ModbusRtuMaster_PcCom

Supported Modbus functions (actions)

• ModbusMaster.ReadCoils
  Modbus function 1 = Read Coils
• Reads binary outputs (coils) from a connected slave. The data is stored in compressed form (8 bit per byte) starting from address pMemoryAddr.
• ModbusMaster.ReadInputStatus
  Modbus function2 = Read Input Status
• Reads binary inputs from a connected slave. The data is stored in compressed form (8 bit per byte) starting from address pMemoryAddr.
• ModbusMaster.ReadRegs
  Modbus function 3 = Read Holding Registers
• Reads data from a connected slave.
• ModbusMaster.ReadInputRegs
  Modbus function 4 = Read Input Registers
• Reads inputs registers from a connected slave.
• ModbusMaster.WriteSingleCoil
  Modbus function 5 = Write Single Coil
  Sends a binary output (Coil) to a connected slave . The data is supposed to be stored in a compressed form (8 bit per byte) starting from address pMemoryAddr.
• ModbusMaster.WriteSingleRegister
  Modbus function 6 = Write SingleRegister
  Sends a single data word to a connected slave
• ModbusMaster.WriteMultipleCoils
  Modbus function 15 = Write Multiple Coils
  Sends binary outputs (Coils) to a connected slave . The data is supposed to be stored in a compressed form (8 bit per byte) starting from address pMemoryAddr.
• ModbusMaster.WriteRegs
  Modbus function 16 = Preset Multiple Registers
  Sends data to a connected slave
• ModbusMaster.Diagnostics
  Modbus function 8 = Diagnostics
  Sends a diagnostics request with a user defined subfunction code to a connected slave. The subfunction code is passed to the function by the MBAddr parameter. Additional data can be passed by pMemoryAddr.

VAR_INPUT

 
VAR_INPUT
    UnitID     : BYTE;
    Quantity    : WORD;
    MBAddr     : WORD;
    cbLength    : UINT;
    pMemoryAddr : DWORD;    
    Execute     : BOOL;
    Timeout     : TIME;
END_VAR
 

UnitID : Modbus station address (1..247). The Modbus slave will only answer if it receives telegrams containing its own station address. Optionally, collective addresses can be used for replying to any requests. Address 0 is reserved for broadcast telegrams and is therefore not a valid station address.

Quantity: Number of data words to be read or written for word-oriented Modbus functions. For bit-oriented Modbus functions, Quantity specifies the number of bits (inputs or coils).

MBAddr: Modbus data address, from which the data are read from the end device (slave). This address is transferred to the slave unchanged and interpreted as a data address.
MBAddr holds a subfuntion code when the Diagnostics function is used.

cbLength : Size of the data variable used for send or read actions in bytes. cbLength must be greater than or equal to the transferred data quantity as specified by Quantity. Example for word access: [cbLength >= Quantity * 2]. cbLength can be calculated via SIZEOF (Modbus data).

pMemoryAddr: Memory address in the PLC, calculated with ADR (Modbus data). For read actions, the read data are stored in the addressed variable. For send actions, the data are transferred from the addressed variable to the end device.

Execute : Start signal. The action is initiated via a rising edge at the Execute input.

Timeout : Timeout value for waiting for a response from the addressed slave.

VAR_OUTPUT

 
VAR_OUTPUT
    BUSY : BOOL;
    Error : BOOL;
    ErrorId : MODBUS_ERRORS;
    cbRead : UINT;
END_VAR
 

Busy: Indicates that the function block is active. Busy becomes TRUE with a rising edge at Execute and becomes FALSE again once the started action is completed. At any one time, only one action can be active.

Error: Indicates that an error occurred during execution of an action.

ErrorId: Indicates an error number in the event of disturbed or faulty communication.

cbRead: Provides the number of read data bytes for a read action

Hardware connection

The data structures required for the link with the communication port are included in the function block. They are displayed in the TwinCAT System Manager once the PLC program has been integrated and can be connected with a COM port. The procedure is as described in Chapter Serial PC Interface.

Requirements