How to Unpack a Modbus Holding Register as 16 Bits Using the Modbus Scanner in a SCADAPack x70 RTU
Some vendors and some users like to bit-pack 16 single Modbus coils into a holding register. This enables a SCADA host or SCADA client RTU/PLC to get 16 bits of information along with other analog data in a single poll to a remote outstation using Modbus function code 03 (Read holding registers).
When a SCADAPack x70 device is acting as Modbus client, it can unpack the bits and place them into individual objects. This is easily done through configuration of the Modbus scanner and no logic is needed to unpack the bits.
When configurating the Modbus scanner in RemoteConnect, the user needs to select “UINT (Digital)” as the data type of what needs to be polled. This will automatically poll the holding register and break it out to individual objects when the data is returned to the SCADAPack. These individual binary objects can be assigned to logic (if needed) or can be assigned individual Modbus/DNP addresses to be polled from a SCADA system or other RTU/PLC client.
Below is an example of the SCADAPack Modbus Scanner settings that are needed to unpack the 16 bits without logic.
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