Micromod MOD: 1750N Output holder for MOD 30ML - Manual (Page 35)

Output Holder Instructions
CONFIGURATION
4.5 THEORY OF OPERATION
When the instrument powers up the process value is read first followed by the RETURN and
SENSE inputs. The expression block OP_T_LGC is executed next with a result that becomes
true when both the SENSE and RUNINIT inputs are true. The SENSE input will always be
true when an output holder is present. The RUNINIT input is true only for a moment when the
instrument powers up. The output of the EX block is connected t the track command input
(TCI) of the PID block. For this brief moment that the EX block’s output is true, the PID
block’s output will track the RETURN input connected to its track source (OPTI). This is the
feedback from the output holder of what it sends to the field. The PID block uses this value
until told to change based on automatic or manual manipulation. This PID output value is
sent to the output block causing the output block’s output current to be equal to that of the
output holder output.
The return input block is set to receive 1 to 5 mA. Its input conditioning block converts this
into 0 to 100 as engineering units that correspond to the PID block output.
The initial value of the SENSE input block and its corresponding input conditioning block are
set to false. As well the output block’s initial value is also set to zero. This permits the output
holder card to maintain the control of the device until the controller’s output is synchronized to
the output holder’s output.
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