We are having a Wago system delivered soon whose "scan time" needs improvement. The PLC is a 750-841 Field Controller, which is also running a web server in addition to its control code.

When going on-line with the programming software (CoDeSys), the scan time varies between 1.4 and 3.5 mS. However, when we toggle an output every scan, the trace on the attached oscilloscope shows it toggling every 6.5 to 8 mS. I'm suspecting that the additional time is being taken up in the overhead tasks of the PLC and is not being reported in the scan time.

Has anyone else seen this before? What can we do to reduce the true scan time down to a manageable level of 5 to 6 mS?

 

Are you using the CoDeSys Task Configuration? If not, create a 5ms cyclic task with a priority of 0, then assign your program to this task. If you need further assistance, call WAGO's free technical support (1-800-DIN-RAIL).