CAN uses short messages – the maximum utility load is 94 bits. There is no explicit address in the messages; instead, the messages can be said to be contents-addressed, that is, their contents implicitly determines their address.
There are four different message types (or “frames”) on a CAN bus:
- the Data Frame
- the Remote Frame
- the Error Frame
- the Overload Frame
The Data Frame
Summary: “Hello everyone, here’s some data labeled X, hope you like it!”
The Data Frame is the most common message type. It comprises the following major parts (a few details are omitted for the sake of brevity):
- the Arbitration Field, which determines the priority of the message when two or more nodes are contending for the bus. The Arbitration Field contains:
- For CAN 2.0A, an 11-bit Identifier and one bit, the RTR bit, which is dominant for data frames.
- For CAN 2.0B, a 29-bit Identifier (which also contains two recessive bits: SRR and IDE) and the RTR bit.
- the Data Field, which contains zero to eight bytes of data.
- the CRC Field, which contains a 15-bit checksum calculated on most parts of the message. This checksum is used for error detection.
- an 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.
Note 1: 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.
Note 2: The Identifier in the Arbitration Field is not, despite of its name, necessarily identifying the contents of the message.
The Remote Frame
Summary: “Hello everyone, can somebody please produce the data labeled X?”
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), and
- 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. 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.
The Error Frame
Summary: (everyone, aloud) “OH DEAR, LET’S TRY AGAIN”
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 Overload Frame
Summary: “I’m a very busy little 82526, could you please wait for a moment?”
The Overload Frame is mentioned here just for completeness. It is very similar to the Error Frame with regard to the format and it is transmitted by a node that becomes too busy. The Overload Frame is not used very often, as today’s CAN controllers are clever enough not to use it. In fact, the only controller that will generate Overload Frames is the now obsolete 82526.