Get_Modbus_Holding_Registers

Overview

The Get_Modbus_Holding_Registers function block reads values from the PLC Modbus Holding Register map and copies them into a local data array. The following steps are required to correctly use the function block:

Select the starting Modbus register
Define the number of registers to read
Select the starting index inside the destination DATA array

💡 Array Size Limitation

The DATA output array has a fixed size of 100 elements (0..99). Make sure MDB_Number_of_registers does not exceed this limit to avoid out-of-bounds errors.

⚠️ What will trigger a fault on the function block?

The following conditions need to be true, in order to correctly run the function block:

MDB_Number_of_registers must be within the range 0..99 (inclusive)
MDB_Start_Register + MDB_Number_of_registers ≤ 1000
DATA_Start_Element + MDB_Number_of_registers ≤ 100

MDB_Start_Register          := (UINT)
MDB_Number_of_registers     := (UINT)
DATA_Start_Element          := (UINT)
DATA                        => (ARRAY [0..99] OF UINT)
STS_Done                    => (BOOL)
STS_Failed                  => (BOOL)
STS_Fault_Code              => (USINT)

Interface

Inputs

Name	Type	Range / Units	Description
MDB_Start_Register	UINT	0 - 999	Start index in the PLC Modbus holding register table.
MDB_Number_of_registers	UINT	1 - 99	Number of consecutive holding registers to copy.
DATA_Start_Element	UINT	0 - 99	Start index inside the destination `DATA` array.

Outputs

Name	Type	Description
DATA	ARRAY[0..99] OF UINT	Destination buffer containing copied holding register values.
STS_Done	BOOL	`TRUE` when the copy completed successfully.
STS_Failed	BOOL	`TRUE` when the operation failed.
STS_Fault_Code	USINT	Diagnostic code indicating the reason for failure.

Status Codes

Code	Meaning	Description
90	Index outside of array bounds	One or more limits were exceeded (max read count, holding register range, or `DATA` range).
91	Unknown error	Unknown error occurred during operation.

Example

PROGRAM PROG_Set_Ethernet_Config
    VAR
        EthCfg : SET_ETHERNET_CONFIG;
        _IP_Address  : ARRAY [0..3] OF USINT := [192, 168, 11, 2];
        _Subnet_Mask : ARRAY [0..3] OF USINT := [255, 255, 255, 0];
        _Gateway     : ARRAY [0..3] OF USINT := [192, 168, 11, 1];
        _DNS         : ARRAY [0..3] OF USINT := [8, 8, 8, 8];
    END_VAR

    EthCfg(
        Network_Operation_Mode := 1,
        IP_Allocation_Method   := 0,
        IP_Address             := _IP_Address,
        Subnet_Mask            := _Subnet_Mask,
        Gateway                := _Gateway,
        DNS                    := _DNS,
        Port                   := 5000
    );
END_PROGRAM

PROGRAM PROG_Get_Modbus_HR
    VAR
        ReadHR  : GET_MODBUS_HOLDING_REGISTERS;
        DestData : ARRAY [0..99] OF UINT;   (* Received Modbus data *)

        _Done    : BOOL;
        _Failed  : BOOL;
        _Fault   : USINT;
    END_VAR

    ReadHR(
        MDB_Start_Register      := 0,
        MDB_Number_of_registers := 10,
        DATA_Start_Element      := 0,
        DATA                    => DestData,
        STS_Done                => _Done,
        STS_Failed              => _Failed,
        STS_Fault_Code          => _Fault
    );
END_PROGRAM

CONFIGURATION Config0
    RESOURCE Res0 ON PLC
        TASK task1_20ms(INTERVAL := T#20ms, PRIORITY := 1);
        PROGRAM Prog_Instance_SET_ETHERNET_CONFIG WITH task1_20ms : PROG_Set_Ethernet_Config;
        PROGRAM Prog_Instance_GET_MODBUS_HR WITH task1_20ms : PROG_Get_Modbus_HR;
    END_RESOURCE
END_CONFIGURATION