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: 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. 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 up 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.

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 highest byte and B1 is the lowest byte of float)
  • F - Float 32 bit (two registers) in little-endian format (bytes B2, B1, B4, B3 will be sent, B4 is highest byte and B1 is the lowest byte of 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 Modbus protocol for data reading, which inserts I/O tag into proper address space:
  • 1 - Coils: binary statuses
  • 2 - Discrete Inputs: binary inputs
  • 3 - Holding Registers: status registers
  • 4 - Input Registers: input registers
• Parameter Address is a 2-bytes address of a register in the range of 0 up to 65535.
• Parameter BitNr optionally specifies a bit of register in the range of 0 up to 15.

Implementation of protocol supports the following functions (commands of 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.
• 2 - Read Discrete Inputs: reading of binary inputs - KOM process sends values of I/O tags of Do type.
• 3 - Read Holding Registers: reading of status registers - KOM process sends values of I/O tags of Co, Ao type (signed/unsigned).
• 4 - Read Input Registers: reading of input registers - KOM process sends values of I/O tags of Co, Ao type (signed/unsigned).
• 5 - Write Single Coil: writing of binary statuses - KOM process writes a received binary value in I/O tag of Di, Do type into system.
• 15 - Write Multiple Coils - KOM process writes all received binary values of I/O tag of Di, Do type into system.
• 6 - Write Single Register: writing of status registers - KOM process writes the received value in I/O tag of Ai, Ao, Ci, Co type into system.
• 16 - Write Multiple registers: writing of multiple registers - KOM process writes all received values in I/O tags of Ai, Ao, Ci, Co type into system.

Note: This is a server type of protocol that is primarily intended for sending the values out of the D2000 system. Therefore the I/O tags should be configured as an output (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 function 5, 15, 6, or 16 from outside.

If the KOM process does not have the valid value of I/O tag or 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).

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)