Applies To:
  • CitectSCADA 3.xx, 4.xx
Summary:
How can I get my trend tags to be polled from the PLC only when the Trend Page is displayed

( Because the Trend Expression is read even if the Trend is "Gated", there seems to be no way to stop the trend from polling )

 

Solution:
The solution to the problem is to Trend a DiskPLC tag that you set to the Real PLC value on a periodic basis (ie : every X seconds when the page is displayed update the Disk PLC tag via an Event)

The use of a "Trigger" is also recommended as it will "Gate" (Hide) the values that would be trended even though the Real PLC is not being read from.

The use of a Disk PLC rather than a Memory PLC means that the system will work across a Citect Network.

You also only need to setup the "I/O Server" to Run Event "Trend"

VARIABLE TAGS (Add 1 Disk tag per PLC Tag + 1 Disk tag per TrendPage)

RealTag1 - Actual I/O Tag
RealTag2 - Actual I/O Tag
etc.

TrendTag1 - Disk PLC Tag (Real)
TrendTag2 - Disk PLC Tag (Real)
etc.

TrendShow1 - Disk PLC Tag (Digital)
TrendShow2 - Disk PLC Tag (Digital)
etc.

TREND TAGS (Change Expression to the Disk tag & add the Trigger of the existing tags)

Trend1     - "Expression" : TrendTag1
           - "Trigger"    : TrendShowX (see the trend page)
           - "Period"     : XX:XX:XX
Trend2     - "Expression" : TrendTag2
           - "Trigger"    : TrendShowX (see the trend page)
           - "Period"     : XX:XX:XX
etc.

TREND PAGE (Add the "Page Entry/Exit functions to the existing pages)

TrendPage1 - "Page Entry" Function : TrendShow1 = TRUE
           - "Page Exit" Function  : TrendShow1 = FALSE
TrendPage2 - "Page Entry" Function : TrendShow2 = TRUE
           - "Page Exit" Function  : TrendShow2 = FALSE
etc.

SYSTEM EVENT (Add one Event for each Trend Tag, but give them all the same Event Name)

Trend      - "Trigger" : TrendShow1
           - "Period"  : XX:XX:XX (see the trend tag)
           - "Action"  : TrendTag1 = RealTag1
Trend      - "Trigger" : TrendShow1
           - "Period"  : XX:XX:XX (see the trend tag)
           - "Action"  : TrendTag2 = RealTag2
etc.

Trend      - "Trigger" : TrendShow2
           - "Period"  : XX:XX:XX (see the trend tag)
           - "Action"  : TrendTag3 = RealTag3
Trend      - "Trigger" : TrendShow2
           - "Period"  : XX:XX:XX (see the trend tag)
           - "Action"  : TrendTag4 = RealTag4
etc.

 
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