Hi All,

The following is my query with regards to Alarm Handling in a AI channel -

I have a Analog Input and Analog Output card connected to a redundant PLC pair. For the AI card, the configured alarm limits for HH, H, L & LL are 90%, 80%, 30% and 20% respectively.

Now, the following is the behavior observed. When the process value connected to the input channel exceeds 90%, both the HH and H alarms flag bits turn ON. Again, when the PV starts decreasing and goes below the HH Limit (for example if PV is 85%), the HH alarm flag bit does not get reset. Again, when the PV falls below the H limit (for example if PV is 75%), both the HH & H flags are reset.

Is this the correct behavior or should the HH flag be reset once the PV goes below the HH limit (ie if PV is 85%)? Kindly help me out with this.

Regards,
Vivek

 

Honestly, you expect us to answer with so little information?

What type of PLC? What software library? Standard or Custom? Can you diagnose the code or not? Do you know what a hysteresis value is? Is there a setting for one? Is there a delay time on the alarms? did you read the manual?

 

Check the hysteresis setting in alarm section

 

> What type of PLC? What software library? Standard or Custom? Can you
> diagnose the code or not? Do you know what a hysteresis value is? Is there a
> setting for one? Is there a delay time on the alarms? did you read the manual?

Hi,
Please find my replies below -

> What type of PLC?
The PLC is MasterLogic PLC from LSIS. The IO card in question is a 4-20mA AI channel card.

> What software library? Standard or Custom?
I am using the default software libraries.

> Do you know what a hysteresis value is?
Yes, and there is no hysteresis setting for this particular card.

> Is there a delay time on the alarms?
No alarm dead band setting.

Thanks & Regards,
Vivek

 

I would say that what you describe is definitely not how you want it to work. There must be an error in the way your alarms are configured.

 

Sounds like the alarm is being latched. It's an alarm handler - not a comparator block. Try resetting ...

YH1_OFF: Output value high 1 section off bit
YH2_OFF: Output value high 2 section off bit
YL1_OFF: Output value low 1 section off bit
YL2_OFF: Output value low 2 section off bit

https://www.honeywellprocess.com/library/support/Public/Documents/ug-instruction-list-200.pdf