Read(TPCANHandle, TPCANMsg)

PCAN-Basic

previous page next page

PCAN-Basic Documentation

Reads a CAN message from the receive queue of a PCAN Channel.

Syntax

Pascal OO

class function Read(
    Channel: TPCANHandle;
    var MessageBuffer: TPCANMsg
    ): TPCANStatus; overload;

public static TPCANStatus Read(
    TPCANHandle Channel,
    out TPCANMsg MessageBuffer);

C++ / CLR

static TPCANStatus Read(
    TPCANHandle Channel,
    TPCANMsg %MessageBuffer);

Visual Basic

Public Shared Function Read( _
    ByVal Channel As TPCANHandle, _
    ByRef MessageBuffer As TPCANMsg) As TPCANStatus
End Function

Parameters

Parameters	Description
Channel	The handle of a PCAN Channel (see TPCANHandle).
MessageBuffer	A TPCANMsg buffer to store the CAN message.

Returns

The return value is a TPCANStatus code. PCAN_ERROR_OK is returned on success. The typical errors in case of failure are:

PCAN_ERROR_ILLPARAMVAL:	Indicates that the parameters passed to the method are invalid. Check the value of the MessageBuffer; it should point to a TPCANMsg structure.
PCAN_ERROR_INITIALIZE:	Indicates that the given PCAN channel was not found in the list of initialized channels of the calling application.
PCAN_ERROR_BUSLIGHT:	Indicates a bus error within the given PCAN Channel. The hardware is in bus-light status.
PCAN_ERROR_BUSHEAVY:	Indicates a bus error within the given PCAN Channel. The hardware is in bus-heavy status.
PCAN_ERROR_BUSOFF:	Indicates a bus error within the given PCAN Channel. The hardware is in bus-off status.
PCAN_ERROR_QRCVEMPTY:	Indicates that the receive queue of the Channel is empty.

Remarks

The Read method returns received messages or status messages from the receive queue. It is important to call Read repeatedly until the queue is empty. In case there are no more messages in queue, the value PCAN_ERROR_QRCVEMPTY is returned. The error code PCAN_ERROR_QRCVEMPTY is also returned if the reception of messages is disabled. See Receive Status Parameter for more information.

The receive queue can contain up to 32767 messages.

If the time when the message was received is needed, use the overloaded Read method.

There are two possibilities for reading messages from the receive queue of a Channel:

Time-Triggered Reading: Consists in periodically calls to the Read method. Typically, an application start a timer that every 50 or 100 milliseconds check for messages, calling the Read method in a loop until the value of PCAN_ERROR_QRCVEMTY or another error condition is reached.

Event-Triggered Reading: Consists in reacting to a notification sent by the PCAN driver to a registered application, when a message is received and inserted in its receive queue. See Using Events to obtain more information about reading with events.

About bus errors / Status messages

If a bus-off error occur, an application cannot use the channel to communicate anymore, until the CAN controller is reset. With PCAN-Basic it is not possible to reset the CAN controller through a method directly. Consider using the PCAN-Basic property PCAN_BUSOFF_AUTORESET which instructs the API to automatically reset the CAN controller when a bus-off state is detected.

Another way to reset errors like BUSOFF, BUSHEAVY, and BUSLIGTH, is to uninitialize and initialise again the channel used. This causes a hardware reset, but only when no more clients are connected to that channel.

The message type (see TPCANMessageType) of a CAN message indicates if the message is a 11-bit, 29-bit, RTR, Error, or Status message. This value should be checked every time a message has been read successfully.

If the bit PCAN_MESSAGE_ERRFRAME is set in the TPCANMsg.MSGTYPE field, the message is an Error frame (see Error Frames).

If the bit PCAN_MESSAGE_STATUS is set in the TPCANMsg.MSGTYPE field, the message is a Status message. The ID and LEN fields do not contain valid data. The first 4 data bytes of the message contain the Error Code. The MSB of the Error Code is in data byte 0, the LSB is in data byte 3. If a status message was read the return value of Read is also the error code.

Examples:

Data0	Data1	Data2	Data3	Error	Error Code	Description
00h	00h	00h	02h	PCAN_ERROR_OVERRUN	0002h	CAN controller has been read out too late.
00h	00h	00h	04h	PCAN_ERROR_BUSLIGHT	0004h	Bus error: An error counter has reached the 'Light' limit.
00h	00h	00h	08h	PCAN_ERROR_BUSHEAVY	0008h	Bus error: An error counter has reached the 'Heavy' limit.
00h	00h	00h	10h	PCAN_ERROR_BUSOFF	0010h	Bus error: CAN Controller is in 'Bus Off' state.

Example

The following example shows the use of method Read on the channel PCAN_USBBUS1. In case of failure, the returned code will be translated to a text (according with the operating system language) in English, German, Italian, French or Spanish, and it will be shown to the user.

Note: It is assumed that the channel was already initialized and that the following code is an OnTimer event handler method.

C#:

TPCANStatus result;
StringBuilder strMsg;
TPCANMsg msg;

strMsg = new StringBuilder(256);

do
{
    // Check the receive queue for new messages
    //
    result = PCANBasic.Read(PCANBasic.PCAN_USBBUS1, out msg);
    if (result != TPCANStatus.PCAN_ERROR_QRCVEMPTY)
    {
        // Process the received message
        //
        MessageBox.Show("A message was received");
        ProcessMessage(msg);
    }
    else
    {
        // An error occurred, get a text describing the error and show it
        //
        PCANBasic.GetErrorText(result, 0, strMsg);
        MessageBox.Show(strMsg.ToString());
        // Here can be decided if the loop has to be  terminated (eg. the bus
        // status is  bus-off)
        //
        HandleReadError(result);
    }
// Try to read a message from the receive queue of the PCAN-USB, Channel 1,
// until the queue is empty
//
}while((result & TPCANStatus.PCAN_ERROR_QRCVEMPTY) != TPCANStatus.PCAN_ERROR_QRCVEMPTY);

C++CLR:

TPCANStatus result;
StringBuilder^ strMsg;
TPCANMsg msg;

strMsg = gcnew StringBuilder(256);

do
{
    // Check the receive queue for new messages
    //
    result = PCANBasic::Read(PCANBasic::PCAN_USBBUS1, msg);
    if (result != TPCANStatus::PCAN_ERROR_QRCVEMPTY)
    {
        // Process the received message
        //
        MessageBox::Show("A message was received");
        ProcessMessage(msg);
    }
    else
    {
        // An error occurred, get a text describing the error and show it
        //
        PCANBasic::GetErrorText(result, 0, strMsg);
        MessageBox::Show(strMsg->ToString());
        // Here can be decided if the loop has to be  terminated (eg. the bus
        // status is  bus-off)
        //
        HandleReadError(result);
    }
// Try to read a message from the receive queue of the PCAN-USB, Channel 1,
// until the queue is empty
//
}while((result & TPCANStatus::PCAN_ERROR_QRCVEMPTY) != TPCANStatus::PCAN_ERROR_QRCVEMPTY);

Visual Basic:

Dim result As TPCANStatus
Dim strMsg As StringBuilder
Dim msg As TPCANMsg

strMsg = New StringBuilder(256)

Do
    ' Check the receive queue for new messages
    '
    result = PCANBasic.Read(PCANBasic.PCAN_USBBUS1, msg)
    If result <> TPCANStatus.PCAN_ERROR_QRCVEMPTY Then
        MessageBox.Show("A message was received")
        ProcessMessage(msg)
    Else
        ' An error occurred, get a text describing the error and show it
        '
        PCANBasic.GetErrorText(result, 0, strMsg)
        MessageBox.Show(strMsg.ToString())
        ' Here can be decided if the loop has to be  terminated (eg. the bus
        ' status is  bus-off)
        '
        HandleReadError(result)
    End If
' Try to read a message from the receive queue of the PCAN-USB, Channel 1,
' until the queue is empty
'
Loop While ((result And TPCANStatus.PCAN_ERROR_QRCVEMPTY) <> TPCANStatus.PCAN_ERROR_QRCVEMPTY)

Pascal OO:

var
    result : TPCANStatus;
    strMsg: array [0..256] of Char;
    msg: TPCANMsg;
begin
    repeat
        // Check the receive queue for new messages
        //
        result := TPCANBasic.Read(TPCANBasic.PCAN_USBBUS1, msg);
        If (result <> PCAN_ERROR_QRCVEMPTY) Then
        begin
            // Process the received message
            //
            MessageBox(0,'A message was received','Success', MB_OK);
            ProcessMessage(msg);
        end
        else
        begin
            // An error occurred, get a text describing the error and show it
            //
            TPCANBasic.GetErrorText(result, 0, strMsg);
            MessageBox(0, strMsg, 'Error',MB_OK);
            // Here can be decided if the loop has to be  terminated (eg. the bus
            // status is  bus-off)
            //
            HandleReadError(result);
        end;
    // Try to read a message from the receive queue of the PCAN-USB, Channel 1,
    // until the queue is empty
    //
    until ((TPCANStatus(Integer(result) AND Integer(PCAN_ERROR_QRCVEMPTY))) = PCAN_ERROR_QRCVEMPTY);