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Kepware Knowledge Base: Solution

Tag Quality in Modbus Drivers Configured for Unsolicited Communications

Last Update: 11/7/2018

The tags configured for unsolicited communications in the Modbus Unsolicited Serial driver and Modbus TCP/IP Ethernet driver have good quality before they've been written to by the Modbus Master. This is because their default setting of the "OPC quality bad until write" option is disabled. Before KEPServerEX version 4.280, the drivers always operated as though this option was off: all tags had good quality from startup. Now, users can choose the behavior as desired through the Device Properties.

If the "OPC quality bad until write" option is enabled, tags in the driver will have bad quality until the memory locations referenced by the tags have been written to by a Modbus command or client application. This option can be helpful in certain scenarios, such as when Modbus master --> Modbus Unsolicited or Ethernet driver --> LinkMaster --> link outputs to another OPC server.

Note: If the "OPC quality bad until write" option is disabled, LinkMaster will write zeros to the link outputs when the system starts and before the Modbus master has written anything to the drivers. Once enabled, LinkMaster will not write to the link outputs until the Modbus master has written to the drivers.

Related Protocols
Modbus Ethernet