História stránky

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• 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 multitask a multitasking TCP server and that is why it is able to operate more clients at the same time.

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

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

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

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

I/O tag address:

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

An independent Every address space of is independent, providing 2-byte addressing, i.e. addresses from 0 up to 65535, is in each address space of given type of register.

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 specifiec specifies a bit of register in the range of 0 upto 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 which that is primary 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 does not make is unable to send a valid value at 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 at call as a response to a read request (MODBUS protocol does not support the transfer of value quality).

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

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Changes and modifications

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• 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)

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