Beckhoff TwinCAT 3 ADS Error 1808: Symbol Not Found after PLC download, what to check?
I'm getting an ADS Error 1808 (0x710) "Symbol Not Found" on my TwinCAT 3 runtime. This started happening after I downloaded a new PLC program to the controller. Only some of the HMI tags are failing, while others on the same connection still work.
ADS Error 1808 (0x710) indicates that the PLC runtime could not find a symbol name requested by a client (like an HMI or SCADA). This often occurs after a PLC download, especially if variables were renamed or removed. The error means the symbol name itself is not recognized, as opposed to a stale handle or no PLC running.
Likely causes and checks
1Symbol path spelled differently from the PLC
- Points to it
- The client is requesting a symbol using a name or path that does not exactly match what is defined in the running PLC project. This includes missing program, GVL, or namespace prefixes.
- Check
- Compare the full symbol path (e.g., MAIN.nCounter, GVL_IO.bStart) used by the client with the symbols available in the currently running PLC project on the target controller.
2Variable not in the symbol file of the running project
- Points to it
- Even if the variable exists in the source code, it might not have been downloaded to the target, or the symbol generation settings excluded it from the symbol file that the client uses to find tags.
- Check
- Verify that the PLC project running on the target controller is indeed the one containing the variable, and confirm that symbol creation settings include this variable.
3Client connected to the wrong PLC runtime port
- Points to it
- The client application might be attempting to connect to an AMS port that does not correspond to the PLC project owning the desired variable.
- Check
- Check that the AMS port (e.g., 851, 852) configured for the client connection matches the specific PLC project that contains the variable you are trying to access.
Next check: Given that only some tags are failing and others on the same port work, start by comparing the full symbol paths of the failing tags with the symbols in your running PLC project to ensure exact matches.
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