Update of process tags in discrete alarms

Hello,

I'm using WinCC RT Advanced V15.1-V16 and a PLC S7-1500.

When configuring a discrete alarm with a process tag in the alarm text, the value shown is not the one at the time of the alarm but the previous value. Is there some documentation explaining how it is working? Has anyone noticed this behavior before?

To better explain the issue:

AlarmText : Error, my tag is equal to <MyProcessTag />

MyProcessTag:  1 2 1 2 1 1 3 3 3 3 3 3

MyAlarmBit:    0 0 0 0 0 0 1 1 1 1 1 1

Displayed alarm: Error, my tag is equal to 1

 Both of the tags come from the PLC with the same acquisition cycle (continuous 1s).

Thanks for your help.

Hi,

for me it seems acquisition mode of MyProcessTag is not "cyclic continuous". If it is though, is the behaviour correct when using acquisition cylce 500ms for MyProcessTag?

Regards,

Frink

It improves things but It is not constant. Moreover, cyclic continuous is heavy on communication.

Also, I've made a few tests.
One of them was to set MyProcessTag and MyAlarmBit as internal tags and modify them using a script.

Simply put, I modify the value of MyProcessTag then I set MyAlarmBit, and again the text shows the old value of MyProcessTag. If I set a delay of 2 seconds in the script between the 2 commands, it shows the correct value. If I set a delay of 1 second, it shows the old value.

So, is WinCC using the value of the process tags stored in its runtime memory when the alarm gets triggered?
I would have thought that it would update the process tag when it gets triggered, especially if the acquisition is set to "Cyclic in operation".

In the help:

SIemens Help

Cyclic in operation

If you select the "Cyclic in operation" acquisition mode, the tag is updated in Runtime while it is displayed in a screen or is logged. The acquisition cycle determines the update cycle for tag value updates on the HMI device. You can either choose a default acquisition cycle or define a user-specific cycle.

Cyclic continuous

If you select the "Cyclic continuous" acquisition mode, the tag will be updated continuously in Runtime, even if it is not in the currently-open screen. This setting is activated for tags that are configured to trigger a function list when their value changes, for example.

Only use the "Cyclic continuous" setting for tags that must truly be updated. Frequent read operations increase communication load.

On demand

If you select the "On demand" acquisition mode, the tag is not updated cyclically. It will only be updated on demand using the "UpdateTag" system function, for example, or by a script.