Dave,
Inorder for the HMI to display a TIMER PV that is changing at a 10 Hz rate (0.1 second period) the following things must happen:
[*] The HMI has to be able to request the TIMER PV at a 20 Hz rate (0.05 second period) to not miss a single count. This is a the Nyquist sampling rate minimum. The total time to send the request from the HMI to the PLC for the TIMER PV, the time for the PLC's low level firmware to respond to this request, the transit time to return the data to be to the HMI and finally the time required by the HMI to process the data and update HMI window, must be less than 0.05 seconds.
[*] The PLC ladder logic scan rate has to equal or exceed 20 Hz for the TIMER's PV to update without missing a count. The TIMERs are implemented in firmware on the PLC and the TIMER PV is only updated at the start of a scan of ladder logic.
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If your requirement is to display time to 0.1 seconds without losing counts on the HMI display, then you need to reduce the time that your PLC program takes to execute.
The PLC scan rate is directly affected by the complexity of the PLC program. You can test this by writing a 2 line PLC program.
The custom function, ScanDelay, is a time waster that can be used to emulate a complex PLC program:
' ScanDelay - CF that wastes time to simulate a complex PLC program
'
' The variable A, determines the number of ms to waste
'
Delay A
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You can change the value of "A" using online-monitoring. You will probably have to exit the on-line monitor to see the affect of slowing down the ladder logic scan rate. The communication between your PC and the PLC to support on-line monitoring will probably slow the PLC to the point that the TIMER PV will appear to miss cycles.
Gary D*ckinson