S Registers are used for step control. Step control arguments must follow the following format.
Step control can be performed in two ways: “Normal Step” and “Reverse Step”. STEPSET is used for “Normal Step”.
STEPSET turns ON the current step, but only if the previous step is ON. STEPSET is processed one step at a time, in sequence, according to the step number. For example, in the ladder above, when P1 turns ON, S0:2 is turned ON, but only if S0:1 is ON.
After S0:2 turns ON, S0:1 is turns OFF. When P2 turns ON, S0:0 is turned ON and all other steps are turned OFF. S0:0, or step 0, is used to reset. All other steps are processed in order.
STEPOUT differs from STEPSET in two ways. First, it is not evaluated in sequence, and second, when a step is turned ON, it remains in that state regardless of its input condition. When using STEP OUT and a step is turned ON, all other steps are turned OFF, so only one step is ON at any given time.
In the ladder above, when P1 turns ON, S0:2 will turn ON. When P0 turns on S0:1 turns ON, and S0:2 turns OFF and S0:1 turns ON. When PO turns OFF, S0:1 is unchanged as each step, when turned ON, remains ON regardless of its input condition. Finally when P2 turns ON, S0:0 turns ON and S0:1 turns OFF.