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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:

Tcp No Delay
Setting Tcp No Delay parameter to YES causes low level socket option TCP_NODELAY being set, thus turning off default packet coalesce feature.
Parameter is immplemented only for line categories TCP/IP-TCP and TCP/IP-TCP Redundant.
YES/NONO

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

ParameterMeaningUnitDefault value
Retry Count
Maximum count of call retry. If no reply returns after calls had been sent, station will be in status of communication error.s2
Retry Timeout
Timeout before retry of call if no reply had not received.s0.1
Wait First Timeout
Delay after sending the request before reading the response.s0.1
Wait Timeout
Timeout between reading the reply.s0.1
Max. Wait Retry
Maximum count of retry of the reply reading.-20
Start Silent Interval
"Start silent interval" before begin of the transmission in RTU mode.ms50
Stop Silent Interval
"Stop silent interval" after ending of the transmission in RTU mode.ms50
Byte mode
Special byte mode of transmission in which the values of registers have length of 1 byte and not 2 bytes as it is defined in Modbus protocol specification.YES/NONO
Variable mode
Special variable mode of transmission in which the values of registers have variable length.

Setting of Variable mode:
Little endian = the lowest bytes are sent first
Big endian = the highest bytes are sent first
OFF = variable mode is switch off

Note 1: Variable and byte mode are incompatible and only one of them can be enabled.
Note 2: Emerson FloBoss 103 device: text strings and time stamps of 6-byte are sent always from the lowest byte.
Note 3: Variable mode is implemented only for Protocol Mode=RTU.
Note 4: A data encoding big-endian is used automatically, according to the default parameter values Byte mod=NO and Variable mode=OFF (i.e. according to MODBUS protocol specification).

OFF
Little endian
Big endian
OFF
Full debug
Log of another debug information about communication on line.YES/NONO
Protocol mode
Protocol mode: RTU or ASCII.

Note: In case of "MODBUS over TCP/IP", the parameter value is ignored and Protocol Mode=RTU is used.

RTU
ASCII
RTU
Addressing model
Sets an address model of MODBUS protocol:
MODBUS PDU
data are addressed from 0 up to 65535.
MODBUS data Model
data are addressed from 1 up to 65536.

Note:
MODBUS PDU is a default value. If MODBUS data Model is set, the object with the address X is addressed as X-1 in MODBUS PDU.
After you change this parameter, restart a proper communication process.
MODBUS PDU
MODBUS data Model
MODBUS PDU
TCP/IP protocol variant
Select a variant of protocol in case of TCP/IP communication:
"MODBUS TCP" is a variant of communication without control checksum. Safeguarding is done by underlying TCP protocol.
"MODBUS over TCP"
is a variant where a payload is MODBUS RTU data containing a checksum.
"MODBUS TCP"
"MODBUS over TCP"
"MODBUS TCP"
Max. Registers
Maximum count of registers that are required in one call.-100
Max. Bytes
Maximum count of bytes that are required in one call (only in "Byte mode").-100
Skip Unconfigured
To require the values from addresses that are not configured, is not allowed.

Description and example:
The calls for data, which are limited by protocol parameter "Max. Registers" or "Max. Bytes", are sent as standard. If I/O tags with addresses "Holding Registers" 1, 2 and 5 has been configured, one call requiring 5 registers from address 1 is sent although the I/O tags with addresses 3 and 4 are not configured. It is more efficiency to gain required data in one call than in two ones even if the unnecessary data are also read.
If the parameter "Skip Unconfigured" is set on YES, two calls are sent, the first one requires two registers from address 1 and the second one requires the one register from address 5.

YES/NONO
Check Receive Length
If this parameter is set to YES, then an extra check is performed when receiving a response to a read request: the length of received data is checked whether it matches amount of registers in a read request:
  • if Byte mode is on (Byte mode=YES), length of received data must equal to number of registers
  • if both Byte mode and variable mode are off, length of received data must equal to double of number of registers
  • if variable mode is on (Variable mode=little-endian or big-endian), check has not been implemented yet
This extra check is reasonable on high-latency and variable-latency lines - e.g. GPRS networks - to detect and avoid the situation when read request (#1) is repeated due to timeouts and then two responses are received, the second of which could be considered to be an answer to another read request (#2), thus causing wrong values being assigned to I/O tags addressed by this read request #2.
YES/NONO

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 MODBUS Client#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.|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, 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 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 registerD2000 attributes
    0 : Invalid valueWeak
    1 : Measured value validValid
    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:

Received bytes of MSB-LSBI/O tag typeValue
0x00 0x01I, U1
0xFF 0xFEI-2
0xFF 0xFEU65534
0x01 0x02B1
0x01 0x02X2

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

Most significant register (ADR address)Least significant register (ADR+1 address)
MSBLSBMSBLSB

Examples:

Received bytes of register (MSB-LSB)Received bytes of register + 1 (MSB-LSB)I/O tag typeValue
0x00 0x000x00 0x01L, S1
0xFF 0xFF0xFF 0xFES-2
0x00 0x010x00 0x02L, S65538
0x3F 0x800x00 0x00f1.0
0xC0 0x000x00 0x00f-2.0

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

Least significant register (ADR address)Most significant register (ADR+1 address)
MSBLSBMSBLSB

Examples:

Received bytes of register (MSB-LSB)Received bytes of register + 1 (MSB-LSB)I/O tag typeValue
0x00 0x010x00 0x00Ll, Sl1
0xFF 0xFE0xFF 0xFFSl-2
0x00 0x020x00 0x01Ll, Sl65538
0x00 0x000x3F 0x80F1.0
0x00 0x000xC0 0x00F-2.0


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 (2 bytes per character), it is read by the function Read Holding Registers from the address 123.
  • a5.3.123 - a text string, length 10 characters (1 byte per character), 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



Blog

You can read blogs about  Modbus protocol (for now, in Slovak language only):


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.

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