Scanning time in plc.

How a PLC scan the programme.what is plc scanning time.
For eg. If scanning time of plc is 10 sec and it scan 10 network in 10 sec. There is no other programme except this.
If the input in network 1 get high at 5th ms of scanning time of plc does it show the status on from off. or its going to show on in next scanning cycle.


) if the scanning time of plc 10 ms and it scan 40 input and output and next scanning cycle it scan 60 input and output. does the scanning time increase or load on processor . expalain .

ravi

The answer to your question is going to vary depending on make and model of PLC.

The most common answer:
1) the PLC will read it's inputs first. It literally makes a copy of the states of the input signals.
2) Solve logic. The program is solved, using the static copy of the input images, the results are written to data tables...not to the outputs themselves at this point.
3) Write the output data tables to the actual output points.
4) Perform housekeeping duties like communication, reset the watchdog timer, etc.
5) Go to step 1

So, with this somewhat more common system, if an input changes states in the middle of step 2, it doesn't matter because the COPY of that input isn't changed until the scan cycle gets back to step 1, and the logic is still being solved with the static copy that was made in the previous input update cycle.

This is off the top of my head and other more professional writing on this subject may be more accurate and detailed, but hopefully it will help you begin to understand. Also, many systems do not follow that classic sequence and may allow input data to change asynchronous to the PLC scan cycle.

thanx