Invalid Modbus Value

What does 3 mean?

The Modbus request included a bit or word value that is outside the permissible range or format for the specified address in the slave device. This indicates the data being written is unacceptable for that particular register. Writing invalid values can cause unexpected behavior, misconfiguration, or failure of the slave device to operate correctly.

• Master controller attempting to write a value exceeding the configured maximum limit for a specific register (e.g., Modbus register 40001, representing parameter P1-10 'Max Frequency', set to 100Hz when drive limit is 60Hz).
• Data type mismatch where the master writes an integer to a bit register or a floating-point value to an integer register without proper scaling (e.g., writing 1.5 to a register expecting 0 or 1).
• Attempting to write data to a Modbus register address that is designated as read-only by the slave device (e.g., writing to a diagnostic status register like P5-01 'Drive Status Word').
• Bitmask operation sending an invalid combination for a control word, leading to an unacceptable state (e.g., setting both 'Start Fwd' and 'Start Rev' bits simultaneously on a VFD's control word register).
• Incorrect scaling factor applied by the master, causing the calculated value to fall outside the permissible operating range for the slave device (e.g., trying to set motor current to 1500% of rated when actual maximum is 200%).

Click steps to track your progress.

1. 1

   1. Verify the Modbus register map for the slave device (e.g., PLC, VFD) against the master's programming for the affected Modbus address (e.g., 40001).

2. 2

   2. Check the permissible data range (minimum and maximum values) for the specific Modbus register being written as per the slave device's technical manual (e.g., P1-10 range is 0 to 6000 representing 0.0-60.0Hz).

3. 3

   3. Confirm the expected data type (e.g., unsigned 16-bit integer, boolean) for the target register on the slave device and ensure the master's write operation matches.

4. 4

   4. Inspect the master controller's program logic (e.g., SCADA system HMI script, PLC ladder logic) to identify the source of the invalid value and correct the calculation or hardcoded limit (e.g., change `MAX_SPEED_CMD = 100` to `MAX_SPEED_CMD = 60`).

5. 5

   5. Attempt to write a known good, in-range value (e.g., 0 or a nominal operating value like 500 representing 50.0Hz for a frequency command) to the specific Modbus register using a Modbus master simulator tool.

6. 6

   6. For bit registers, confirm the master's bitmask logic aligns with the slave's expected control word structure (e.g., ensuring only one direction bit is set for motor control in Modbus register 40000).