BASIC OF MPX (Multiplex Communication)

The PRIUS multiplex communication system uses serial communication, which converts multiple pieces of information into serial communication data. As a result, they can be transmitted through a single communication wire.

Conceptual Drawing 

Fig 1: Serial Communication Wiring Diagram

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• Serial communication data consists of bits and frames. A bit is the basic unit that represents the amount of information. A bit is represented by binary values "0" or "1". A frame is a body of data that is transmitted together. A frame contains a "header" that indicates the beginning of the data and an "end" message that indicates the end of the data.
  Fig 2: Identifying Frame Containing Header Indicates Beginning Of Data & End Message Indicates End Of Data

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Based on serial communication, various ECUs are connected on a network to exchange various pieces of information. Such a system is called a multiplex communication system.

• The CAN communication on the PRIUS uses the multi star style bus connection. In this style of network, the ECUs are connected in a star shape. The ECUs cannot establish communication with other ECUs without passing through the master ECUs. For the Prius, the 2 junction connectors are the master ECUs and they are connected with main bus lines. Each of the junction connectors extends sub bus lines and communicates between all the connected ECUs.
  Fig 3: CAN Communication Block Diagram

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• The BEAN on the PRIUS uses the ring and bus styles of networks to connect ECUs. The connection method is called a daisy chain. In a daisy chain, communication can be maintained even if there is an area that has an open circuit. The ECUs are connected in a ring form, which enables a signal that is output by a transmitting ECU to circle the ring and return to its original ECU. This ring also functions as a single common communication wire. The transmitting ECUs output signals through a common communication wire and the receiving ECUs input data through a common communication wire.
  Fig 4: BEAN Communication Block Diagram

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• The AVC-LAN in the PRIUS uses a star style. This style is centered on a master ECU, which holds a central control function. The ECUs are connected in a star shape. The ECUs cannot establish communication with other ECUs without passing through the master ECU. For the Prius, the multi display or audio head unit functions as a master ECU.
  Fig 5: AVC-LAN Communication Block Diagram

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Data for respective protocols (CAN, BEAN and AVC-LAN) are arranged differently. Therefore, it is not possible to exchange data between different protocols. For this reason, data arrangement must be converted before transferring data to communication bus (CAN, BEAN and AVC-LAN). This conversion is called gateway function, which is performed by the gateway ECU.

*: "bps" stands for bit per second.

Although BEAN and AVC-LAN communicate at almost the same speed, the CAN communication speed is much faster than BEAN or AVC-LAN. When the vehicle control system uses BEAN or AVC-LAN, which have a slower communication speed than CAN, it means that the system control could be delayed by the slow speed. For this reason, the vehicle control system uses CAN, which can send and receive a large volume of data at one time, in addition to being able to transmit at a faster speed.

A single, automobile vinyl (AV) wire is used for BEAN communication. A twisted-pair wire is used for CAN and AVC-LAN communication.

Fig 6: Communication Wire Chart

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