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Entry type: FAQ Entry ID: 63112120, Entry date: 06/10/2013
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Edge function in MSS ES 2008

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QUESTION:  

For certain applications it can be necessary to evaluate the change of a signal (from high to low or vice versa). This edge-function can be done by the use of the counter elements of the MSS software parameterization. The counter limit value will be set to “1”. When the limit is reached it will be reset automatically. The following image shows the logic and the parameter settings.

Note:
The connection of the „count down“ input of the counter elements is mandatory. But for the edge evaluation it’s not applicable. Though, the input will be connected by a permanent 0-signal. Therefore a device-status element can be used, that is not evaluating any device status.


ANSWER:

To realize a Set/Reset function (e.g. for Starting and Stopping with a single push button) two Counter-elements will be connected upstream to a SR-element. These function blocks evaluate a positive edge at their CU inputs. A “Device status” block generates a permanent 0-signal and provides it to the CD input of the counters. Therefore the parameter “Status type” remains in its default-setting “No status information”.
The evaluated logic result is available at the output “Q” of the SR-element and can be used in the further software logic.
The following grafic displays the complete solution.


Evaluation of positive edge


63112120_Evaluation_of_pos._edge.xml (4 KB) ( 4 KB )



Evaluation of negative edge

  

63112120_Evaluation_of_neg._edge.xml (4 KB) ( 4 KB )