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AO_SOFT/AO_SCADA COMPOSITE

Hello,

 

We have used the AO_SOFT/AO_SCADA composite for writing analog output to PLC through VIM. AO_SOFT/AO_SCADA composite is not showing modbad alarm when analog output register is bad. Is anyone used these composites and faced same issue? 

 

Is anyone used these composites for third party communication and faced any issue related to BAD? 

Prasad Lakkala

• 12 Mar 2025 8:22 AM
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1 Reply

• Andre Dicaire

  

  • 12 Mar 2025 3:27 PM

  The ModBad Alarm is typically tied to either MError. With IO blocks, the BlockError for an AO or DO block can be flagged to set Bad_Active on the block, which in turn is flagged in MError to trigger the Bad_Active parameter of the module. The ModBad is often configured to Bad_Active.
  
  The recommended connection to serial or bus cards is through an External Reference parameter. This is because an AO or DO block suspend writes to a Bad signal (channel). This allows the block to initialize to the field state, in case that changed. Serial data does not have any Fault State behavior and the last value written remains in the Serial card register until status goes good, and this last good value written may not be appropriate.
  
  With an External Reference, there is no status initialization and as such, the module can adjust the value in the Serial card even when the status is bad. This allows a Module to align the output value based on expected field behavior, such as a watchdog timer in the device tripping the motor so DeltaV does the same so the Serial card does not inappropriately send a start.
  
  Anyway, the Modbad Alarm needs to be configured such that the MError or MStatus masks respond to the appropriate flag when the External reference reports a Bad Status. Setting the MASK parameters for that condition will set Bad_Active, which if used to drive ModBad alarm will trigger the alarm.
  
  Alternatively, you can use a CND block to derive whatever conditions you consider to indicate a bad module and set the ModBad alarm to be set by that CND/OUT_D value. This would also provide a delay filter on the OUT_D to address chattering or fleeting conditions.
  
  Sorry I don't know the exact options in MError or MStatus that are set on a Bad External Reference. Since these are parameters, the Block Error settings do not play a part.

  Andre Dicaire

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