Enumerations

This section documents all enumerations used in the libmodbus library.

🎮 Function Codes

ModbusFunctionCode

Defines all supported Modbus function codes.

enum class ModbusFunctionCode : uint8_t

Modbus function codes as defined in the Modbus specification.

This enumeration contains all the standard Modbus function codes supported by this library. Each function code corresponds to a specific operation that can be performed on Modbus data.

Values:

enumerator NONE: No function code (invalid/uninitialized)

enumerator READ_COILS: Read coils (discrete outputs) - FC 01.

enumerator READ_DISCRETE_INPUTS: Read discrete inputs - FC 02.

enumerator READ_HOLDING_REGISTERS: Read holding registers - FC 03.

enumerator READ_INPUT_REGISTERS: Read input registers - FC 04.

enumerator WRITE_SINGLE_COIL: Write single coil - FC 05.

enumerator WRITE_SINGLE_REGISTER: Write single register - FC 06.

enumerator READ_EXCEPTION_STATUS: Read exception status - FC 07.

enumerator DIAGNOSTICS: Diagnostics - FC 08.

enumerator WRITE_MULTIPLE_COILS: Write multiple coils - FC 15 (0x0F)

enumerator WRITE_MULTIPLE_REGISTERS: Write multiple registers - FC 16 (0x10)

🚨 Exception Codes

ModbusExceptionCode

Defines all Modbus exception codes for error responses.

enum class ModbusExceptionCode : uint8_t

Modbus exception codes returned in error responses.

When a Modbus request cannot be processed successfully, the server responds with an exception frame containing one of these exception codes to indicate the specific error condition.

Values:

enumerator NONE: No exception (success)

enumerator ILLEGAL_FUNCTION: Function code not supported.

enumerator ILLEGAL_DATA_ADDRESS: Data address not valid.

enumerator ILLEGAL_DATA_VALUE: Data value not valid.

enumerator SLAVE_DEVICE_FAILURE: Unrecoverable error in slave device.

enumerator ACKNOWLEDGE: Request accepted, processing.

enumerator SLAVE_DEVICE_BUSY: Slave device busy.

enumerator NEGATIVE_ACKNOWLEDGMENT: Request cannot be performed.

enumerator MEMORY_PARITY_ERROR: Memory parity error.

enumerator GATEWAY_PATH_UNAVAILABLE: Gateway path unavailable.

enumerator GATEWAY_TARGET_DEVICE_FAILED_TO_RESPOND: Gateway target device failed to respond.

🔧 Diagnostic Codes

ModbusDiagnosticsCode

Defines sub-function codes for the Diagnostics function (FC 08).

enum class ModbusDiagnosticsCode : uint16_t

Diagnostic sub-function codes for function code 08 (Diagnostics)

The diagnostics function (FC 08) supports various sub-functions for testing communication and retrieving diagnostic information from Modbus devices. This enumeration defines the standard sub-function codes.

Values:

enumerator RETURN_QUERY_DATA: Echo back query data.

enumerator RESTART_COMMUNICATIONS_OPTION: Restart communications option.

enumerator RETURN_DIAGNOSTIC_REGISTER: Return diagnostic register.

enumerator CHANGE_ASCII_INPUT_DELIMITER: Change ASCII input delimiter.

enumerator FORCE_LISTEN_ONLY_MODE: Force listen only mode.

enumerator CLEAR_COUNTERS_AND_DIAGNOSTIC_REGISTER: Clear counters and diagnostic register.

enumerator RETURN_BUS_MESSAGE_COUNT: Return bus message count.

enumerator RETURN_BUS_COMMUNICATION_ERROR_COUNT: Return bus communication error count.

enumerator RETURN_BUS_EXCEPTION_ERROR_COUNT: Return bus exception error count.

enumerator RETURN_SLAVE_MESSAGE_COUNT: Return slave message count.

enumerator RETURN_SLAVE_NO_RESPONSE_COUNT: Return slave no response count.

enumerator RETURN_SLAVE_NAK_COUNT: Return slave NAK count.

enumerator RETURN_SLAVE_BUSY_COUNT: Return slave busy count.

enumerator RETURN_BUS_CHARACTER_OVERRUN_COUNT: Return bus character overrun count.

🖼️ Frame Types

ModbusFrameType

Defines the type of Modbus frame being processed.

enum class ModbusFrameType

Enumeration of Modbus frame types.

This enumeration identifies the type of Modbus frame being processed, which determines how the frame should be interpreted and what response should be generated.

Values:

enumerator NONE: Uninitialized or invalid frame.

enumerator REQUEST: Request frame from client to server.

enumerator RESPONSE: Normal response frame from server to client.

enumerator EXCEPTION: Exception response frame indicating an error.