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 executes a slaver communication with arbitrary devices that support standard MODBUS RTU in versions of serial communication and MODBUS over TCP/IP.

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, KOM process listens 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 operate more 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, 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.