A remote Modbus device might return an exception error for various reasons. Should an exception occur, Geo SCADA Expert will set the affected Modbus scanner to ‘Failed’. The alarm that is raised will indicate the reason for the failure. With the Simple Modbus driver, an exception might be generated if any of the following occur:
- Illegal Function—Triggered if a function code is invalid or not supported by the remote device. Such an exception might also occur if the remote device is in the wrong state to process a function code.
For information on the function codes supported by Geo SCADA Expert, see Modbus Function Codes Supported by the Geo SCADA Expert Simple Modbus Driver.
- Illegal Data Address—Triggered if the address specified in the request is outside of the range supported by the remote device. Such an exception might occur, for example, should a ‘Read Input Registers’ request be sent with a start register lower than the start range supported by the device, or should the request be for a larger number of registers than that supported by the device.
For more information on register ranges, see Modbus Register Ranges.
- Illegal Data Value—Triggered if the remainder of a request is invalid.
- Slave Device Failure—Triggered if there is a problem with the remote device.
- Slave Device Busy—Triggered if the remote device cannot process the request at the time of receipt.
- Memory Parity Error—Returned if supported by a function code, should a memory parity error occur.
A Modbus scanner’s Comms Statistics status attribute indicates if any exception errors have occurred since that scanner was last restarted.