Entry type: FAQ, Entry ID: 5505008, Entry date: 06/28/2010
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How do you configure the "Sample_T" input?

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Description
The "Sample_T" input can be found on blocks DIF_T, CTRL_PID and PT1_P among others. These blocks have one thing in common: they simulate the behavior of continuous transfer elements. The input/output signal behavior of these transfer elements is defined by parameters such as TN, TV and GAIN.

Processing in digital computers however is time discrete. This means that calculation of the transfer behavior takes a finite time or is only done at discrete intervals. This time is not negligible and it is imperative to include it in the calculation. This is what the "Sample_T" input is for. You use this input to inform the block of the time base used to call it.
Always call these blocks in a cyclic interrupt OB. Only there will the block be processed in a cycle of constant duration and you can specify the cycle time of this cyclic interrupt OB directly at the "Sample_T" input parameter.

In PCS 7 V4.0x you must determine the calling cyclic interrupt OB and specify the cycle time configured for this OB at the "Sample_T" input.

As from version V5 of PCS 7 the "Sample_T" input is automatically parameterized with the correct sampling time if this has been selected in the "Compile" dialog. The CFC recognizes this from the system attribute "S7_sampletime = 'true' " at the "Sample_T" input parameter.


Fig. 01

As from version V7 of PCS 7 the "Sample_T" input is parameterized automatically each time you compile.

Warning:
Do not use the "Sample_T" parameter to change the I/O behavior of the transfer element. Only ever use the parameters provided for that purpose.