Not sure if you are asking about converting or plug in. here is a link to a flip / flop or latching relay. In other words -set -re-set -right? When one contact is "set" nothing happens until the other contact is "set"
If you look under the code the difference between SR and RS is actually the order of execution. In Siemens PLC where you can see the code represented as either ladder, FBD, or STL ( Statement List) it is clear.
A set dominant instruction in a PLC has two inputs, SET and RESET, with SET having precedence. Regardless of the condition of the RESET input, the instruction will turn its output true on the rising edge of SET and will remain on as long as SET is true, regardless of RESET state, and the output will continue on when SET is off until RESET is true. Reset dominant works the same way except opposite. The ease of converting set dominant to reset dominant will depend on what kind of logic you have in front. It should be possible to invert the instruction input and output logic to achieve the same result. Remember that when inverting, ANDs become ORs and ORs become ANDs. Or...., you could abandon the set/reset dominant instructions and build it using latches. Depending on who is maintaining this program after you are done with it, it may be more readable for them if you use latch/unlatch pairs. I personally like to take full advantage of the PLC instruction set, but I also don't like 3:00 am phone calls to explain something to a technician who could otherwise read simpler RLL.
Remember that PLC programs are cyclic, and that, excepting immediate commands, nothing actually happens until the end of a cycle. Therefore simply use the set and reset commands, the latter command will be the dominant one.
>>Some PLC do not have separate set and reset dominant flip flops.
>>How to convert a set dominant flip flop to reset dominant one?
>Remember that PLC programs are cyclic, and that, excepting immediate
>commands, nothing actually happens until the end of a cycle. Therefore
>simply use the set and reset commands, the latter command will be the
>dominant one.
"nothing actually happens until the end of a cycle" is true for many PLCs but not all of them.
For example, Allen-Bradley uses asynchronous IO refresh in many of their architectures (even the ControlLogix uses asynchronous IO). If an IO refresh occur between the set and the reset, a
glitch will happen on your output. A workaround may be to latch in internal memory and copy to IO
bit at the end of the scan.
