MODBUS Server communication protocol

Supported device types and versions
Communication line configuration
Communication station configuration
Line protocol parameters
I/O tag configuration
Literature
Changes and modifications
Document revisions

Supported device types and versions

The protocol implements a server (slave) communication with arbitrary devices that support the MODBUS RTU standard, either in a serial communication version or a MODBUS over TCP/IP variant.

Communication line configuration

• Line category: Serial (serial communication), SerialOverUDP Device Redundant (serial communication).
• Line category TCP/IP-TCP (MODBUS over TCP/IP). Use a symbolic address ALL or *, in order for the KOM process to listen on a selected TCP port on all existing network interfaces. TCP port 502 is commonly used, but any of the ports can be used.
  Line number - set the value, for example, 1.
  Note: The KOM process works as a multitasking TCP server, and that is why it is able to handle multiple clients at the same time.

Line protocol parameters

Configuration dialog box - table Parameters.
They influence some optional parameters of the protocol. The following line protocol parameters can be set:

Table 1

Communication station configuration

• Communication protocol "Modbus Server".
• The station address is a decimal figure in the range of 1 to 247. Address 0 is reserved for broadcast.

It is possible to configure more stations with different addresses on one line; the KOM process will reply on behalf of every configured station. See also the line protocol parameters Single Server.

Station protocol parameters

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

Table 2

I/O tag configuration

Possible value types: Ai, Ao, Di, Do, Ci, Co.

I/O tag address:

In the MODBUS protocol, the basic address space is divided into registers of Coils type (reading/writing), Discrete Inputs (reading), Holding Registers (reading/writing), and Input Registers (reading).

Every address space is independent, providing 2-byte addressing, i.e., addresses from 0 up to 65535.

The I/O tag with an address starting with %IGNORE will be ignored.

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 the highest byte and B1 is the lowest byte of the float)
  • F - Float 32-bit (two registers) in little-endian format (bytes B2, B1, B4, B3 will be sent, B4 is the highest byte and B1 is the lowest byte of the 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 the status and speed of Modbus communication.
• Parameter Fn is a function of the Modbus protocol for data reading, which inserts the I/O tag into the proper address space:
  • 1 - Coils: binary statuses (for Modbus Server, these are digital output I/O tags)
  • 2 - Discrete Inputs: binary inputs (for Modbus Server, these are digital output I/O tags)
  • 3 - Holding Registers: status registers (for Modbus Server, these are integer/analog output I/O tags)
  • 4 - Input Registers: input registers (for Modbus Server, these are integer/analog output I/O tags)
• Parameter Address is a 2-byte address of a register in the range of 0 to 65535.
• Parameter BitNr optionally specifies a bit of a register in the range of 0 up to 15.
  Note: coexistence of an I/O tag without the BitNr parameter and multiple I/O tags with the BitNr parameter having the same Address is possible.

Implementation of the protocol supports the following functions (commands of the MODBUS Client for a D2000 KOM process):

• 1 - Read Coils: reading of binary status - KOM process sends values of I/O tags of Do type as a response to the Read Coils request.
• 2 - Read Discrete Inputs: reading of binary inputs - KOM process sends values of I/O tags of Do type as a response to the Read Discrete Inputs request.
• 3 - Read Holding Registers: reading of status registers - KOM process sends values of I/O tags of Co, Ao type (signed/unsigned) as a response to the Read Holding Registers request.
• 4 - Read Input Registers: reading of input registers - KOM process sends values of I/O tags of Co, Ao type (signed/unsigned) as a response to the Read Input Registers request.
• 5 - Write Single Coil: writing of binary statuses - KOM process writes a received binary value in the I/O tag of Di, Do type into the system as a response to the Write Single Coil request.
• 15 - Write Multiple Coils - KOM process writes all received binary values of the I/O tag of Di, Do type into the system as a response to the Write Multiple Coils request.
• 6 - Write Single Register: writing of status registers - KOM process writes the received value in the I/O tag of Ai, Ao, Ci, Co type into the system as a response to the Write Single Register request.
• 16 - Write Multiple Registers: writing of multiple registers - KOM process writes all received values in the I/O tags of Ai, Ao, Ci, Co type into the system as a response to the Write Multiple Registers request.
• 22 - Mask Write Register: writing the register with the AND/OR mask - KOM process writes the received value in the I/O tag of Ai, Ao, Ci, Co type into the system as a response to the Mask Write Register request.

Note: This is a server-type protocol that is primarily intended for sending the values out of the D2000 system. Therefore, the I/O tags should be configured as outputs (Ao, Co, Do) because of the manipulation of their values directly or by control objects. If the I/O tag is configured as input (Ai, Ci, Di), the KOM process is unable to send a valid value in a reply to reading by functions 1 - 4 until the value is written by functions 5, 15, 6, 16. or 22 from outside.

If the KOM process does not have the valid value of an I/O tag or a request to read a nonexistent I/O tag is received, an implicit value False or 0 is sent as a response to a read request (MODBUS protocol does not support the transfer of value quality). Protocol exceptions are not generated.

Literature

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

Changes and modifications

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Document revisions

• Ver. 1.0 - April 24th, 2009 - document creating
• Ver. 1.1 - November 21st, 2010 - document update.
• Ver. 1.2 - November 11th, 2011 - document update.
• Ver. 1.3 - July 22th, 2019 - Implementation of signed/unsigned long values (L, Ll, S, Sl)