5.7.3 Precautions When Creating Software Wiring
Software wiring varies input values in an application but its operation is somewhat
different from real analog and digital inputs.
For instance, in the case of real analog outputs in a plant, it takes time for PV values
to vary after the MV value of, say, a control valve, has been manually changed, and
input values do not vary in a definite manner with variance in analog output values.
Similarly for real digital outputs in a plant, it may take some time for real open/close
signals or operation stop inputs to respond to open/close or start/stop signals.
To model these response delays and the magnitude of input value variance with
analog output variance, first order lag time constant, gain and bias can be specified
accordingly.
Even so, variance by software wiring of analog I/O is definite – after a change in
output value, the input value is always increased by the specified gain and bias after
a lapse of time specified by the first-order lag time constant.
Similarly, a digital signal always responds to a change in an ON/OFF signal after a
lapse of time specified by the first-order lag time constant.
On the other hand, the response of a PV value to a manual change of the MV value
of, say, a control valve in an actual plant, may take a longer or shorter time than
expected.
Extremely large or extremely small variance in PV value may follow after a manual
change of MV value.
Moreover, after an open/close or start/stop digital signal output, equipment may fail
to work for whatever reason so that the open/close signal or "operation has stopped”
signal does not turn on.
These phenomena do not happen with software wiring, so the operation of the
application under such circumstances cannot be tested during debugging using only
software wiring.
To perform such tests, remove software wiring, and enter analog/digital input values
directly from the Watch window instead.
As described above, debugging using only software wiring means that function test
cannot be done for deviations from the behavior of software wiring.
To overcome this shortcoming, use software wiring in conjunction with input from the
Watch window during debugging.
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