MB_WriteBinary

MB_WriteBinary writes binary values to a Modbus RTU slave using Write Single Coil or Write Multiple Coils.

Ladder Representation

Block Structure

Variable Type	Name	Data Type	Description
VAR_INPUT	Execute	BOOL	Enables block execution.
VAR_INPUT	SlaveAddress	BYTE	Modbus RTU slave address.
VAR_INPUT	Function#	BYTE	Function code used for the write request.
VAR_INPUT	InitialDataAddress	WORD	Initial bit address where the data will be written.
VAR_INPUT	NumberOfData	BYTE	Number of bits to write (1 to 128).
VAR_INPUT	Timeout#	WORD	Maximum time to wait for the response, in milliseconds.
VAR_INPUT	Offset#	BOOL	Indicates whether the block must subtract 1 from InitialDataAddress.
VAR_INPUT	Value	BOOL	Variable that stores the binary data to write.
VAR_OUTPUT	Done	BOOL	Indicates that the block completed successfully.
VAR_OUTPUT	Active	BOOL	Indicates that the block is waiting for a response.
VAR_OUTPUT	Busy	BOOL	Indicates that the RS485 interface is busy with another request.
VAR_OUTPUT	Error	BOOL	Indicates an execution error.
VAR_OUTPUT	ErrorID	BYTE	Execution error identifier.
VAR	MB_WRITEBINARY	MB_WRITEBINARY	Block structure access instance.

Operation

On a rising edge of Execute, the block checks whether the Modbus RTU slave at SlaveAddress is free for a request. If Busy is FALSE, it writes the values from Value to the bit range starting at InitialDataAddress, using NumberOfData and the function selected in Function#.

The block sets Active while it waits for the slave response and resets Active when the response is received. If the slave is busy, the block waits until Busy becomes FALSE before sending the request.

When Execute is FALSE, Done remains FALSE. Done is TRUE only after the block completes successfully and remains TRUE until Execute becomes FALSE.

Request Cancellation

If Execute becomes FALSE while Busy is still TRUE, the request is canceled.

Value Size

Value must be an array with the same size as NumberOfData. Check this compatibility to avoid block errors.

Error Codes

Code	Description
0	Executed successfully.
1	Invalid input data.
2	Master not enabled.
4	Response timeout from the slave.
5	Slave returned an error.

Execution Flowchart

Examples

Ladder

This example writes the data contained in VALUE, with size DATA_COUNT, starting at INIT on the Modbus RTU slave address SLAVE using the Write Single Coil function. When the block finishes successfully, Done is activated.

Structured Text

The following Structured Text code applies the same logic shown in the Ladder example.

Structured Text

VAR
    SLAVE : BYTE := 1;
    INIT : WORD;
    VALUE : ARRAY [0..4] OF BOOL;
    MB_WRITEBINARY_INST_0 : FB_MB_WriteBinary;
END_VAR

MB_WRITEBINARY_INST_0.Execute := DI1;
MB_WRITEBINARY_INST_0(
    SlaveAddress:=SLAVE,
    Function:=5,
    InitialDataAddress:=INIT,
    NumberOfData:=4,
    Timeout:=30,
    Offset:=1,
    Value:=VALUE);
DO1 := MB_WRITEBINARY_INST_0.Done;