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Address format of I/O tag:
Address format is [I|U|L|Ll|S|Sl|f|F|C|D]Fn.Address[.BitNr] in which:

  • The first optional character defines the type of I/O tag:
    • I - Integer 16 bit
    • U - Unsigned 16 bit (default)
    • L - Unsigned long (4 bytes = 2 registers) - two registers with Address and Address+1 are read, unsigned and transmitted as big-endian (see Note)
    • Ll - Unsigned long (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as little-endian, unsigned (see Note)
    • S - Signed long (4 bytes = 2 registers) - two registers with Address and Address+1 are read, signed and transmitted as big-endian (see Note)
    • Sl - Signed long (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as little-endian, signed (see Note)
    • f - Float 32 bit (two registers) in big endian format (bytes B4, B3, B2, B1 will be sent, B4 is highest byte and B1 is lowest byte of float)
    • F - Float 32 bit (two registers) in little endian format (bytes B2, B1, B4, B3 will be sent, B4 is highest byte and B1 is lowest byte of float)
    • C - Request counter up (16 bit unsigned, which is incremented by every read request). Works only for Fn=3 or Fn=4
    • D - Request counter down (16 bit unsigned, which is decremented by every read request). Works only for Fn=3 or Fn=4
      Note: Request counter up/down can be used to configure a "watchdog" I/O tag to monitor status and speed of Modbus communication.
  • Parameter Fn is a function of Modbus protocol for data reading, which inserts I/O tag into proper address space:
    • 1 - Coils: binary statuses
    • 2 - Discrete Inputs: binary inputs
    • 3 - Holding Registers: status registers
    • 4 - Input Registers: input registers
  • Parameter Address is 2-bytes address of register in the range of 0 up to 65535.
  • Parameter BitNr optionally specifiec a bit of register in the range of 0 upto 15.

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