I confirmed my OPC connection was good, then I configured my tags as described in the documentation…and I got a configuration error.

This error was strange considering I had other tags already reading properly over this OPC server. So, I scoured the internet for an explanation of why this occurred. All of the forums I read through, however, pointed to the same Ignition documentation I mentioned earlier for how to correctly address these tags.

If I couldn’t find the solution online, then I would find the solution myself—one of DMC’s core values is “Make Things Happen,” after all. After experimenting, I eventually discovered that syntax was the problem. With a few adjustments, the following format yielded correct addresses for reading the OPC tags within Ignition:

Component

Description

{ConnectionName}

The connection name, specific to the PLC (can be found by browsing the OPC server).

{MemoryArea}

The memory area where the desired tag is located. Examples include:

• Inputs – I
• Datablocks – DB{n}
• Outputs – Q

{Offset}

The numerical offset within the memory area

{Type}

The tag data type. Examples include:

• Real – R
• Double Word – DW
• I – Integer
• C – character
• X{n} – bit {n} of the word specified in {Offset}

Examples

The following are some examples I found in this project that illustrate this format:

• Bit 1 at offset 164 in Inputs:

• Double word at offset 42 in datablock 947:

• Real at offset 12 in datablock 2701: 

• Character at offset 92 in datablock 945 (interestingly, this came across as the ASCII code for the character, which I then had to use a script to convert into an actual character):

These are the example areas/data types I came across while working with this system, and they should work for other areas/data types. If you’ve run into other examples, please let us know in the comments below and let’s share some knowledge!

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