Function SET , RESET for PLC Programming
Often, an output is to be switched on by briefly pressing a start button, e.g. the motor of a mixing plant remains switched on when the plant is switched on.
How SR-Latches and RS-Latches work
A short set signal sets the output of the latch to logic "1". A reset signal resets the latch element back to logic "0". At this point we distinguish between SR-latches and RS-latches: Latches reset-dominant and set-dominat dominant in PLC programming.
What is the difference?
SR elements are reset dominant - RS elements are set dominant. If there is a 1-signal at both inputs at the same time, reset is performed for reset-dominant (SR elements) and set is performed for set-dominant (RS elements) elements.
In general, dangerous outputs, e.g. motors should be programmed reset-dominant. At this point the wire break safety must be considered. Signal elements can also be programmed set-dominant, depending on the application.
To explain why SR-latches are reset dominant and RS-latches are set dominant, we must look at the way the PLC works. We know that the PLC works serially, i.e. the instructions of the program are processed from "top" to "bottom" one after the other, i.e. the last instruction is valid.
Let´s have a look, how these latches can be programmed in IL (instruction list):