SCADA Question concerning alarming

This is a fundamental question about SCADA design.

I am looking for the pros and cons of having alarms originate at the PLC vs alarms originating at the HMI.

I have worked with systems in the past where the alarm is generated at outstation PLC. I am now asked to create an HMI that takes the data from the outstation and processes the alarm.

To me, this seems very fundamental and I am hoping to get more insight on the reasons that you would do either over the other.

Hope this wasnt too confusing.
Thanks!

Generate the alarm in the PLC always. HMI's, SCADAs are for display only. Something to tell the operator what the alarm is.

I generally agree with Mickey, although there are times when a non-critical alarm is easier generated in the HMI....Something like a large storage tank is nearing low level. It's easier to configure the HMI tag to just alarm on that low level. To create it in the pLC of course takes additional code, and an additional bit (low level alarm) in the PLC to HMI traffic.

up_onus said:

This is a fundamental question about SCADA design.

I am looking for the pros and cons of having alarms originate at the PLC vs alarms originating at the HMI.

I have worked with systems in the past where the alarm is generated at outstation PLC. I am now asked to create an HMI that takes the data from the outstation and processes the alarm.

To me, this seems very fundamental and I am hoping to get more insight on the reasons that you would do either over the other.

Hope this wasnt too confusing.
Thanks!

Click to expand...

Assuming we are talking SCADA PCs.
I always beleive a less reliable device (PC) should not control a more reliable device (PLC or are they PAC now?)

Chances are the PLC logic will need to know the status of equipment or a particular parameter. That should be in the PLC and it is a very dangerous practice to have the PLC/PAC expect the HMI to tell the PLC/PAC logic a dependent motor has tripped and other motors needs to shut down.

The SCADA PC/HMI should be able to reset, (Windoze Reboot) without impacting the PLC program.

but....why?
Pros and cons?
reasons?
antyhing besides "this is the way"?

I want all alarms originating in the PLC. Like Mickey said, the HMI/SCADA should only be a window into the process. The system should be able to function without the HMI/SCADA... yes, it might be ugly, but it should be able to run...

up_onus said:

but....why?
Pros and cons?
reasons?
antyhing besides "this is the way"?

Click to expand...

Reliably and operators are not pushing buttons on the PLC.
HMI's and SCADAs are in the most hazardous of environments,
where operators hang out. That's reason enough for me.

Another reason is scan time. The PLC can scan the state of it's I/O much faster than a remote device can query the PLC for that same information. If you need to alarm on data that can change states very quickly, then this is critically important.

Generate the alarms in the PLC. I have found that all HMI packages from all vendors (espcially on Windows) are nowhere nearly as reliable as a PLC, and have a lot less 'up time'. In some processes, it is acceptable to run with the HMI out of action (note I say some) and that way if you have the HMI offline (breakdown / maintenance / reboot) you will still have the alarm triggered. Ofcourse, you may not be able to see the alarm message (cos that comes up on the HMI) but at least the operators know there is something wrong and can take action.

In HMI - Pros

- Quicker to configure - many have friendly dialog boxes etc. May be able to use CSV files for mass changes
- Saves memory in the PLC
- Less comms between PLC and HMI

In HMI - Cons
- Less reliable - cos the HMI will have lower up-time
- More chance of errors (I wave seen errors between PLC and HMI values)
- More nuisance alarms when taking HMI on and off line (some packes release a short burst)
- Less scalable (if you have multiple HMIs)
- Slower (Windows might be busy doing something)
- HMI now becomes more critical so taking it offline for a reboot because more risky
- Cannot use alarm bits in PLC code (interlocks etc)

Hope that helps