Recently, I needed to access tags on a Siemens S7-300 PLC in Ignition 7.9 for one of my projects. These tags were communicated using an intermediate Siemens SIMATIC NET OPC Server. Since I know Ignition’s documentation is usually quite stellar, I consulted their guide to help configure my OPC tags.
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:
ns=3;s={ConnectionName}.{MemoryArea}.{Offset},{Type}
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Component
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Description
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{ConnectionName}
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The connection name, specific to the PLC (can be found by browsing the OPC server).
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{MemoryArea}
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The memory area where the desired tag is located. Examples include:
- Inputs – I
- Datablocks – DB{n}
- Outputs – Q
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{Offset}
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The numerical offset within the memory area
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{Type}
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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}
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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:
ns=3;s=S7 connection LA2141.DB947.42,DW
- Real at offset 12 in datablock 2701:
ns=3;s=S7 connection LF4431.DB2701.12,R
- 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):
ns=3;s=S7 connection LA2141.DB945.92,C
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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