FB_MBReadRegs (Modbus function 3)

VAR_INPUT

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

sIPAddr: Is a string containing the IP address of the target device.

nTCPPort: Port number of the target device.

nUnitID: Identification number of a serial sub-network device. If a device is addressed directly via TCP/IP, this value must be 16#FF.

nQuantity: Number of output registers (data words) to be read. The value of nQuantity must be > 0.

nMBAddr: Start address of the output registers to be read (word offset).

cbLength: Contains the max. byte size of the destination buffer. The minimum buffer byte size must be: nQuantity * 2.

pDestAddr: 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 found with the ADR operator.

bExecute: The function block is activated by a rising edge at this input.

tTimeout: States the length of the timeout that may not be exceeded by execution of the ADS command.

VAR_OUTPUT

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

bBusy : When the function block is activated this output is set. It remains set until an acknowledgement is received.

bError : If an ADS error should occur during the transfer of the command, then this output is set once the bBusy output is reset.

nErrId : Supplies the ADS error number when the bError output is set.

cbRead: Contains the number of bytes currently read.

Example of calling the 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 in the arrData array:

Requirements