A flip flop circuit in a PLC usually has one input and two outputs. When the input is activated, the two outputs latch on/off opposite to each other alternately. Basically, it is used to toggle (latch) an output on and off with just one input. In the PLC it is a single input that will toggle an output on and off each time the input signal is activated.
The PLC program will be a little different than the relays because of the way in which the PLC scans. Scanning takes place from left to right, top to bottom. The output conditions from the logic are available to the next rung as the logic is solved. Outputs and inputs are read usually only once at the end of the scan. Remember to think of the outputs in the PLC as make before break. This is the opposite of the relay logic presented above which is break before make.
Let’s look at the logic. This is programmed using the Do-More Programming Software which comes with a simulator. This full programming package is free of charge and can be downloaded here.
The input is on leading-edge instruction. (One Scan) If output 2 is on then it will set output 1. If output 2 is not on then it will reset output 1. The third line of code will determine the state of output 2 based upon output 1.
You may be asking yourself why do we not just use the conditions from output 1 to control output 1. This is because if we substituted output 1 for the conditions on the input then the output 1 would never turn on/off. The output conditions are available for the next line of PLC code. This would allow the output to be set and reset within the scan without being updated. Using output 2 is the only way in which this logic would work.
Here is an automated picture to show the input toggling on / off and the outputs flip-flopping.
Note: An emergency condition can be added to the set or reset rungs to automatically control the output either way.
Watch on YouTube: Creating a Flip Flop Circuit in the PLC
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