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CAN Channel Message Filtering

This example shows you how to use CAN message filters to allow only messages that contain specified identifiers to pass through a channel. It uses MathWorks Virtual CAN channels connected in a loopback configuration.

Create Transmitting and Receiving Channels

One channel will transmit messages the other. Filters are set on the receiving channel.

txCh = canChannel('MathWorks', 'Virtual 1', 1);
rxCh = canChannel('MathWorks', 'Virtual 1', 2);

Create Messages

These messages are sent to the receiving channel multiple times throughout the example. Note that one of the messages has an extended identifier.

txMsgs(1) = canMessage(250, false, 8);
txMsgs(2) = canMessage(500, false, 8);
txMsgs(3) = canMessage(1000, false, 8);
txMsgs(4) = canMessage(1500, true, 8);
txMsgs(5) = canMessage(2000, false, 8);

Receive Messages on an Open Filter

Set the channels online, transmit the messages, and receive them. Note that all messages sent were received. The filter settings on a newly created channel are fully open for both standard and extended identifiers.

start(rxCh);
start(txCh);
transmit(txCh, txMsgs);
pause(0.5);
rxMsgs1 = receive(rxCh, Inf, 'OutputFormat', 'timetable')
stop(rxCh);
stop(txCh);
rxMsgs1 = 

       Time       ID     Extended    Name       Data        Length      Signals       Error    Remote
    _________    ____    ________    ____    ___________    ______    ____________    _____    ______

    0.476 sec     250    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    0.476 sec     500    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    0.476 sec    1000    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    0.476 sec    1500    true        ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    0.476 sec    2000    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 

Plot the identifiers of the received messages to see that all messages sent were received by the channel.

plot(1, rxMsgs1.ID, 'x')
h_gca = gca;
h_gca.XTick = 0:1:2;
h_gca.XTickLabel = {'', 'Transmit 1', ''};
axis([0 2 0 2047])
xlabel('Message Transmits')
ylabel('CAN Identifiers')

Receive Messages on a Configured Filter

Use the filterAllowOnly command to specify messages to receive by CAN identifier and identifier type.

filterAllowOnly(rxCh, [500 2000], 'Standard');

Display the channel to view the configured state of the message filters.

rxCh
rxCh = 

Summary of CAN Channel using 'MathWorks' 'Virtual 1' Channel 2.

  Channel Parameters:  Bus Speed is 500000.
                       Bus Status is 'N/A'.
                       Transceiver name is 'N/A'.
                       Serial Number of this device is 0.
                       Initialization access is allowed.
                       No database is attached.

              Status:  Offline - Waiting for start.
                       0 messages available to receive.
                       0 messages transmitted since last start.
                       5 messages received since last start.

      Filter History:  Standard ID Filter: Allow Only | Extended ID Filter: Allow All

Transmit the messages again to the receiving channel. Note that fewer messages were received this time.

start(rxCh);
start(txCh);
transmit(txCh, txMsgs);
pause(0.5);
rxMsgs2 = receive(rxCh, Inf, 'OutputFormat', 'timetable')
stop(rxCh);
stop(txCh);
rxMsgs2 = 

       Time        ID     Extended    Name       Data        Length      Signals       Error    Remote
    __________    ____    ________    ____    ___________    ______    ____________    _____    ______

    2.4401 sec     500    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    2.4401 sec    1500    true        ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    2.4401 sec    2000    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 

Add the new receive data to the plot to see which messages are blocked by the filter. The message with the extended identifier was numerically supposed to be blocked, but it passed the filter because the filter was only configured for standard identifiers.

plot(1, rxMsgs1.ID, 'x', 2, rxMsgs2.ID, 'x');
h_gca = gca;
h_gca.XTick = 0:1:3;
h_gca.XTickLabel = {'', 'Transmit 1', 'Transmit 2', ''};
axis([0 3 0 2047])
xlabel('Message Transmits')
ylabel('CAN Identifiers')

Reset the Message Filter

You can reset the message filters to their default open state with the filterAllowAll command.

filterAllowAll(rxCh, 'Standard');

Display the channel to view the configured state of the message filters.

rxCh
rxCh = 

Summary of CAN Channel using 'MathWorks' 'Virtual 1' Channel 2.

  Channel Parameters:  Bus Speed is 500000.
                       Bus Status is 'N/A'.
                       Transceiver name is 'N/A'.
                       Serial Number of this device is 0.
                       Initialization access is allowed.
                       No database is attached.

              Status:  Offline - Waiting for start.
                       0 messages available to receive.
                       0 messages transmitted since last start.
                       3 messages received since last start.

      Filter History:  Standard ID Filter: Allow All | Extended ID Filter: Allow All

Transmit and receive a third time to see that all messages are once again passing through the filter and into the channel.

start(rxCh);
start(txCh);
transmit(txCh, txMsgs);
pause(0.5);
rxMsgs3 = receive(rxCh, Inf, 'OutputFormat', 'timetable')
stop(rxCh);
stop(txCh);
rxMsgs3 = 

       Time        ID     Extended    Name       Data        Length      Signals       Error    Remote
    __________    ____    ________    ____    ___________    ______    ____________    _____    ______

    3.3821 sec     250    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    3.3821 sec     500    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    3.3821 sec    1000    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    3.3821 sec    1500    true        ''      [1×8 uint8]    8         [0×0 struct]    false    false 
    3.3821 sec    2000    false       ''      [1×8 uint8]    8         [0×0 struct]    false    false 

With the new data added to the plot, observe that the first and third transmits are identical as the message filters were fully open in both cases.

plot(1, rxMsgs1.ID, 'x', 2, rxMsgs2.ID, 'x', 3, rxMsgs3.ID, 'x')
h_gca = gca;
h_gca.XTick = 0:1:4;
h_gca.XTickLabel = {'', 'Transmit 1', 'Transmit 2', 'Transmit 3', ''};
axis([0 4 0 2047])
xlabel('Message Transmits')
ylabel('CAN Identifiers')

Filter Messages By Name

The filterAllowOnly command can also reference messages by name when using a CAN database file.

db = canDatabase('demoVNT_CANdbFiles.dbc');
rxCh.Database = db;
filterAllowOnly(rxCh, 'EngineMsg');
rxCh
rxCh = 

Summary of CAN Channel using 'MathWorks' 'Virtual 1' Channel 2.

  Channel Parameters:  Bus Speed is 500000.
                       Bus Status is 'N/A'.
                       Transceiver name is 'N/A'.
                       Serial Number of this device is 0.
                       Initialization access is allowed.
                       'demoVNT_CANdbFiles.dbc' database is attached.

              Status:  Offline - Waiting for start.
                       0 messages available to receive.
                       0 messages transmitted since last start.
                       5 messages received since last start.

      Filter History:  Standard ID Filter: Allow Only | Extended ID Filter: Allow All

Blocking All Messages of a Specific Type

The filterBlockAll command lets you quickly set the filter to block all messages of either standard or extended identifier type.

filterBlockAll(rxCh, 'Extended');
rxCh
rxCh = 

Summary of CAN Channel using 'MathWorks' 'Virtual 1' Channel 2.

  Channel Parameters:  Bus Speed is 500000.
                       Bus Status is 'N/A'.
                       Transceiver name is 'N/A'.
                       Serial Number of this device is 0.
                       Initialization access is allowed.
                       'demoVNT_CANdbFiles.dbc' database is attached.

              Status:  Offline - Waiting for start.
                       0 messages available to receive.
                       0 messages transmitted since last start.
                       5 messages received since last start.

      Filter History:  Standard ID Filter: Allow Only | Extended ID Filter: Block All

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