Example: Multi-channel Thermocouple

The Thermocouple Monitoring example reads one channel. This variation reads all four channels of an SLM-THM-4 using a FOR loop and arrays, with per-channel burnout tracking.

Hardware

Slot	Module
1	SLM-THM-4 (4-channel thermocouple input)

Wire thermocouples on channels 1 through 4.

Backplane configuration

Same as the base example. Configure each channel’s sensor type to match the fitted thermocouples.

Slot	Channel	Address	Alias
1	TC1	%ID0	TC1_Temp
1	TC1 Burnout	%IX0.0	TC1_Burnout
1	TC2	%ID1	TC2_Temp
1	TC2 Burnout	%IX0.1	TC2_Burnout
1	TC3	%ID2	TC3_Temp
1	TC3 Burnout	%IX0.2	TC3_Burnout
1	TC4	%ID3	TC4_Temp
1	TC4 Burnout	%IX0.3	TC4_Burnout

The program

Variables

Variables table

Name	Class	Type	Location	Initial Value	Documentation
tc1	Local	REAL	TC1_Temp		Channel 1
tc2	Local	REAL	TC2_Temp		Channel 2
tc3	Local	REAL	TC3_Temp		Channel 3
tc4	Local	REAL	TC4_Temp		Channel 4
tc1_bo	Local	BOOL	TC1_Burnout		Channel 1 burnout
tc2_bo	Local	BOOL	TC2_Burnout		Channel 2 burnout
tc3_bo	Local	BOOL	TC3_Burnout		Channel 3 burnout
tc4_bo	Local	BOOL	TC4_Burnout		Channel 4 burnout
temps	Local	ARRAY [1..4] OF REAL			Gathered readings
burnouts	Local	ARRAY [1..4] OF BOOL			Gathered status bits
i	Local	INT			Loop counter
any_fault	Local	BOOL			TRUE if any channel is in burnout

Text mode

  VAR
    tc1 : REAL AT TC1_Temp;
    tc2 : REAL AT TC2_Temp;
    tc3 : REAL AT TC3_Temp;
    tc4 : REAL AT TC4_Temp;
    tc1_bo : BOOL AT TC1_Burnout;
    tc2_bo : BOOL AT TC2_Burnout;
    tc3_bo : BOOL AT TC3_Burnout;
    tc4_bo : BOOL AT TC4_Burnout;
    temps : ARRAY [1..4] OF REAL;
    burnouts : ARRAY [1..4] OF BOOL;
    i : INT;
    any_fault : BOOL;
  END_VAR

Code body

temps[1] := tc1;    burnouts[1] := tc1_bo;
temps[2] := tc2;    burnouts[2] := tc2_bo;
temps[3] := tc3;    burnouts[3] := tc3_bo;
temps[4] := tc4;    burnouts[4] := tc4_bo;

any_fault := FALSE;
FOR i := 1 TO 4 DO
  IF burnouts[i] THEN
    any_fault := TRUE;
  END_IF;
END_FOR;

A located variable can also be declared as an array, bound to consecutive addresses (OpenPLC Editor 4.3.0 or newer):

temps AT %ID0 : ARRAY [1..4] OF REAL;      (* %ID0 to %ID3 *)
burnouts AT %IX0.0 : ARRAY [1..4] OF BOOL; (* %IX0.0 to %IX0.3 *)

This example binds each channel through its alias instead and copies it into the array, so the program keeps reading the correct channels if the IEC addresses change. Arrays and other custom data types are covered in the OpenPLC data types documentation.