Section 6.1 of the CAN spec:
BIT ERROR: A unit that is sending a bit on the bus also monitors the
bus. A BIT ERROR has to be detected at that bit time, when the bit
value that is monitored is different from the bit value that is sent.
An exception is the sending of a ’recessive’ bit during the stuffed
bit stream of the ARBITRATION FIELD or during the ACK SLOT.
So, the node which first transmits a '1' when the other is transmitting a '0' will note a Bit Error and then signal an error as normal - by transmitting an error-flag (see Section 3.1.3) , as described formally in Section 6.2.
Informally, if that node is error-active (which should be the usual case) it will transmit an error flag of 6 dominant bits, which all other nodes will also detect (as a stuff error). This has the effect of destroying that message completely:
- no-one will receive it
- none of the transmitters will think they have successfully transmitted it.
Each transmitter will then attempt to retransmit - depending on the precise timing of the retransmissions, one may start sufficiently before the other the gain control of the bus. Otherwise, the same sequence may happen again. (Or another higher-priority message may put them both off for a while!)