Problem Description

Runtime Test Cases
PLC Program to Call a Subroutine for a Different Process
This is a PLC Program to Call a Subroutine for a Different Process.

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Problem Description
Add temperature outputs of two temperature transmitters and display the total temperature. Update
display every 10 seconds when this system is active using Subroutine program.
Problem Solution

Using subroutine has an advantage that process is not affected by any other data in subroutine program
unless and until the subroutine is called.

This is very useful in displaying various data on the same display with a particular time delay.

For example, we can use the same display to show Pressure readings for the first 5 seconds and
Temperature reading for the next five seconds and so on.

When we use subroutine instruction, Subroutine address must be same in JSR instruction of main program
and RET instruction of subroutine program.
PLC Program
Here is PLC program to Call a Subroutine for a Different Process, along with program explanation and run
time test cases.
List of Inputs and Outputs
I:1/0
= Start
(Input)
I:1/1
= Stop
(Input)
O:2/0
= Master Coil
(Output)
T4:0
= Time delay to update data
(Timer)
O:6
= Display address
(Output)
N7:0 & N7:1 = Temperature data of transmitters 
(Register)
T4:0/DN = Update display calling Subroutine, reset timer
(Timer Bit)
U:3
= Address of Subroutine
(Subroutine)

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Ladder
 
Diagram
 
to
 
call
 
subroutine
Program Description

Main program comprises of simple Master Coil rung, rung for timer and rung to call subroutine.

When Start PB I:1/0 is pressed, Master coil energizes and the TON timer T4:0 is enabled.

After 10 seconds when PRESET=ACCUMULATOR, T4:0/DN bit goes high calling subroutine.

Subroutine has a program that adds two data from addresses N7:0 and N7:1 which are outputs from
Temperature transmitters. These outputs are added and output of addition is converted into BCD and sent to
address O:6 which is connected to Display.

Output data are in Hexadecimal and Displays accept BCD inputs. So TOD instruction is used here to
convert data of temperature addition to BCD.

T4:0/DN is also used as an XIO input to Timer itself, hence it automatically resets to 0 whenever preset and
accumulator values are equal or in other words, it updates the display every 10secs resetting timer.