- Qt 6.7
- Qt Serial Bus
- C++ Classes
- QModbusServer
- Qt 6.7.0 Reference Documentation

# QModbusServer Class

The QModbusServer class is the interface to receive and process Modbus requests. More...

Header: | #include <QModbusServer>
CMake: | find_package(Qt6 REQUIRED COMPONENTS SerialBus) target_link_libraries(mytarget PRIVATE Qt6::SerialBus)
qmake: | QT += serialbus
Inherits: | QModbusDevice
Inherited By: | QModbusRtuSerialServer and QModbusTcpServer

- List of all members, including inherited members

## Public Types

enum | Option { DiagnosticRegister, ExceptionStatusOffset, DeviceBusy, AsciiInputDelimiter, ListenOnlyMode, …, UserOption }

## Public Functions

| QModbusServer(QObject *parent = nullptr)
bool | data(QModbusDataUnit *newData) const
bool | data(QModbusDataUnit::RegisterType table, quint16 address, quint16 *data) const
virtual bool | processesBroadcast() const
int | serverAddress() const
bool | setData(const QModbusDataUnit &newData)
bool | setData(QModbusDataUnit::RegisterType table, quint16 address, quint16 data)
virtual bool | setMap(const QModbusDataUnitMap &map)
void | setServerAddress(int serverAddress)
virtual bool | setValue(int option, const QVariant &newValue)
virtual QVariant | value(int option) const

## Signals

void | dataWritten(QModbusDataUnit::RegisterType table, int address, int size)

## Protected Functions

virtual QModbusResponse | processPrivateRequest(const QModbusPdu &request)
virtual QModbusResponse | processRequest(const QModbusPdu &request)
virtual bool | readData(QModbusDataUnit *newData) const
virtual bool | writeData(const QModbusDataUnit &newData)

## Detailed Description

Modbus networks can have multiple Modbus servers. Modbus Servers are read/written by a Modbus client represented by QModbusClient. QModbusServer communicates with a Modbus backend, providing users with a convenient API.

## Member Type Documentation

### enum QModbusServer::Option

Each Modbus server has a set of values associated with it, each with its own option.

The general purpose options (and the associated types) are:

Constant | Value | Description
--- | --- | ---
`QModbusServer::DiagnosticRegister` | `0` | The diagnostic register of the server. `quint16`
`QModbusServer::ExceptionStatusOffset` | `1` | The exception status byte offset of the server. `quint16`
`QModbusServer::DeviceBusy` | `2` | Flag to signal the server is engaged in processing a long-duration program command. `quint16`
`QModbusServer::AsciiInputDelimiter` | `3` | The Modbus ASCII end of message delimiter. `char`
`QModbusServer::ListenOnlyMode` | `4` | Flag to set listen only mode of the server. This function is typically supported only by Modbus serial devices. `bool`
`QModbusServer::ServerIdentifier` | `5` | The identifier of the server, not the server address. `quint8`
`QModbusServer::RunIndicatorStatus` | `6` | The run indicator of the server. `quint8`
`QModbusServer::AdditionalData` | `7` | The additional data of the server. `QByteArray`
`QModbusServer::DeviceIdentification` | `8` | The physical and functional description of the server. `QModbusDeviceIdentification`

User options:

Constant | Value | Description
--- | --- | ---
`QModbusServer::UserOption` | `0x100` | The first option that can be used for user-specific purposes.

For user options, it is up to the developer to decide which types to use and ensure that components use the correct types when accessing and setting values.

## Member Function Documentation

### `[explicit]` QModbusServer::QModbusServer(QObject *parent = nullptr)

Constructs a Modbus server with the specified parent.

### bool QModbusServer::data(QModbusDataUnit *newData) const

Returns the values in the register range given by newData.

newData must provide a valid register type, start address and valueCount. The returned newData will contain the register values associated with the given range.

If newData contains a valid register type but a negative start address the entire register map is returned and newData appropriately sized.

See also setData().

### bool QModbusServer::data(QModbusDataUnit::RegisterType table, quint16 address, quint16 *data) const

Reads data stored in the Modbus server. A Modbus server has four tables (table) and each have a unique address field, which is used to read data from the desired field. See QModbusDataUnit::RegisterType for more information about the different tables. Returns `false` if address is outside of the map range or the register type is not even defined.

See also QModbusDataUnit::RegisterType and setData().

### `[signal]` void QModbusServer::dataWritten(QModbusDataUnit::RegisterType table, int address, int size)

This signal is emitted when a Modbus client has written one or more fields of data to the Modbus server. The signal contains information about the fields that were written:

- Register type (table) that was written,
- address of the first field that was written,
- and size of consecutive fields that were written starting from address.

The signal is not emitted when the to-be-written fields have not changed due to no change in value.

### `[virtual protected]` QModbusResponse QModbusServer::processPrivateRequest(const QModbusPdu &request)

This function should be implemented by custom Modbus servers. It is called by processRequest() if the given request is not a standard Modbus request.

Overwriting this function allows handling of additional function codes and subfunction-codes not specified in the Modbus Application Protocol Specification 1.1b. Reimplementations should call this function again to ensure an exception response is returned for all unknown function codes the custom Modbus implementation does not handle.

This default implementation returns a `QModbusExceptionResponse` with the request function code and error code set to illegal function.

See also processRequest().

### `[virtual protected]` QModbusResponse QModbusServer::processRequest(const QModbusPdu &request)

Processes a Modbus client request and returns a Modbus response. This function returns a QModbusResponse or QModbusExceptionResponse depending on the nature of the request.

The default implementation of this function handles all standard Modbus function codes as defined by the Modbus Application Protocol Specification 1.1b. All other Modbus function codes not included in the specification are forwarded to processPrivateRequest().

The default handling of the standard Modbus function code requests can be overwritten by reimplementing this function. The override must handle the request type in question and return the appropriate QModbusResponse. A common reason might be to filter out function code requests for data values to limit read/write access and function codes not desired in particular implementations such as serial line diagnostics on ethernet or Modbus Plus transport layers. Every other request type should be forwarded to this default implementation.

Note: This function should not be overridden to provide a custom implementation for non-standard Modbus request types.

See also processPrivateRequest().

### `[virtual]` bool QModbusServer::processesBroadcast() const

Subclasses should implement this function if the transport layer shall handle broadcasts. The implementation then should return `true` if the currently processed request is a broadcast request; otherwise `false`. The default implementation returns always `false`.

Note: The return value of this function only makes sense from within processRequest() or processPrivateRequest(), otherwise it can only tell that the last request processed was a broadcast request.

### `[virtual protected]` bool QModbusServer::readData(QModbusDataUnit *newData) const

Reads the values in the register range given by newData and writes the data back to newData. Returns `true` on success or `false` if newData is `0`, the newData range is outside of the map range or the registerType() does not exist.

Note: Sub-classes that implement reading from a different backing store then default one, also need to implement setMap() and writeData().

See also setMap() and writeData().

### int QModbusServer::serverAddress() const

Returns the address of this Modbus server instance.

See also setServerAddress().

### bool QModbusServer::setData(const QModbusDataUnit &newData)

Writes newData to the Modbus server map. Returns `false` if the newData range is outside of the map range.

If the call was successful the dataWritten() signal is emitted. Note that the signal is not emitted when the addressed register has not changed. This may happen when newData contains exactly the same values as the register already. Nevertheless this function returns `true` in such cases.

See also data().

### bool QModbusServer::setData(QModbusDataUnit::RegisterType table, quint16 address, quint16 data)

Writes data to the Modbus server. A Modbus server has four tables (table) and each have a unique address field, which is used to write data to the desired field. Returns `false` if address outside of the map range.

If the call was successful the dataWritten() signal is emitted. Note that the signal is not emitted when data has not changed. Nevertheless this function returns `true` in such cases.

See also QModbusDataUnit::RegisterType, data(), and dataWritten().

### `[virtual]` bool QModbusServer::setMap(const QModbusDataUnitMap &map)

Sets the registered map structure for requests from other ModBus clients to map. The register values are initialized with zero. Returns `true` on success; otherwise `false`.

If this function is not called before connecting, a default register with zero entries is setup.

Note: Calling this function discards any register value that was previously set.

### void QModbusServer::setServerAddress(int serverAddress)

Sets the address for this Modbus server instance to serverAddress.

See also serverAddress().

### `[virtual]` bool QModbusServer::setValue(int option, const QVariant &newValue)

Sets the newValue for option and returns `true` on success; `false` otherwise.

Note: If the option's associated type is `quint8` or `quint16` and the type of newValue is larger, the data will be truncated or conversation will fail.

Key | Description
--- | ---
QModbusServer::DiagnosticRegister | Sets the diagnostic register of the server in a device specific encoding to newValue. The default value preset is `0x0000`. The bit values of the register need device specific documentation.
QModbusServer::ExceptionStatusOffset | Sets the exception status byte offset of the server to newValue which is the absolute offset address in the coils (0x register). Modbus register table starting with `0x0000h`. The default value preset is `0x0000`, using the exception status coils similar to Modicon 984 CPUs (coils 1-8). The function returns `true` if the coils register contains the 8 bits required for storing and retrieving the status coils, otherwise `false`.
QModbusServer::DeviceBusy | Sets a flag that signals that the server is engaged in processing a long-duration program command. Valid values are `0x0000` (not busy) and `0xffff` (busy). The default value preset is `0x0000`.
QModbusServer::AsciiInputDelimiter | The newValue becomes the end of message delimiter for future Modbus ASCII messages. The default value preset is `\n`.
QModbusServer::ListenOnlyMode | Ss the server's listen only state to newValue. If listen only mode is set to `true`, messages are monitored but no response will be sent. The default value preset is `false`.
QModbusServer::ServerIdentifier | Sets the server's manufacturer identifier to newValue. Possible values are in the range of `0x00` to 0xff. The default value preset is `0x0a`.
QModbusServer::RunIndicatorStatus | Sets the servers' run indicator status to newValue. This data is used as addendum by the QModbusPdu::ReportServerId function code. Valid values are `0x00` (OFF) and `0xff` (ON). The default value preset is `0xff` (ON).
QModbusServer::AdditionalData | Sets the server's additional data to newValue. This data is used as addendum by the QModbusPdu::ReportServerId function code. The maximum data size cannot exceed 249 bytes to match response message size restrictions. The default value preset is `Qt Modbus Server`.
QModbusServer::DeviceIdentification | Sets the server's physical and functional description. By default there is no additional device identification data set.
QModbusServer::UserOption | Sets the value of a user option to newValue. Note: For user options, it is up to the developer to decide which types to use and ensure that components use the correct types when accessing and setting values.

See also value().

### `[virtual]` QVariant QModbusServer::value(int option) const

Returns the value for option or an invalid `QVariant` if the option is not set.

Option | Description
--- | ---
QModbusServer::DiagnosticRegister | Returns the diagnostic register value of the server. The diagnostic register contains device specific contents where each bit has a specific meaning.
QModbusServer::ExceptionStatusOffset | Returns the offset address of the exception status byte location in the coils register.
QModbusServer::DeviceBusy | Returns a flag that signals if the server is engaged in processing a long-duration program command.
QModbusServer::AsciiInputDelimiter | Returns a end of message delimiter of Modbus ASCII messages.
QModbusServer::ListenOnlyMode | Returns the server's listen only state. Messages are monitored but no response will be sent.
QModbusServer::ServerIdentifier | Returns the server manufacturer's identifier code. This can be an arbitrary value in the range of `0x00` to 0xff.
QModbusServer::RunIndicatorStatus | Returns the server's run indicator status. This data is used as addendum by the QModbusPdu::ReportServerId function code.
QModbusServer::AdditionalData | Returns the server's additional data. This data is used as addendum by the QModbusPdu::ReportServerId function code.
QModbusServer::DeviceIdentification | Returns the server's physical and functional description.
QModbusServer::UserOption | Returns the value of a user option. Note: For user options, it is up to the developer to decide which types to use and ensure that components use the correct types when accessing and setting values.

See also setValue().

### `[virtual protected]` bool QModbusServer::writeData(const QModbusDataUnit &newData)

Writes newData to the Modbus server map. Returns `true` on success, or `false` if the newData range is outside of the map range or the registerType() does not exist.

Note: Sub-classes that implement writing to a different backing store then default one, also need to implement setMap() and readData(). The dataWritten() signal needs to be emitted from within the functions implementation as well.

See also setMap(), readData(), and dataWritten().