Principle of Operation: CAN-Bus: Notes
The CAN-Bus consists of a twisted pair of wires:
- CAN High (CAN-H, yellow and black)
- CAN Low (CAN-L, yellow and brown).
Both Can-H and CAN-L have a standing voltage of 2.5 volts. CAN-L is pulled low to approximately 1.5 volts while CAN-H is pulled high to approximately 3.5 volts.
With CAN-H and CAN-L both at 2.5 volts there is no potential difference (voltage) between them. This is known as the recessive state and is equivalent to logic 1. With CAN-H pulled to 3.5 volts and CAN-L pulled to 1.5 volts there is a potential difference of 2 volts between them. This is known as the dominant state and is equivalent to logic 0. CAN-H and CAN-L always switch together. These two states (0 or 1) are the only two possible.
When a control unit transmits a signal, it is made up of a series of dominant and recessive states. The signal is a combination of the two possible states, in effect 0 and 1, hence a digital signal.
For correct operation CAN-Bus must be terminated at both ends (EMS 2000 and IKE) with a control unit resistance of 120 ohms, connected between CAN-H and CAN-L. These terminations ensure that bit errors due to signal reflections are avoided.
Data transmitted from any subscriber on a CAN-Bus does not contain addresses. Instead the content of the message (RPM, TD, Temp, etc) is labeled with an identifier code unique throughout the CAN. All of the subscribers receive the message and each one checks the message to see if it is relevant to its own operation.
If the message is relevant then it will be processed, if not, it will be ignored. The identifier code also determines the priority of the message. In a case where two control units attempt to send a message over a free bus line, the message with the higher priority will be transmitted first. The protocol of the CAN ensures that no message is lost, but stored by the responsible control unit and then re-transmitted later when it is possible.