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    Invalid slave id

    Modbus4J general discussion
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    • F
      fmunhoz
      last edited by

      As I recall Modbus only has an 8-bit address, so one master can only talk to 255 slaves via 1 port, so valid addresses are in the range 0..247 where 0 is used as a broadcast address.

      However you can use another port, though.

      If have any doubts about it I recommend you google for modbus documentation.

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      • M
        mlohbihler
        last edited by

        Actually, max slave id is 240.

        Best regards,
        Matthew

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        • H
          HenryDUT
          last edited by

          Thanks for fmunhoz's recommendation.

          In MODBUS Messaging on TCP/IP Implementation Guide, it says that

          On TCP/IP, the MODBUS server is addressed using its IP address; therefore, the MODBUS Unit Identifier is useless. The value 0xFF has to be used.

          When addressing a MODBUS server connected directly to a TCP/IP network, it’s recommended not using a significant MODBUS slave address in the “Unit Identifier” field. 0xFF is recommended for the “Unit Identifier" as non-significant value.

          But 0xFF is greater than 247 (decimal), how can I assign 0xFF to the slave device address?

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          • H
            HenryDUT
            last edited by

            @mlohbihler said:

            Actually, max slave id is 240.

            really?

            Is it not assigned from 1 to 247 (decimal) ?

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            • H
              HenryDUT
              last edited by

              @HenryDUT said:

              Thanks for fmunhoz's recommendation.

              In MODBUS Messaging on TCP/IP Implementation Guide, it says that

              On TCP/IP, the MODBUS server is addressed using its IP address; therefore, the MODBUS Unit Identifier is useless. The value 0xFF has to be used.

              When addressing a MODBUS server connected directly to a TCP/IP network, it’s recommended not using a significant MODBUS slave address in the “Unit Identifier” field. 0xFF is recommended for the “Unit Identifier" as non-significant value.

              But 0xFF is greater than 247 (decimal), how can I assign 0xFF to the slave device address?

              Who can help me?

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              • M
                mlohbihler
                last edited by

                Is the slave id actually causing problems for you?

                Best regards,
                Matthew

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                • V
                  villadaniel_750
                  last edited by

                  Amazing. Here i have PLC General Electric F650, with slave Modbus TCP and the slave id is 254 (HEX FE). Great job Modbus4J assuming a slave id between 0 to 247. JAR to TRASH

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                  • phildunlapP
                    phildunlap
                    last edited by phildunlap

                    I will refer you to the protocol specification.
                    http://modbus.org/docs/PI_MBUS_300.pdf

                    Page 19 (#10) is probably the page you'd be interested in, first paragraph. Some people suggest there are extra functions mapped onto units addressed 248-255, but I can't find a spec for that.

                    http://www.modbus.org/docs/Modbus_over_serial_line_V1.pdf
                    page 8

                    http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf
                    page 23

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                    • C
                      craig
                      last edited by

                      villadaniel_, since it is a PLC why don't you change its slave id to something in the range that will work?

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