CAN Data Frames¶
The CAN Data Frame, represented by the
Frame object, is the most
common message type, which consists of the following major parts (a few details
are omitted for the sake of brevity):
- CAN identifier:
The CAN identifier, or Arbitration Field, determines the priority of the message when two or more nodes are contending for the bus. The CAN identifier contains for:
CAN 2.0A, an 11-bit Identifier and one bit, the RTR bit, which is dominant for data frames.
CAN 2.0B, a 29-bit Identifier, with the EXT bit set, (which also contains two recessive bits: SRR and IDE) and the RTR bit.
- Data field:
The Data field contains zero to eight bytes of data.
- Data Length Code:
The DLC field specifies the number of data bytes in the Data field.
- CRC Field:
The CRC Field contains a 15-bit checksum calculated on most parts of the message. This checksum is used for error detection.
- Acknowledgement Slot:
Any CAN controller that has been able to correctly receive the message sends an Acknowledgement bit at the end of each message. The transmitter checks for the presence of the Acknowledge bit and retransmits the message if no acknowledge was detected.
It is worth noting that the presence of an Acknowledgement Bit on the bus does not mean that any of the intended addressees has received the message. The only thing we know is that one or more nodes on the bus has received it correctly. The Identifier in the Arbitration Field is not, despite of its name, necessarily identifying the contents of the message.
CAN FD Data Frames¶
A standard CAN network is limited to 1 MBit/s, with a maximum payload of 8 bytes per frame. CAN FD increases the effective data-rate by allowing longer data fields - up to 64 bytes per frame - without changing the CAN physical layer. CAN FD also retains normal CAN bus arbitration, increasing the bit-rate by switching to a shorter bit time only at the end of the arbitration process and returning to a longer bit time at the CRC Delimiter, before the receivers send their acknowledge bits. A realistic bandwidth gain of 3 to 8 times what’s possible in CAN will particularly benefit flashing applications.
Nearly all hardware platforms support detection of Error Frames. If an Error
Frame arrives, the flag
ERROR_FRAME is set in the
Frame. The identifier is garbage if an Error Frame is received, but
for LAPcan it happens to be 2048 plus the error code from the SJA1000.
Simply put, the Error Frame is a special message that violates the framing rules of a CAN message. It is transmitted when a node detects a fault and will cause all other nodes to detect a fault - so they will send Error Frames, too. The transmitter will then automatically try to retransmit the message. There is an elaborate scheme of error counters that ensures that a node can’t destroy the bus traffic by repeatedly transmitting error frames.
The Error Frame consists of an Error Flag, which is 6 bits of the same value (thus violating the bit-stuffing rule) and an Error Delimiter, which is 8 recessive bits. The Error Delimiter provides some space in which the other nodes on the bus can send their Error Flags when they detect the first Error Flag.
The Remote Frame is just like the Data Frame, with two important differences:
It is explicitly marked as a Remote Frame (the RTR bit in the Arbitration Field is recessive)
There is no Data Field.
The intended purpose of the Remote Frame is to solicit the transmission of the corresponding Data Frame. If, say, node A transmits a Remote Frame with the Arbitration Field set to 234, then node B, if properly initialized, might respond with a Data Frame with the Arbitration Field also set to 234.
Remote Frames can be used to implement a type of request-response type of bus traffic management. In practice, however, the Remote Frame is little used. It is also worth noting that the CAN standard does not prescribe the behaviour outlined here. Most CAN controllers can be programmed either to automatically respond to a Remote Frame, or to notify the local CPU instead.
There’s one catch with the Remote Frame: the Data Length Code must be set to the length of the expected response message even though no data is sent. Otherwise the arbitration will not work.
Sometimes it is claimed that the node responding to the Remote Frame is starting its transmission as soon as the identifier is recognized, thereby “filling up” the empty Remote Frame. This is not the case.
Overload Frames aren’t used nowadays. Certain old CAN controllers (Intel 82526) used them to delay frame processing in certain cases.
Other frame features of interest¶
There are some other frame features of interest: