circuit is required.

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  • Software:
    • The rPi communicates to the MAX485 module with serial communication (using an onboard UART).
      rPi serial port (UART0) uses GPIO14 (Tx) and GPIO15 (Rx)

    • The rPi-Tx signal is a serial data stream that is output from a UART on the rPi.
      The RS485 definition/protocol does not specify anything about this data stream,
      so the programmer must decide what it will be when he writes the software code.
      Baud rate, packet size, start/stop bits, error correction techniques, basically everything about the data
      must be defined by the program.

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