Outstation Communications Alarm on Server Changeover

[MichaelAur]

Hello All,

We have setup a calculation point to monitor and alarm when the state of a (modbus) station is no longer in healthy (11) and we haven't received a message for longer than 30 seconds (to avoid alarms in spotty signal areas). This works fine in normal operation, but on server change over the state and time are reset which causes the alarm state to clear.

("....Communications.Modbus Outstation.State" <> 11) AND ("....Communications.Modbus Outstation.State" <> 0) 
AND
("~Templates.Time.CurrentTime.Value" - "....Communications.Modbus Outstation.Time" > 0.000347) 
{If current state is not healthy and not establishing comms and time since last update is greater than 30 seconds (0.00347 days) then consider failed}

 Ideally I would set this up so that in Connecting state (9) we dont update the state of the point. However as this point is setup to execute on tag processed and quality update, I can't use an IIF statement where it sets the output to its current state as this would cause a circular reference. Would anyone have an idea on how I could make this point only calculate when not establishing comms, or some other way of preserving the points state after change over? Thanks
For reference we are running 2021 (Build 84.8852) March 2024. 

[geoffpatton · Accepted solution]

You could move the logic into an ST program that sets the state of your digital point. Convert your digital point to an Internal Digital.

[MichaelAur]

Thanks for your response @geoffpatton. I had thought the same thing, but was wondering if there may have been some clever alternative option as it seemed rather excessive to write logic for this. But its always reassuring to know that someone else is thinking the same way as you. 

Would you happen to know if there is much performance difference between Logic and Expressions. I know that logic you need to be aware of overruns, this has never been something that I have considered for Expressions though (perhaps incorrectly!)

[geoffpatton]

I have not had a problem running scripts for a similar alarm. This is still in use as its still helpful for sites wit poor signals but when we first put in DNP3 on 3G cellular the site would randomly disappear from the network and the comm alarms were a nuisance. So we set the actual comm alarm to a severity that the users are not assigned and they instead see an internal points alarm state. There are around 600 stations and the logic runs every 10 seconds. I have a timer of 600 seconds before it sets the internal point to alarm and then a time of 200 seconds before it sets the internal point to off. They run on interval because of the timers.

That system is bumping 75000 points currently and almost every point has history turned on.

This system also has a ST program on every site that runs a small SQL query every 20 seconds. SQL Queries in ST I know can be a problem, but with these the system run fine.

[JChamberlain1]

IMO the way to do this kind of thing is to set the outstation alarm to a low severity (eg make a custom 'Ignore' severity and set it to 1 and bump Low up to 2) and use a change severity redirection after the desired delay. Then you can filter out the low severity as desired.

[MichaelAur]

Thats a good option too. But when we change server sides the outstation gets set back to 'Establishing comms' so the alarm state would get reset and cause the operator to have to re-acknowledge the alarm when it times out again. 

[JChamberlain1]

If you filter out the ignore severity by default, they will never even know there was an alarm.

[MichaelAur]

Ahh, I think I understand what you're saying is to enable alarms on the outstation element and have them set to Inhibited (or some other similar level that doesnt show to operators).
However, in playing with this option it looks like you can't just enable just the Communication State - Failed state. So my redirection will apply on any other alarms that might occur on the outstation which we wouldn't want to clutter to our main users (eg when I set up a modbus simulation I got a 'Error when Reading Digital Outputs - ' which to be fair is a legitimate error, but not something we would want to present to our normal users). 
Do you know if there is any way to enable only alarms for the 'OutstationComms' alarm object?