FB_MBReadRegs (Modbus function 3)

This function is used for reading 1 to 128 output registers (16 bit). The first byte contains the lower eight bits and the second byte the upper eight bits.

Inputs

VAR_INPUT
    sIPAddr       : STRING(15);
    nTCPPort      : UINT:= MODBUS_TCP_PORT;
    nUnitID       : BYTE:=16#FF;
    nQuantity     : WORD;
    nMBAddr       : WORD;
    cbLength      : UDINT;
    pDestAddr     : 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 output registers (data words) to be read. The value zero is not permitted.
nMBAddr	WORD	Start address of the output register (word offset) to be read.
cbLength	UDINT	Contains the maximum byte size available in the destination buffer for the register values to be read. The buffer must have at least the byte size: nQuantity 2*.
pDestAdd	POINTER OF BYTE	Contains the address of the destination buffer into which the data are to be read. 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;
    cbRead     : 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.
cbRead	UDINT	Contains the number of bytes currently read.

Sample of calling the function block in FBD

PROGRAM Test
VAR
    fbReadRegs         : FB_MBReadRegs;
    bReadRegs          : BOOL;
    bReadRegsBusy      : BOOL;
    bReadRegsError     : BOOL;
    nReadRegsErrorId   : UDINT;
    nReadRegsCount     : UDINT;
    nQuantity          : WORD:=2;
    nMBAddr            : WORD:=24;
    arrData            : ARRAY [1..2] OF WORD;
END_VAR

After a rising edge of "bExecute" and successful execution of the ReadRegs command, the content of registers 25 and 26 is located in the arrData array:

Register	Array offset	Status
25	1	0x1234 ( as byte 0x34 0x12)
26	2	0x5563 ( as byte 0x63 0x55)

Requirements

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