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MX Foundation 4
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MX Foundation 4
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CAN is a multi-master serial bus standard for connecting Electronic Control Units [ECUs] also known as nodes. Two or more nodes are required on the CAN network to communicate.
All nodes are connected to each other through a two wire bus. The wires are 120 Ω nominal twisted pair.
ISO 11898-2, also called high speed CAN, uses a linear bus terminated at each end with 120 Ω resistors.
High speed CAN signalling drives the CAN high wire towards 5V and the CAN low wire towards 0V when transmitting a dominant (0), and does not drive either wire when transmitting a recessive (1). The dominant differential voltage is a nominal 2V. The termination resistor passively returns the two wires to a nominal differential voltage of 0V. The dominant common mode voltage must be within 1.5 to 3.5V of common and the recessive common mode voltage must be within +/-12 of common.
Each node is able to send and receive messages, but not simultaneously. A message or Frame consists primarily of the ID (identifier), which represents the priority of the message, and up to eight data bytes (classical CAN) or sixty four data bytes (CAN FD). A CRC, acknowledge slot [ACK] and other overhead are also part of the message.
CAN data transmission uses a lossless bitwise arbitration method of contention resolution. This arbitration method requires all nodes on the CAN network to be synchronized to sample every bit on the CAN network at the same time.
The CAN specifications use the terms "dominant" bits and "recessive" bits where dominant is a logical 0 (actively driven to a voltage by the transmitter) and recessive is a logical 1 (passively returned to a voltage by a resistor). The idle state is represented by the recessive level (Logical 1). If one node transmits a dominant bit and another node transmits a recessive bit then there is a collision and the dominant bit "wins". This means there is no delay to the higher-priority message, and the node transmitting the lower priority message automatically attempts to re-transmit six bit clocks after the end of the dominant message.
A CAN network can be configured to work with two different message (or "frame") formats: the standard or base frame format (described in CAN 2.0 A and CAN 2.0 B), and the extended frame format (only described by CAN 2.0 B). The only difference between the two formats is that the "CAN base frame" supports a length of 11 bits for the identifier, and the "CAN extended frame" supports a length of 29 bits for the identifier, made up of the 11-bit identifier ("base identifier") and an 18-bit extension ("identifier extension"). The distinction between CAN base frame format and CAN extended frame format is made by using the IDE bit, which is transmitted as dominant in case of an 11-bit frame, and transmitted as recessive in case of a 29-bit frame. CAN controllers that support extended frame format messages are also able to send and receive messages in CAN base frame format. All frames begin with a start-of-frame (SOF) bit that denotes the start of the frame transmission.
CAN has four frame types:
MAX Technologies CAN bus interface is compatible with ARINC 825 and ISO 11898-5 standards and also compliant with CAN 2.0A and CAN 2.0B Specification controllers.
