set/reset relay

SR Flipflop is set dominant, so parky's example would not work. There would also be potential timing issues depending on the platform with the outputs.


Set Q
+-] [------+----( )
| Q Reset|
+-] [--]/[-+

Below is another way, similar to parky's although I prefer this less as there are now two places where Q is written and two places where Set is read. It takes me extra time to think of the scenario where Set changes between rungs. All the changes I make to rung 1, I have to do the opposite on rung 2. The opposite isn't always obvious.
Set Q
--] [-----------(S)
Set Reset Q
--]/[--] [------(R)

 
It does work, it is only a quick example to show a set/reset because the reset is after the set if the input (or memory) X0 stays on and X1 is on it will reset it strait away you could say it's a safe mode to ensure the y is off regardless of X0 if X1 is on
You could use one shot rising if required, most PLC's refresh outputs on end of scan so setting it before reset if reset bit true it would always be off at update of outputs
 
Last edited:
Like I said before, It was a very simple example, most logic would not be so simple and there are many ways to achieve this for example one shot rising so could only be set or reset on transition from low to high, or as I do, put any logic onto internal markers and use these in a block at end of program (or call block to outputs last) and use markers to drive outputs (I rarely set or reset outputs within the code blocks, it makes debugging easier, by putting all outputs in one call you can monitor and work back to find what is holding outputs on or what is stopping them from turning on) I have been programming for nearly 40 years as an OEM on many different platforms.
 

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