QCanSignalDescription Class

Each CAN frame can contain multiple values. The rules to extract the values from a CAN frame include the following:

• Data source (frame ID or payload).

• Data endianness. See Data Endianness Processing section for more details.

• Data format.

• Start bit position.

• Data length in bits.

• Multiplexing options.

Start bit position is specified relative to the selected data source. The bits are counted starting from the LSB.

Once the data is extracted, it might require conversion to an application-defined format. The following parameters can be used for that:

• Various parameters for converting the extracted value to a physical value (factor, offset, scale).

• Expected data range.

• Data units.

The QCanSignalDescription class provides methods to control all those parameters.

Little endian and big endian data is encoded differently. For big endian values, start bit positions are given for the most significant bit. For little endian values, the start position is that of the least significant bit.

Let's consider two examples. In both examples we will encode two 12-bit values in the 3-byte payload.

I-2-1. Little Endian▲

For the little endian case the data layout can be represented by the following image:

Here the columns represent bit numbers, and the rows represent byte numbers. LSB marks the first (least significant) bit of the value, and MSB marks the last (most significant) bit of the value. The blue color marks the first value, and the orange color marks the second value.

The information about these values will be encoded in QCanSignalDescription in the following way:

 

Sélectionnez

QCanSignalDescription signal1;
signal1.setDataEndian(QSysInfo::Endian::LittleEndian);
signal1.setStartBit(0);
signal1.setBitLength(12);
// other parameters for signal1

QCanSignalDescription signal2;
signal2.setDataEndian(QSysInfo::Endian::LittleEndian);
signal2.setStartBit(12);
signal2.setBitLength(12);
// other parameters for signal2

I-2-2. Big Endian▲

The following image represents the value layout for the big endian case:

The values can be represented in QCanSignalDescription in the following way:

 

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QCanSignalDescription signal1;
signal1.setDataEndian(QSysInfo::Endian::BigEndian);
signal1.setStartBit(7);
signal1.setBitLength(12);
// other parameters for signal1

QCanSignalDescription signal2;
signal2.setDataEndian(QSysInfo::Endian::BigEndian);
signal2.setStartBit(11);
signal2.setBitLength(12);
// other parameters for signal2

Note how the start bits are different from the little endian case. Also the values are aligned differently.

There are two common ways to encode the data in the CAN payload:

• Each range of bits always represents the same signal. For example, Bytes 0-1 in a payload can represent an engine speed (in rpm), and Bytes 2-3 can represent the vehicle speed (in km/h).

• The same range of bits can represent different data, depending on the values of some other bits in the payload. For example, if Byte 0 has the value 0, the Bytes 1-2 represent an engine speed (in rpm), and if Byte 0 has the value 1, the same Bytes 1-2 represent a vehicle speed (in km/h).

The second case uses signal multiplexing. In the provided example we will have three signals. The first signal represents the value of Byte 0 and acts like a multiplexor signal. The other two signals represent an engine speed and a vehicle speed respectively, but only one of them can be extracted from the CAN payload at a time. Which signal should be extracted is defined by the value of the multiplexor signal.

In more complicated cases the payload can have multiple multiplexor signals. In such cases the signal can be extracted from the payload only when all multiplexors contain the expected values.

In many cases the signals transferred over CAN bus cannot hold the full range of the physical values that they represent. To overcome these limitations, the physical values are converted to a smaller range before transmission, and can be restored on the receiving end.

The following formulas are used to convert between the physical value and the signal's value:

physicalValue = scaling * (signalValue * factor + offset);
signalValue = (physicalValue / scaling - offset) / factor;

The factor and scaling parameters cannot be equal to 0.

If any of the parameters equals to qQNaN(), it is not used during the conversion. If all of the parameters are equal to qQNaN() (which is the default), the conversion is not performed.

Creates an empty signal description.

Creates a signal description with the values copied from other.

Creates a signal description by moving from other.

Destroys this signal description.

Adds a new multiplexor signal for this signal. The name parameter contains the name of the multiplexor signal, and the ranges parameter contains the desired value ranges.

If this signal already has desired value ranges for the multiplexor signal name, the ranges are overwritten.

III-5-1. See Also▲

This is an overloaded function.

This is a convenience overload for the case when the multiplexor signal is expected to have only one specific value, not a range of values.

The name parameter contains the name of the multiplexor signal, and the value parameter contains the desired value.

If this signal already has desired value ranges for the multiplexor signal name, the ranges are overwritten.

III-6-1. See Also▲

Returns the bit length of the signal's value.

III-7-1. See Also▲

Removes all multiplexor signals for this signal.

III-8-1. See Also▲

Returns the comment for the signal.

This parameter is introduced only for extra description. It's not used during signal processing.

III-9-1. See Also▲

Returns the data endian of the signal's value.

By default, BigEndian is used.

III-10-1. See Also▲

Returns the data format of the signal's value.

By default, SignedInteger is used.

III-11-1. See Also▲

Returns the data source of the signal's value.

By default, Payload is used.

III-12-1. See Also▲

Returns the factor that is used to convert the signal's value to a physical value and back.

By default the function returns qQNaN(), which means that a factor is not used.

The Value Conversions section explains how this parameter is used.

III-13-1. See Also▲

Returns true when the signal description is valid and false otherwise.

A valid signal description must fulfill the following conditions:

• have a non-empty name()

• have bitLength() == 32 if the dataFormat() is Float

• have bitLength() == 64 if the dataFormat() is Double

• the bitLength() must be a multiple of 8 if the dataFormat() is AsciiString

• the bitLength() must be greater than 0 and less than or equal to 64.

III-14-1. See Also▲

Returns the maximum supported value for the signal.

By default the function returns qQNaN(), which means that there is no maximum value.

III-15-1. See Also▲

Returns the minimum supported value for the signal.

By default the function returns qQNaN(), which means that there is no minimum value.

III-16-1. See Also▲

Returns the multiplexor signals and their desired values that are used to properly identify this signal.

The returned hash contains signal names as keys and respective desired ranges of values as values.

This signal's value can be extracted from the payload only when all the signals from the hash have the expected values.

III-17-1. See Also▲

Returns the multiplex state of the signal.

See the Multiplexed Signals Explained section for more details on multiplexed signals.

By default this method returns None.

III-18-1. See Also▲

Returns the name of the signal.

III-19-1. See Also▲

Returns the offset that is used to convert the signal's value to a physical value and back.

By default the function returns qQNaN(), which means that an offset is not used.

The Value Conversions section explains how this parameter is used.

III-20-1. See Also▲

Returns the physical unit (e.g. km/h) of the signal's value or an empty string if the unit is not set.

This parameter is introduced only for extra description. It's not used during signal processing.

III-21-1. See Also▲

Returns the receiver node for this signal.

This parameter is introduced only for extra description. It's not used during signal processing.

III-22-1. See Also▲

Returns the scaling that is used to convert the signal's value to a physical value and back.

By default the function returns qQNaN(), which means that scaling is not used.

The Value Conversions section explains how this parameter is used.

III-23-1. See Also▲

Sets the bit length of the signal's value to length.

III-24-1. See Also▲

Sets the comment for the signal to text.

This parameter is introduced only for extra description. It's not used during signal processing.

III-25-1. See Also▲

Sets the data endian of the signal's value to endian.

III-26-1. See Also▲

Sets the data format of the signal's value to format.

III-27-1. See Also▲

Sets the data source of the signal's value to source.

III-28-1. See Also▲

Sets the factor that is used to convert the signal's value to a physical value and back to factor.

Pass qQNaN() to this method to skip this parameter during the conversion.

The factor cannot be 0. An attempt to set a zero factor is equivalent to setting it to qQNaN().

The Value Conversions section explains how this parameter is used.

III-29-1. See Also▲

Sets the multiplexor signals for this signal to multiplexorSignals.

The multiplexorSignals hash must contain signal names as keys and respective desired value ranges as values.

III-30-1. See Also▲

Sets the multiplex state of the signal to state.

See the Multiplexed Signals Explained section for more details on multiplexed signals.

III-31-1. See Also▲

Sets the name of the signal to name.

The signal's name must be unique within a CAN message.

III-32-1. See Also▲

Sets the offset that is used to convert the signal's value to a physical value and back to offset.

Pass qQNaN() to this method to skip this parameter during the conversion.

The Value Conversions section explains how this parameter is used.

III-33-1. See Also▲

Sets the physical unit (e.g. km/h) of the signal's value.

This parameter is introduced only for extra description. It's not used during signal processing.

III-34-1. See Also▲

Sets the minimum and maximum for the signal's value.

Setting one or both of the parameters to qQNaN() means that the corresponding limit will not be used.

III-35-1. See Also▲

Sets the receiver node for this signal.

This parameter is introduced only for extra description. It's not used during signal processing.

III-36-1. See Also▲

Sets the scaling that is used to convert the signal's value to a physical value and back to scaling.

Pass qQNaN() to this method to skip this parameter during the conversion.

The scaling cannot be 0. An attempt to set zero scaling is equivalent to setting it to qQNaN().

The Value Conversions section explains how this parameter is used.

III-37-1. See Also▲

Sets the start bit of the signal's value in the dataSource() to bit.

III-38-1. See Also▲

Returns the start bit of the signal's value in the dataSource().

III-39-1. See Also▲

Assigns the values from other to this signal description.

Move-assigns the values from other to this signal description.