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The protocol implements client (master) communication with arbitrary devices which supports support MODBUS RTU or MODBUS ASCII standards (serial communication) as well as MODBUS over TCP/IP. Moreover, it supports two extensions:

• Byte mode - allows working with devices that implement the registers as 1-byte variables (in contrast with Modbus standard in which the register value is 2 bytes).
• Variable mode - allows working with devices that implement the registers with different sizes than standard 2 bytes. It was implemented because of support of the flowmeter FloBoss 103 made by Fisher Controls International (at this time a part of Emerson Process Management): 1-byte variables, 4-byte unsigned/signed integers, text strings of length 10,12,20,40 characters, a 6-byte time stamp, and other.

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• First 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 considered processed (1st in sequence)
  • Ul - Unsigned16 - one register is read, unsigned, only the lower byte is considered 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 number of 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.

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