This means that the controller can run multiple program loops at the same time. When developing structured ladder logic, the strategy that is employed is to develop a bulletproof state transition diagram. This state transition diagram graphically displays all of the states, what happens in each state, and how the program transitions from one state to another state. Once the state transition diagram is complete, the ladder logic is entered into the PLC using Allen Bradley's 6200 software. The ladder logic is in reverse order, with the last state rung being first and the first state rung being last.
Ladder logic is structured in rungs, with each rung being a state.
Within the rung there are a series of examine on bits, examine off bits, and input conditions. Inputs come from the machine to the PLC and include different photo sensors, proximity switches, and thermocouples. There are also comparison statements in the rungs that examine different counters and temperature readings. These comparison statements are greater than, less than, and equal to. These bits control when the PLC moves from one state to the next.