PLC programs scan usually from left to right, top to bottom. The outputs from the previous rung are available for the next. This is the operation of a PLC. Program control instructions allow us to add or remove parts of our program during each scan of the controller. The Horner APG XL4 has subroutines which we can achieve this control. We will look at how to divide your main logic up and at how subroutines work. Continue Reading!