Fail-safe Countermeasure Guidelines
Section
3-5
137
3-5-2
Measures for a CPU Unit fatal error
When a fatal error occurs on the CPU Unit (including during execution of the
FALS command), the Loop Control Unit also stops running. To hold the
analog output to the previous value before the stop occurred, and to set the
analog output to either the minimum value or maximum value, use the output
hold function of the Analog Output Unit or Analog Input/Output Unit.
Note
The Loop Control Unit continues to run even if the Output OFF flag causes
output refreshing of the CPU Unit to stop. Note, however, that in this case
contacts are not output and turn OFF, and the output hold function of the
Analog Output Unit holds analog output to a specific value. For this reason,
the Loop Control Unit only performs internal operations, and does not
perform control on the outside.
3-5-3
Required Conditions for the Man-Machine Interface
The Loop Control Interface itself does not have a man-machine interface. So,
an external interface such as CX-Process Monitor that satisfies the following
conditions must be provided:
Loop Control Unit run/stop command function and run status display
function (operation of ITEM014 to ITEM017 of the System Common block)
Loop Control Unit error status display function (operation of ITEM091 to
ITEM095 of System Common block)
Basic display function for Set Point, PV, MV and other values
Basic setting function for Set Point and other settings
Function for changing P, I, D and other parameters
Auto/Manual switching and manual operation functions