FB_MBWriteCoils (Modbus function 15)

This function is used to write 1 to 2048 digital outputs (coils). A digital output corresponds to one bit of the written data bytes.

Inputs

VAR_INPUT
    sIPAddr       : STRING(15);
    nTCPPort      : UINT:= MODBUS_TCP_PORT;
    nUnitID       : BYTE:=16#FF;
    nQuantity     : WORD;
    nMBAddr       : WORD;
    cbLength      : UDINT;
    pSrcAddr      : POINTER OF BYTE;
    bExecute      : BOOL;
    tTimeout      : TIME;
END_VAR

Name	Type	Description
sIPAddr	STRING (15)	Is a string containing the IP address of the target device.
nTCPPort	UINT	Port number of the target device.
nUnitID	BYTE	Identification number of a serial sub-network device. If a device is addressed directly via TCP/IP, this value must be 16#FF.
nQuantity	WORD	Number of digital outputs to be written (data bits). The value zero is not permitted.
nMBAddr	WORD	Start address of the digital outputs to be written (bit offset).
cbLength	UDINT	Contains the maximum available byte size of the source buffer that contains the data to be written. The buffer must have at least the byte size: (nQuantity + 7) / 8.
pSrcAdd	POINTER OF BYTE	Contains the address of the source buffer that contains the data to be written. The buffer can be a single variable, an array or a structure, whose address can be determined with the ADR operator.
bExecute	BOOL	The function block is enabled by a rising edge at this input.
tTimeout	TIME	States the length of the timeout that may not be exceeded by execution of the ADS command.

Outputs

VAR_OUTPUT
    bBUSY      : BOOL;
    bError     : BOOL;
    nErrId     : UDINT;
END_VAR

Name	Type	Description
bBusy	BOOL	When the function block is enabled, this output is set and remains set until a feedback is received.
bError	BOOL	If an ADS error should occur during the transfer of the command, then this output is set once the bBusy output is reset.
nErrId	UDINT	Returns the ADS error number when the bError output is set.

Sample of calling the function block in FBD

PROGRAM Test
VAR
    fbWriteCoils        : FB_MBWriteCoils;
    bWriteCoils         : BOOL;
    bWriteCoilsBusy     : BOOL;
    bWriteCoilsError    : BOOL;
    nWriteCoilsErrorId. : UDINT;
    nWriteCoilsCount    : UDINT;
    nQuantity           : WORD := 10;
    nMBAddr             : WORD := 14;
    arrData             : ARRAY [1..2] OF BYTE := 16#75,16#03;
END_VAR

After a rising edge of "bExecute" and successful execution of the ReadCoils command, the content of the arrData array is written to outputs 15 - 24:

Bit	0	1	1	1	0	1	0	1	0	0	0	0	0	0	1	1
Output	22	21	20	19	18	17	16	15	X	X	X	X	X	X	24	23

Requirements

Development environment	Target system type	PLC libraries to be linked
TwinCAT v3.0.0	PC or CX (x86)	Tc2_ModbusSrv