Documentation

CAN Read Strategy

The "CAN Read" block reads one message from one channel of the module input queue. The "Data present" output port shows if there are more messages available in the queue.

The CAN receiver requires a do-while construction to ensure that all messages in the buffer are read at a given sample step. It is important to estimate the number of messages expected so that the correct sample time of the do-while subsystem is selected. The sample time of the subsystem is defined by Simulink inheritance rules: if the subsystem does not have input ports, it will inherit the fundamental sample time of the model. Alternatively, the sample time of the subsystem can be controlled by adding an input port. The sample time of the block connected to the input port is inherited by the subsystem.

The do-while construction iterates the CAN Read operation until the receiver buffer is empty (this information is provided by the Data present port). In the same subsystem, we can filter the messages required by the application.

The sample time must be chosen in order to avoid buffer overflow during a sample step. Furthermore, if multiple messages with the same Message ID are received, only the most recent messages will appear in the do-while subsystem output. To read all messages with a certain ID, the do-while subsystem must be run faster than the frequency of the message(s) in question.

For example, if Message ID 100 is expected with a frequency of 10 ms, and all these messages must be read, the do-while subsystem sample time must be faster than 10 ms, for example, 5 ms.

If there are other messages in the line, a check must be carried out to ensure that there is no message overflow; for example, with a buffer size of 501 messages (IO612), if 500 messages are expected within 500 ms, a sample time of 500 ms or slower may lead to the messages in the buffer being overwritten.

Top Level View

Inside the While Iterator Subsystem

The "Sample Time" input port defines the rate at which the subsystem is executed. Otherwise it will run on the base rate.

Initialization and Termination CAN Messages

The IO602 Setup driver block allows you to define CAN messages to be sent during initialization and termination of the real-time application. The driver block can send messages at each application start and stop. The main purpose for sending these messages is to initialize or terminate other CAN nodes on the network, such as CANopen or DeviceNet nodes. Even if the CAN layers are not directly supported, the model can usually communicate with these nodes with standard CAN messages, provided the nodes are initialized. The initialization and termination fields of the Setup blocks are intended for this purpose.

You define the initialization and termination CAN messages by using MATLAB^® structure arrays with CAN-specific field names. This approach was also used for the RS-232 and general counter driver blocks found in the Simulink Real-Time™ I/O library. Refer to these driver blocks and their help for additional information about this basic concept.