Use a tag's Value as a part of another OPC Tag's path

Ignition
1

Hello Everybody.

I was wondering if it would be possible to construct a Tag’s source path (e.g. an OPC Tag’s path) by using a combination of strings and dynamic Values from other tags.

For instance, I have two UDT instances called Plc, stored in the Device1 and Device2 folder respectively, like so:

MyTags/Devices/Device1/Plc/DeviceName
MyTags/Devices/Device2/Plc/DeviceName

The Plc UDT contains a tag called PlcDeviceName.

Now, each PLC contains a software module called ModuleA, which I model like so:

MyTags/Devices/Device1/Modules/Cooker/Pressure
MyTags/Devices/Device2/Modules/Cooker/Pressure

In both cases, Cooker are instances of a UDT called Cooker, which contains an OPC Tag called Pressure.

What I’m trying to do is to define a dynamic OPC path for Pressure, like so:

[{[.]…/…/Plc/DeviceName}]DB100,I8

which references integer 8 within data block 100 of the underlying SIEMENS PLC (Device1 or Device2 respectively).

I understand the above syntax doesn’t work; I just used it to illustrate my question with an example.
My questions in short:

  • Is it possible to define dynamic OPC paths?
  • If so, how?
  • If not directly possible, can we use some advanced scripting technique to achieve the same results?

Thanks.

2

Here’s an update of what I’ve been able to accomplish so far.

Based on the following example, this is the result:

screenshot-01.jpg1053×245 22.2 KB

I’ve been able to construct a script string by referencing other tags, using the relative tag path, like so:

OpcItemPath Expression Tag’s expression:

"[" + tag('[.]../../Plc/DeviceName') + "]" + "DB733,I8"

Script Expression Tag’s expression:

"shared.pcsmis.opc.readValue('" + tag('[.]OpcItemPath') + "')"

The returned string stored in the “Script” tag is:
shared.pcsmis.opc.readValue('[S1P01]DB733,I8')

This is the global script that returns the OPC value, and it works great as I tested it on its own:

def getOpcServerName():
	return 'Ignition OPC-UA Server' 
	
def readValue(item_path):
	return system.opc.readValue(getOpcServerName(), item_path)

The only problem I still have is, how to run the script successfully from the Pressure Expression Tag.

Pressure Expression Tag’s expression:

runScript( toString( tag('[.]Script') )  )

This is the error displayed under Tag Diagnostics:

screenshot-02.jpg729×545 47.8 KB

Anyone knows what this error means?

Thanks.