MODBUS Client communication protocol

Supported device types and versions
Communication line configuration
Line protocol parameters
Station configuration
I/O tag configuration
Note to FloBoss 103 device
Note to Honeywell
Literature
Changes and modifications
Document revisions

Supported device types and versions

The protocol executes client (master) communication with arbitrary devices which supports a standard MODBUS RTU and ASCII in the versions of serial communication as well as MODBUS over TCP/IP. Moreover it supports two extension:

• Byte mode - allows to work with devices that get back the values of registers as 1 byte (in contrast with Modbus standard in which the register value is 2 bytes).
• Variable mode - allows to work with devices that get back values of registers with different size as normal 2 bytes. It was implemented because of support 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, 6-byte time stamp and other.

Communication line configuration

• Line category Serial (serial communication)
• Line category SerialOverUDP Device Redundant (serial communication).
• Line category TCP/IP-TCP and TCP/IP-TCP Redundant (MODBUS over TCP/IP). Reserved TCP port 502 is used in common, but it is possible to use any other one according to setting of device. Number of line is not used, for example set the value 1.

Line protocol parameters

A dialog window of communication line configuration - Protocol parameters tab.
They influence some optional protocol parameters.

The line protocol contains the following parameters:

Station configuration

• Communication protocol "Modbus Client".
• Station address is decimal figure mostly in the range of 1 up to 247. Address 0 is reserved as broadcast.

Station protocol parameters

Configuration dialog box - tab Parameter.
They influence some optional parameters of protocol. Following station protocol parameters can be set:

Table 1

I/O tag configuration

Possible types of I/O tag values for invariable mode: Ai, Ao, Di, Do, Ci, Co, TxtI.

Possible types of I/O tag values for variable mode: Ai, Ao, Di, Do, Ci, Cout, TxtI, TxtO, TiA.

I/O tag address:

The main address space in the protocol MODBUS is divided into following registers:

• Coils type (reading/writing)
• Discrete Inputs (reading)
• Holding Registers (reading/writing)
• Input Registers (reading)

An independent address space of 2 byte, i.e. addresses from 0 up to 65535 (so called MODBUS PDU addressing model), is in each address space of given type of register. Some of devices work with address space starting with 1 (so called MODBUS data Model). In this case it is necessary to deduct -1 in address at configuration I/O tags in D2000 system or change the setting of parameter Addressing model to MODBUS data Model.

I/O tag address can acquire basic or extended format (for a variable mode).

Basic format of I/O tag address:
Address format is [I|U|Uu|Ul|f|F|L|Ll|S|Sl|B|X|sn.|an.][d][b][s]RdFn[-WrFn[d]].Address[.BitNr] in which:

• 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 upper byte is considered (1st in sequence)
  • Ul - Unsigned16 - one register is read, unsigned, only lower byte is considered (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 upper 8 bits of the register value
  • X - Byte unsigned, only 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, n registers with Address up to Address+n-1 are read
• Modifier d indicates that the number is an 8-byte number (4 consecutive registers). It can be used for types L, Ll, S, Sl, F,f and it allows configuration of 8 byte integer with/without a sign as well as an 8-byte float (variants big endian and little endian).
  Note: when using a modifier d, I/O tag must be of Analog type (Ai), because Integer type (Ci) in D2000 is implemented as a 4-byte variable and overflow might occur.
• 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. Following table shows values of 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 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 Modbus protocol for a data writing. 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: it influences only value of the particular bit BitNr of status register. Usable only for Do types with the address parameter BitNr.
• Parameter d activates the function "delayed write". Sending of the value is delayed until the request to write value of the object without parameter d comes. All accumulated request waiting to be written are sent. If the function WrFn is set on "Write Multiple Registers", the values are sent in one packet.
• Parameter Address is 2-byte address of 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 number of bit 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 about the byte and register order

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

2. When values are read from two registers as big-endian the received bytes are analysed in this way:

Examples:

3. When values are read from two registers as little-endian the received bytes are analysed in this way:

Examples:

Example of configuration:

• 1.10 - the function Read Coils reads the binary status value with address 10.
• 3.1 - 16-bit number, it is read by the function Read Holding Registers from the address 1 (it can be also in the form I3.1).
• U3.1 - 16-bit number without sign that is read by the function Read Holding Registers from address 1.
• I3-6.1000 - 16-bit number with sign that is read by the function Read Holding Registers from address 1000 and written by the function Write Single Register (as this function is default, address could be also I3.1000).
• S3.321 - 32-bit number with sign, it is read by the function Read Holding Registers from the registers 321 and 322.
• B1.20.0 - bit that is read by function Read Coils from address 20 as 0-bit in byte.
• s10.3.123 - a text string, length 10 characters, it is read by the function Read Holding Registers from the address 123.
• U0-6.456 - 16-bit number, unsigned, is written to the register 456, it is written by Write Single Register, a register reading is not executed.

Extended format of I/O tag address:

Address format is [xN][I|U|F|B|C|T][b]RdFn[-WrFn].Address[.BitNr] in which:

• xN indicates the number of bytes that read or write. Valid values N are 1, 2, 4 (in combination with I, U, F, 6 for T type and optional figure for C type).
• A letter defines the type of I/O tag. In comparison with standard I, U, F, B, there are two extra types:
  • C - text string of fixed length (e.g. x10.C3.1001 is 10-sign string on address 1001)
  • T - time stamp with length 6 bytes (ss:mi:hh dd:mm:yy)
• A meaning of other parameters is in compliance with invariable mode.

See the example of configuration in next section.

Note to FloBoss 103 device

• configuration software ROCLINK800
• default login LOI, password 1000
• logging in FloBoss 103: click on DirectConnect (connection through COM1, on the side of FloBoss 103 it is connected to LOI-local interface)
• menu Configure->Modbus->Configuration
  set the parameter "Variable Mode" on station in D2000 according to setting "Byte Order":
  • if "Least Significant Byte first" then "Little endian"
  • if "Most Significant Byte first" then "Big endian"
• I/O tags are configured through menu Configure -> Modbus -> Registers on FloBoss 103
• following types are supported (n means 16-bit address):
  • Binary input:
    • address in D2000: 1.n, e.g. 1.1001, variable of Di/Dout type
    • address in FloBoss 103: variable of BIN type
      Function: 1
      Starting/ending register: n
  • Binary output:
    • address in D2000: 1.n, e.g. 1.1001, variable of Dout type
    • address in FloBoss 103: variable of BIN r/w
      Function: 1 (for reading)
      Starting/ending register: n
      Function: 5 (for reading)
      Starting/ending register: n
  • Unsigned Int 8 bits input:
    • address in D2000: x1.B3.n, e.g. x1.B3.1003, variable of Ci/Co type
    • address in FloBoss 103: variable of UINT8 type
      Function: 3A or 3B
      Starting/ending register: n
  • Unsigned Int 8 bits output:
    • address in D2000: x1.B3.n, e.g. x1.B3.1003, variable of Co type
    • address in FloBoss 103: variable of UINT8 r/w type
      Function: 3A or 3B
      Starting/ending register: n
      Function: 6
      Starting/ending register: n
  • Unsigned Int 16 bits input:
    • address in D2000: x2.U3.n, e.g. x2.U3.1004, variable of Ci/Co type
    • address in FloBoss 103: variable of UINT16 type
      Function: 3A or 3B
      Starting/ending register: n
  • Unsigned Int 16 bits output:
    • address in D2000: x2.U3.n, e.g. x2.U3.1004, variable of Co type
    • address in FloBoss 103: variable of UINT16 r/w type
      Function: 3A or 3B
      Starting/ending register: n
      Function: 6
      Starting/ending register: n
  • Signed Int 16 bits input:
    • address in D2000: x2.I3.n, e.g. x2.I3.1005, variable of Ci/Co type
    • address in FloBoss 103: variable INT16 type
      Function: 3A or 3B
      Starting/ending register: n
  • Signed Int 16 bits output:
    • address in D2000: x2.I3.n, e.g. x2.I3.1005, variable of Co type
    • address in FloBoss 103: variable of INT16 r/w type
      Function: 3A or 3B
      Starting/ending register: n
      Function: 6
      Starting/ending register: n
  • Unsigned Int 32 bits input:
    • address in D2000: x4.U3.n, e.g. x4.U3.1006, variable of Ci/Co type
    • address in FloBoss 103: variable of UINT32 type
      Function: 3A or 3B
      Starting/ending register: n
  • Unsigned Int 32 bits output:
    • address in D2000: x4.U3.n, e.g. x4.U3.1006, variable of Co type
    • address in FloBoss 103: variable of UINT32 r/w type
      Function: 3A or 3B
      Starting/ending register: n
      Function: 6
      Starting/ending register: n
  • Float 32 bits input:
    • address in D2000: x4.F3.n, e.g. x4.F3.1008, variable of Ai/Ao type
    • address in FloBoss 103: variable of FL type
      Function: 3A or 3B
      Starting/ending register: n
  • Float 32 bits output:
    • address in D2000: x4.F3.n, e.g. x4.F3.1008, variable of Co type
    • address in FloBoss 103: variable of FL r/w type
      Function: 3A or 3B
      Starting/ending register: n
      Function: 6
      Starting/ending register: n
  • String (N bytes) input:
    • address in D2000: x1N.C3.n, e.g. x10.C3.1010, variable of TxtI/TxtO type
    • address in FloBoss 103: variable of ACm(AC10,AC12,AC20,AC30,AC40) type
      Function: 3A or 3B
      Starting/ending register: n
  • String (N bytes) output:
    • address in D2000: xN.C3.n, e.g. x10.C3.1010, variable of Co type
    • address in FloBoss 103: variable of ACN r/w type (AC10,AC12,AC20,AC30,AC40)
      Function: 3A or 3B
      Starting/ending register: n
      Function: 6
      Starting/ending register: n
  • Time and date 6 bytes input:
    • address in D2000: x6.T3.n, e.g. x6.T3.1010, variable of TiA/TxtI type
    • address in FloBoss 103: variable of DT6 type
      Function: 3A or 3B
      Starting/ending register: n
    • Note 1: FloBoss 103 supports local and monotonous time - that is why the configuration of station in D2000 must correspond to configuration of FloBoss.
    • Note 2: It is possible to set time and date but it requires to configure extra the I/O tags for second, minute, hour, day, month and year as Unsigned Int 8 bits and after that to write into them.

Note to Honeywell controllers

The basic parameters and current data of these controllers are not normally read by means of functions 0x01 up to 0x04. There is necessary to use the function 0x14/0x15 Read / write configuration reference data. These controllers use "big-endian" byte order. Therefore, for proper functionality is not necessary to modify parameters that changes byte mode and endianness.

Examples of I/O tag configuration:

20.039 - 16-bit number from address 39(0x27)
f20.040 - 32-bit real number from address 40(0x28)

Literature

• MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1b, December 28, 2006. http://www.modbus.org

Changes and modifications

-

Document revisions

• Ver. 1.0 - November 27th, 2006 - document creating.
• Ver. 1.1 - November 21st, 2007 - document update.
• Ver. 1.2 - April 24th, 2009 - document update.
• Ver. 1.3 - November 3rd, 2010 - document update.
• Ver. 1.4 - December 6th, 2010 - document update.