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Problem:
When attempting to read data from data blocks configured in an S7-1200 device, the quality is Bad. This causes the server's Event Log to post the error "Unable to read <block size> bytes starting at address '<address>' on device '<device name>'".

Solution:
The "Symbolic Access only" block attribute restricts access to the data block from external connections. This setting is checked by default in the controller ladder software. To prevent this error, uncheck the "Symbolic Access only" block attribute. Then, re-load the ladder project to the controller.