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• The first character defines a type of I/O tag:
  • I - Integer16 (default) - one register is read, signed
  • U - Unsigned16 - one register is read, unsigned
  • Uu - Unsigned16 - one register is read, unsigned, only the upper byte is processed (1st in sequence)
  • Ul - Unsigned16 - one register is read, unsigned, only the lower byte is processed (2nd in sequence)
  • f - Float (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as big-endian (see Note).
  • F - Float (4 bytes = 2 registers) - two registers with Address and Address+1 are read and transmitted as little-endian (so-called Modicon format), (see Note)
  • 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)
  • B - Byte unsigned, only the upper 8 bits of the register value
  • X - Byte unsigned, only the lower 8 bits of the register value
  • sn. - Text string with the length of n characters, one register is one character, n registers with Address up to Address+n-1 are read
  • an. - Text string with the length of 2*n characters, one register is two ASCII characters, characters are transmitted in the same order as they appear in the string,  n registers with Address up to Address+n-1 are read
  • An. - Text string with the length of 2*n characters, one register is two ASCII characters, characters are transmitted in big-endian order (i.e. "1234" is transmitted as "2143"),  n registers with Address up to Address+n-1 are read
• Modifier d indicates that a number is an 8-byte number (4 consecutive registers). It can be used for types L, Ll, S, Sl, F,f, and it is used for configuration of signed/unsigned 8-byte integer as well as an 8-byte float (big-endian <B8>..<B1> and little-endian <B1>..<B8> formats).
  Modifier D indicates that a number is an 8-byte number (4 consecutive registers). It can be used for types Ll, Sl, F and it is used for configuration of signed/unsigned 8-byte integer as well as an 8-byte float (little-endian format <B2><B1><B4><B3><B6><B5><B8><B7>).
  Note: when using a modifier d or D, the I/O tag must be of Analog type (Ai), because the Integer type (Ci) in D2000 is implemented as a 4-byte variable, and overflow might occur. Writing of Integer type (Co) as an 8-byte number is supported.
• Modifier b indicates that figure is coded by BCD. It can be used for I/O tags of I, U, B, L, Ll types.

• Modifier s indicates that a status register (Unsigned16) located on address Address is followed by a big-endian Float value located on address Address+1 .. Address+2. This indicator is used for type f and it is implemented for calorimeter Endress+Hauser RMS621. The following table shows the values of the status register and their mapping to D2000 attributes.

  Status register	D2000 attributes
  0: Invalid value	Weak
  1: Measured value valid	Valid
  2: Overflow warning 3: Overflow error 4: Underflow warning 5: Underflow error 6: Saturated steam alarm 7: Error in differential pressure calculation 8: Wrong medium for DP calculation 9: Wrong value range - DP calculation inaccurate 10: Differential pressure - general error 11: Range overshoot (Tsat > 350 etc.) on 12: Change in state of aggregation 26 : Differential pressure --> general error 99: No measured value is assigned to the register in the setup of the ModBus	Weak

  
  

• Parameter RdFn is a function of the Modbus protocol for a data reading. The following functions are implemented:
  • 1 - Read Coils: binary status reading
  • 2 - Read Discrete Inputs: binary input reading
  • 3 - Read Holding Registers: status register reading (Integer16/Unsigned16 and Float32 - reads two successive registers)
  • 4 - Read Input Registers: input register reading (Integer16/Unsigned16 and Float32 - reads two successive registers)
  • 0 - A value is not read, it is only written. The function for writing (WrFn) must be set.
• Parameter WrFn is the function of the Modbus protocol for data writing. The following functions are implemented:
  • 5 - Write Single Coil: binary status writing (default for Read Coils)
  • 6 - Write Single Register: status register writing (default for Read Holding Registers)
  • 16 - Write Multiple registers: multiple registers writing, it must be used when 2-register type is written (e.g. Float, Unsigned long, etc.).
    Note: function can be used to write more than two registers at once if a text string is used. Example:
    if we have an I/O tag with address a3.0-16.#8A00 (i.e. text string covering 3 registers, having length of 6 characters) and we write a string '123456', then hexadecimal values 0x3132, 0x3334 and 0x3536 (ASCII code for '1' is 0x31, for '2' is 0x32 etc) will be written to registers 0x8A00, 0x8A01 and 0x8A02.
  • 22 - Mask Write Register:  write affects only the value of the particular bit BitNr of the status register. It is usable only for Do value types with the address parameter BitNr.
• Parameter d activates the function "delayed write". Sending of the value is delayed until the request to write the value of the object without parameter d comes. All accumulated requests waiting to be written are sent. If the function WrFn is set to "Write Multiple Registers", the values are sent in one packet.
• Parameter Address is a 2-byte address of the register (0-65536). See also the protocol parameter Addressing model.
  Note: address can be specified as a hexadecimal number using a number sign (#), e.g. #50CE
• Parameter BitNr is a bit's position in a word. The values 0-7 are allowed to be used for binary statuses and inputs, values 0-15 are allowed to be used for reading of bit from 16-bit status or input registers.
  Note: coexistence of an I/O tag without BitNr parameter and multiple I/O tags with BitNr parameter having the same Address is possible.

Note on writing: if only a part of the register (lower/higher byte or selected bit) is written and the I/O tag also has RdFn (function for a data reading) configured, then when writing, the value of the entire register is read first, and then the relevant part of the register is modified and the entire register is written.

1. MODBUS protocol uses the big-endian, i.e. the most significant byte (MSB) is transmitted as first. Examples:

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