- Qt 6.4
- Qt Serial Bus
- Using PassThruCAN Plugin
- Qt 6.4.2 Reference Documentation

# Using PassThruCAN Plugin

The Pass-Thru CAN plugin accesses CAN adapters via the SAE J2534 Pass-Thru API. SAE J2534 is a standard for accessing vehicle buses from an x86 Windows PC. Although the API is specified only for 32-bit Windows, some vendors also provide implementations for 64-bit Windows and other operating systems such as Linux.

## PassThruCAN usage

To use PassThruCAN, the corresponding vendor drivers for the CAN adapter must be installed. The vendor must also provide an implementation of the J2534 API by way of a shared library. Currently, only version 04.04 of the Pass-Thru API is supported.

When using an x64 build of Qt, this plugin only works if the CAN device vendor also provides a 64-bit version of the J2534 Pass-Thru interface library. If the vendor provides only a 32-bit J2534 interface, a 32-bit build of Qt is required to make use of it.

For automatic device discovery, the vendor software must also list and describe the available adapters in the Windows registry. On systems other than Windows, automatic discovery is currently not supported.

## Creating CAN Bus Devices

At first it is necessary to check that QCanBus provides the desired plugin:

     if (QCanBus::instance()->plugins().contains(QStringLiteral("passthrucan"))) {
         // plugin available
     }

Where passthrucan is the plugin name.

On Windows, automatic device discovery should be used to list the available CAN adapters accessible via the Pass-Thru API:

     const auto adapters = QCanBus::instance()->
             availableDevices(QStringLiteral("passthrucan"));
     for (const QCanBusDeviceInfo &info : adapters) {
         // List available adapter in the user interface.
         uiListBox->addItem(info.name());
     }

On other operating systems, the list of discovered adapters will be empty. Instead, the full path to the vendor-provided J2534 interface library should be provided in lieu of the device name:

     QCanBusDevice *device = QCanBus::instance()->createDevice(
         QStringLiteral("passthrucan"), QStringLiteral("/path/to/libj2534-vendor.so"));

For special needs, it is also possible to pass a vendor-specific device name argument when opening the Pass-Thru adapter:

     QCanBusDevice *device = QCanBus::instance()->createDevice(
         QStringLiteral("passthrucan"), info.name() + QChar::fromLatin1('%') + deviceName);

All operations on the Pass-Thru CAN bus device are executed asynchronously, including connect and disconnect. In order to be notified when the device is ready for reading and writing CAN frames, connect to the stateChanged(QCanBusDevice::CanBusDeviceState state) signal:

     if (!device) {
         // Error handling goes here
     } else {
         connect(device, &QCanBusDevice::stateChanged,
                 this, &MyClass::canStateChanged);
         device->connectDevice();
     }

state() will return ConnectedState once the CAN adapter has been successfully connected to. The device is then open for writing and reading CAN frames:

     QCanBusFrame frame;
     frame.setFrameId(8);
     frame.setPayload(QByteArray("\xA3\x6E\x74\x9C", 4));
     device->writeFrame(frame);

The reading can be done using the readFrame() method. The framesReceived() signal is emitted when at least one new frame is available for reading:

     QCanBusFrame frame = device->readFrame();

The Pass-Thru CAN plugin supports the following configuration options controllable via setConfigurationParameter():

Configuration parameter key | Description
--- | ---
QCanBusDevice::LoopbackKey | When enabled, if a CAN frame is transmitted on the CAN bus, a local echo of this frame will be received by the CAN adapter. The echo frames are marked with QCanBusFrame::hasLocalEcho(). By default, loopback mode is disabled.
QCanBusDevice::RawFilterKey | This option allows setting up filters for incoming CAN bus messages. If provided, the value should be a QList<QCanBusDevice::Filter>. Only data frame ID filters are supported. By default, data frames with any ID are accepted.
QCanBusDevice::BitRateKey | The bit rate of the CAN bus as an unsigned integer, in bit/s. The default bit rate is 500000 (500 kbit/s). Setting the bit rate after the device has already been connected may trigger an implicit reinitialization of the CAN interface.

The Pass-Thru CAN plugin supports extended frame format (29-bit IDs), but not flexible data-rate (CAN FD).