Scan time

[skj1402]

Could you please tell me what the typical scan time is for a program of approx 6000 words (max capacity of PLC), and would it be half that time for a program of 3000 words i.e. is scan time directly proportional to program length?

[support]

Ladder logic scan time is about 10 microsecond per step, whereas TBASIC is approximately 100 to 500 microsecond per command depending on the command type and number of parameters. There is no simple way to compute the scan time since most TBASIC are executed in {dCusF} which is a differentiated instruction and so is not executed every scan of the ladder program.

If you need to handle fast I/O response, use the interrupt inputs or use them as high speed counter or Pulse Measurement input.

[whenry]

How would one go about monitoring / measuring scan time within the program?

[support]

create a ladder circuit as follow:

    Out1           Out1
|---|/|---------(OUT)

Then measure the output waveform with an oscilloscope or frequency counter. The scan time is half the period of the output square wave.

[sengbeng]

Hi,

Thanks for your input...

regards
sengbeng

[WonderY]

Hi Guys,

what if there's not enough interupt inputs or high speed input? Get a few more MD and network together? If I need to put together a high speed machine with 4 servo motor, can I use the MD?

[support]

You certain can use more than one MD to handle 4 high speed interrupt tasks. The two PLC each will have its own code to handle its inputs but one of them can be the master and the other the slave unit and then can communicate via RS485.

[Roonster]

With an MD PLC running a fixed program that has both Ladder logic and Basic Functions Does the scan time vary from scan to scan depending on which special functions are called?  Or is the scan time the same every cycle regardless of which branch operations are executed?

[support]

The scan varies depending on which custom function is executed and how much time the custom function consumes. So scan time is not a very meaningful measurement of the PLC performance since not every custom function is executed at every scan of the ladder program.