8 Jump and call

This chapter considers the jump instruction which enables part of a program to be jumped over and the way in which subroutines in ladder programmes can be called up. Subroutines enable commonly occurring operations in a program to be repeatedly called up and used over again.

8.1 Jump

A function often provided with PLCs is the conditional jump. We can describe this as:

IF (some condition occurs) THEN
perform some instructions
ELSE
perform some other instructions.

Such a facility enables programs to be designed such that if certain conditions are met then certain events occur, if they are not met then other events occur. Thus, for example, we might need to design a system so that if the temperature is above 60°C a fan is switched ...

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