Functions of the IM 178-4
6-3
IM 178-4
C79000-G7076-C178-01
Detecting the Actual Reference Value Dependent on I0
By setting parameters (edge selection I0), you decide when the actual reference
value will be detected, as follows:
S
On the falling edge of I0
S
On the rising edge of I0
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On both edges of I0
The following mechanism is used to detect the actual reference value for each
channel:
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You trigger the function by assigning the value TRUE to the REF0 control bit.
This is only possible when at this time the REF0_Q status bit has the value
FALSE.
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The next time that the selected edge condition is satisfied by I0, the IM 178-4
generates the actual reference value and enters it in the user data area of the
inputs. The module then sets the REF0_Q status bit of the corresponding
channel.
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You can now evaluate the actual reference value and terminate detection of the
actual reference value by resetting the REF0 control bit.
S
Following this, the module resets the REF0_Q status bit. Detection of the actual
reference value can now be started again.
Digital
input I0
User terminates
actual reference
value detection
Control bit
REF0
Status bit
REF0_Q
User requests
actual reference
value detection
IM 178-4 signals
existence of the new
actual reference value
t
t
t
Figure 6-1
Handshaking between the user program and IM 178-4 during actual reference
value detection with a rising edge at I0.