TABLE OF CONTENTS 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.
A function often provided with PLCs is the conditional jump. We can describe this as:
IF (some condition occurs) THEN perform some instructionsELSE 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 on, and if below that temperature no action occurs.
Thus, if the appropriate conditions are met, this function enables part of a ladder program to be jumped over. Figure 8.1 illustrates this in a general manner. When there is an input to In 1, its contacts close and there is an output to the jump relay. This then results in the program jumping to the rung in which the jump end occurs, so missing out intermediate program rungs. Thus, in this case, when there is an input to Input 1, the program jumps to rung 4 and then proceeds with rungs 5, 6, etc. When there is no input to Input 1, the jump relay is not energised and the program...
