CAN Channel Message Filtering

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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 =

  5x8 timetable

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

    0.0091838 sec     250     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0091875 sec     500     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0091922 sec    1000     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0091959 sec    1500     true       {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0091991 sec    2000     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 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 = 

  Channel with properties:

   Device Information
            DeviceVendor: 'MathWorks'
                  Device: 'Virtual 1'
      DeviceChannelIndex: 2
      DeviceSerialNumber: 0
            ProtocolMode: 'CAN'

   Status Information
                 Running: 0
       MessagesAvailable: 0
        MessagesReceived: 5
     MessagesTransmitted: 0
    InitializationAccess: 1
        InitialTimestamp: 23-Feb-2021 17:16:16
           FilterHistory: 'Standard ID Filter: Allow Only | Extended ID Filter: Allow All'

   Channel Information
               BusStatus: 'N/A'
              SilentMode: 0
         TransceiverName: 'N/A'
        TransceiverState: 'N/A'
       ReceiveErrorCount: 0
      TransmitErrorCount: 0
                BusSpeed: 500000
                     SJW: []
                   TSEG1: []
                   TSEG2: []
            NumOfSamples: []

   Other Information
                Database: []
                UserData: []

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 =

  3x8 timetable

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

    0.0057236 sec     500     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0057319 sec    1500     true       {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0057391 sec    2000     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 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 = 

  Channel with properties:

   Device Information
            DeviceVendor: 'MathWorks'
                  Device: 'Virtual 1'
      DeviceChannelIndex: 2
      DeviceSerialNumber: 0
            ProtocolMode: 'CAN'

   Status Information
                 Running: 0
       MessagesAvailable: 0
        MessagesReceived: 3
     MessagesTransmitted: 0
    InitializationAccess: 1
        InitialTimestamp: 23-Feb-2021 17:16:18
           FilterHistory: 'Standard ID Filter: Allow All | Extended ID Filter: Allow All'

   Channel Information
               BusStatus: 'N/A'
              SilentMode: 0
         TransceiverName: 'N/A'
        TransceiverState: 'N/A'
       ReceiveErrorCount: 0
      TransmitErrorCount: 0
                BusSpeed: 500000
                     SJW: []
                   TSEG1: []
                   TSEG2: []
            NumOfSamples: []

   Other Information
                Database: []
                UserData: []

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 =

  5x8 timetable

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

    0.0082374 sec     250     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.00824 sec       500     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0082424 sec    1000     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0082445 sec    1500     true       {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 struct}    false    false 
    0.0082466 sec    2000     false      {0x0 char}    {[0 0 0 0 0 0 0 0]}      8       {0x0 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 = 

  Channel with properties:

   Device Information
            DeviceVendor: 'MathWorks'
                  Device: 'Virtual 1'
      DeviceChannelIndex: 2
      DeviceSerialNumber: 0
            ProtocolMode: 'CAN'

   Status Information
                 Running: 0
       MessagesAvailable: 0
        MessagesReceived: 5
     MessagesTransmitted: 0
    InitializationAccess: 1
        InitialTimestamp: 23-Feb-2021 17:16:19
           FilterHistory: 'Standard ID Filter: Allow Only | Extended ID Filter: Allow All'

   Channel Information
               BusStatus: 'N/A'
              SilentMode: 0
         TransceiverName: 'N/A'
        TransceiverState: 'N/A'
       ReceiveErrorCount: 0
      TransmitErrorCount: 0
                BusSpeed: 500000
                     SJW: []
                   TSEG1: []
                   TSEG2: []
            NumOfSamples: []

   Other Information
                Database: [1x1 can.Database]
                UserData: []

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 = 

  Channel with properties:

   Device Information
            DeviceVendor: 'MathWorks'
                  Device: 'Virtual 1'
      DeviceChannelIndex: 2
      DeviceSerialNumber: 0
            ProtocolMode: 'CAN'

   Status Information
                 Running: 0
       MessagesAvailable: 0
        MessagesReceived: 5
     MessagesTransmitted: 0
    InitializationAccess: 1
        InitialTimestamp: 23-Feb-2021 17:16:19
           FilterHistory: 'Standard ID Filter: Allow Only | Extended ID Filter: Block All'

   Channel Information
               BusStatus: 'N/A'
              SilentMode: 0
         TransceiverName: 'N/A'
        TransceiverState: 'N/A'
       ReceiveErrorCount: 0
      TransmitErrorCount: 0
                BusSpeed: 500000
                     SJW: []
                   TSEG1: []
                   TSEG2: []
            NumOfSamples: []

   Other Information
                Database: [1x1 can.Database]
                UserData: []

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