Detailed description of the operation of the MV_DIA function block
(Original Document)
About us
Introduction
As soon as one of the inputs monitored is no longer parameterized within the DFB, the DFB signals that there is a fault while updating output.
Any faults detected during a single monitoring cycle are picked up as they appear (STATUS word bit is set to 1, corresponding with output update).
At the end of a monitoring cycle (falling Edge ED input), the ERROR andSTATUS outputs are reinitialized to 0.
Trend diagram
The following chart shows how the MV_DIA function block works .
Operation
The following table describes the different phases illustrated by the chart below:
Phase
Description
1
When the EVENT input is different from the VALUE public variable (ENABLE = 1), a fault is detected.
2
The ERROR output changes to 0 when the EVENT input takes on the values of the VALUE public variable.
3
A fault is detected when the EVENT input becomes unstable.
This type of fault appears after the status of EVENT input changes twice in the same monitoring cycle.
The EVENT input unstable fault (bit 8 of Status word goes to 1) , becomes an EVENT different from VALUE fault (bit 0 of Status word goes to 1) if there are more than 1000 PLC cycles before a new fault is detected.
The EVENT input unstable fault disappears if there are more than 1000 PLC cycles, and also if the EVENT input is always equal to the value specified by VALUE.
4
A fault is detected when COND input is anything other than 1.
5
ERROR output changes to 0 when COND input takes on the value of 1.
6
EVENT input is different from the VALUE public variable: there is no fault as ENABLE input = 0.
7
ERROR output changes to 0 when ED input takes on the value of 0.
Illustration of DFB operation, EVENT_T0 and EVENT_T1 inputs
The following chart shows how the MV_DIA function block works .
Description of operation for EVENT_T0 and EVENT_T1
The following table describes the different phases illustrated by the chart below:
Phase
Description
1
A fault is detected when the EVENT_T0 input does not change to 1 during T0 time.
2
ERROR output changes to 0 when EVENT_T0 input takes on the value of 1.
3
A fault is detected when EVENT_T1 input has not changed to 1 during T1 time.
4
ERROR output changes to 0 when EVENT_T1 input takes on the value of 1.
Illustration of DFB operation, EVENT_T0 and EVENT_T1 inputs
The following chart shows how the MV_DIA function block works .
Description of operation for EVENT_T0 and EVENT_T1 inputs
The following table describes the different phases illustrated by the chart below:
Phase
Description
1
A fault is detected when EVENT_T0 input has not remained at 1 after T0 time.
2
ERROR output changes to 0 when EVENT_T0 input takes on the value of 1.
3
A fault is detected when EVENT_T1 input has not remained at 1 after T1 time.
4
ERROR output changes to 0 when ENABLE input changes to 0.
Time base
The time base for counting T0 , T1 , MMIN and MMAX actual time is defined by BASE . A change in the BASE value is not taken into account during the monitoring cycle which is running at the time. It will be taken into account at the start of the next cycle.
DFB operation during power outage
During a cold restart, the DFB initializes the parameters and public variables :