Programmable Logic Controllers (PLC) will scan very quickly. This can be anywhere from 1 to 20 ms, which translates into 1000 to 500 times each second. But what exactly is a scan?
A scan is when the PLC will complete the following:
Look at all of the inputs to the programmable controller. Digital, Analog, Communication
Solve the logic to determine the output status. PLCs generally will solve the logic from left to right, top to bottom. The output of the rung before is available for the next rung.
This is like some of the popular bands of PLCs like Mitsubishi, Allen Bradley, Siemens, Omron, Automation Direct, etc. There are some exceptions like older Modicon models which solve the logic top to bottom, left to right. Always refer to the manufactures manual to ensure the program execution method.
Diagnostics and Communication:
The PLC will do a self check. It will verify that no errors exists in memory, cards attached, etc. This is critical because the PLC in an industrial application can have devastating effects if something malfunctions and the machine continues to function erratically. The PLC will stop executing, return the outputs to a normal state and indicate an error has occurred.
Communication will happen to the remote I/O, operator panels, etc.
Outputs are set according to the PLC program. (Digital, Analog) This is where the physical items will start moving. (Motors, Valves etc.)
To understand the scan, lets take a look at an example.
The following program will look at input X0 and set an internal bit for one scan one the rising edge of the input and one on the trailing edge of the input. The rising edge is when the input transitions from off to on and the trailing edge is when the input transitions from on to off.
The bits will only be on for one scan so we will increment an internal word by one when the bits go on. This way we will be able to see the bit increment in the word.
Leading edge one shot (one scan) bit. When the input signal goes on (X0) and C1 is not on, then C0 is turned on. The next rung will have C0 and X0 on so C1 turns on.
Remember: The PLC will scan from left to right, top to bottom and the outputs from the previous rung are available for the next.
C0 is on so the increment will add one to D0.
The next scan X0 is still on, C1 is now on so output C0 is turned off. C0 has been now on for one scan from the transition from off to on.
Trailing edge one shot (one scan) bit. When the input signal goes ooff (X0) and C3 is not on, then C2 is turned on. The next rung will have C2 and not X0 on so C3 turns on.
C2 is on so the increment will add one to D1.
The next scan X0 is still off, C3 is now on so output C2 is turned off. C2 has been now on for one scan from the transition from on to off.
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