Technician Handbook 673 Electronic & Computer Controlled Systems Section Topics Overview of Multiplex Communication Technical Training • Why Use Multiplexing? • How ECUs Communicate • Communication Protocols • Multiplex Topology • Single Wire vs Twisted Pair 23 673 Electronic & Computer Controlled Systems Technician Handbook Why Use Multiplexing? One multiplex circuit does the work of many conventional circuits • • • • • • • Fewer wires Lighter wiring harnesses Simpler, more reliable wiring Fewer components Fewer connections Lower cost Self diagnostics Applications of Multiplexing Multiplexing (or MPX) is a method for communicating between multiple components over a single one-wire or two-wire communication line Without multiplexing, inter-module communication requires dedicated, pointto-point wiring between all components resulting in bulky, expensive, complex, and difficult-to- install wiring harnesses Using multiplexing reduces the number of wires by combining many signals on a single wire Control modules use the data received to control functions such as anti-lock braking, turn signals, power windows, dashboard displays, and audio systems Benefits of Multiplexing In-vehicle networking provides a number of benefits: • Each function requires fewer dedicated wires, reducing the size of the wiring harness This yields improvements in system cost, weight, reliability, serviceability, and installation cost • Common sensor data, such as vehicle speed, engine temperature, etc are available on the network, so data can be shared, thus eliminating the need for redundant sensors or multiple connecting wires • Networking allows greater vehicle flexibility because functions can be added through software changes in the ECU Without multiplexing, systems require an additional module or additional terminals for each function added 24 Technical Training Technician Handbook 673 Electronic & Computer Controlled Systems What is Multiplexing (MPX)? Multiplexing is a way to use one wire to communicate between many devices Conventional wiring between components Light Motor Heater Solenoid Switch Discrete signals L M H Light S Motor ECU MPX communication line ECU Heater Solenoid Switch Multiplexing In conventional electrical circuits, each voltage signal between components requires its own dedicated wire The presence, absence, or amount of voltage on the wire (supplied by a switch or a sensor, for example) controls the operation of a component on the other end In a multiplex circuit, a computer chip on one end of a single wire can transmit a series of coded voltage signals that can be interpreted by a computer chip on the other end The computer chips are inside electronic control units (ECUs), and the coded voltage signals are data packets A data packet may instruct the receiving ECU to: • Turn on a light • Start a power window motor • Activate a solenoid Because the data packets are sent in series, multiplexing is also referred to as serial communication or serial networking, and the communication line is called a serial data bus Technical Training 25 Technician Handbook 673 Electronic & Computer Controlled Systems How ECUs Communicate ECUs communicate by sending voltage pulses in a coded sequence ECU Logic Circuit: • Controls the ON/OFF signal • “Reads” the data on the MPX line • Performs self diagnosis ECU To communication line: • Supply voltage when transistor is OFF • Ground voltage when transistor is ON Voltage Supply 0v Time ECU Communication In the ECU, a switching transistor in the logic circuit controls the transmitting of multiplex signals When the transistor is OFF, no current flows Referring to the diagram above, if you were to measure the available voltage on the communication line, you would find supplied voltage When the transistor is turned ON, current flows and all of the available voltage is dropped across the resistor Now the voltage measurement on the communication line (after the resistor) is ground voltage By turning the transistor ON and OFF in a timed sequence, the ECU can send a message to another ECU, similar to sending a message in Morse code Part of the message, called a data packet, indicates which ECU the message is addressed to Other ECUs listening to these messages ignore the ones not intended for them NOTE 26 The ECU communication line is powered through a resistor that acts as a load in the circuit This is commonly called a pull-up resistor If the circuit is grounded, the resistor protects the ECU from damage Technical Training Technician Handbook 673 Electronic & Computer Controlled Systems Signaling Between ECUs When one ECU signals another, the one sending the signal is not necessarily the one supplying the power to the circuit Sender Supplies B+ ECU ECU 12V Sends signal Receiver Supplies B+ ECU ECU 12V Sends signal Signaling Between ECUs Technical Training In diagnosing ECU controlled circuits, don’t make the assumption that the ECU sending a signal is the one supplying the circuit voltage As shown in the illustrations above, it’s possible for the ECU receiving a signal to be the one providing power to the circuit 27 Technician Handbook 673 Electronic & Computer Controlled Systems Communication Protocols A “protocol” is the set of rules and standards for communication between components Protocol BEAN (TOYOTA Original) CAN (ISO Standard) LIN (Consortium) AVC-LAN (TOYOTA Original) Application Body Electrical Power Train Body Electrical Audio Communication Speed 10 kbps 500 kbps (HS)* 250 kbps (MS) 20 kbps 17.8 kbps AV Single Wire Twisted-pair wire AV Single Wire Twisted-pair wire Drive Type Single Wire Voltage Drive Differential Voltage Drive Single Wire Voltage Drive Differential Voltage Drive Voltage 10+ volts 2.5v to 3.5v CANH 2.5v to 1.5v CANL volts 2v to 3v TX+ 2v to 3v TX- Configuration Ring/Daisy Chain Bus Star Star Sleep/Wake-up Available Available Available N.A Communication Wire BEAN: Body Electronics Area Network * Up to Mbps CAN: Controller Area Network LIN: Local Interconnect Network AVC-LAN: Audio Visual Communication - Local Area Network Communication Protocols The rules and standards for transmitting and receiving data packets between ECUs are called a protocol Some protocols provide faster exchange of messages between components and more reliable operation than others As speed and reliability increases, so does the cost The chart above compares some of the characteristics of the different protocols found in Toyota vehicles • BEAN is the earliest protocol used by Toyota Based on early technology, it is also one of the slowest protocols BEAN is typically used for body electrical systems such as lights, locks, windows, and air conditioning • AVC-LAN is another early protocol developed by Toyota as a faster alternative to BEAN for audio, video, and navigation components • CAN, the ISO standard for automotive applications, is a high-speed protocol for critical vehicle systems such as engine control, braking, precollision, and SRS systems • LIN is an alternate, low-speed standard protocol developed in later years and used by many manufacturers Because it is a common standard, it is slightly lower in cost, and because it is a newer standard, it is slightly faster than BEAN In later model Toyota vehicles, LIN replaces BEAN for control of some body electrical systems such as windows and seats NOTE 28 Network speeds are measured in bits per second (bps) A “bit” (represented as ON or OFF, or or 1) is the smallest unit of the code used in a data packet Kbps stands for kilobits (1000 bits) per second Mbps stands for megabits (one million bits) per second Technical Training Technician Handbook 673 Electronic & Computer Controlled Systems Multiplex Topology Bus Style All ECUs are connected to a single common communication line Daisy Chain Style The ECUs are connected in a combination ring and bus form ECU ECU ECU ECU ECU ECU ECU ECU ECU ECU ECU Applies to CAN ECU Applies to LIN and AVC-LAN ECU Master ECU Applies to BEAN ECU ECU Star Style Each ECU is connected directly to a master ECU with a central control function Multiplex Topology Topology describes the pattern of physical connections between components on a network This may also be called network architecture Multiplex networks can be configured in a variety of designs Toyota networks are arranged using primarily three styles: the bus, the ring, and the star • Bus In the bus style, multiple ECUs are connected to a single common communication line, allowing each ECU to transmit or receive signals directly with any other ECU on the network • Ring ECUs connected in a ring have two network lines to provide a backup path for communication If one communication line is disconnected, the ECU can still receive network communications on the other line • Star The star style uses a central ECU called a master to control the other ECUs in the network (slaves) In this configuration, slaves cannot communicate directly with one another without passing the message through the master • Daisy Chain Sometimes a multiplex circuit can combine two design types An example is a BEAN circuit with both ring and bus topologies NOTE Technical Training Components on a network are referred to as nodes ECUs are not the only possible nodes Sensors with multiplex communication capability can also be nodes on a network Examples are steering angle sensors and yaw rate sensors 29 Technician Handbook 673 Electronic & Computer Controlled Systems Ring Topology In a ring network, a single open circuit in the loop does not affect performance Communication lines (bus) One open wire does not affect network operation Ring Topology 30 When network components are connected in a ring, every component has two paths for sending messages to another component The advantage of ring topology is added reliability because the network continues to operate normally in the event of an open wire anywhere in the multiplex circuit Technical Training Technician Handbook 673 Electronic & Computer Controlled Systems Ring Topology Two open connections in a ring network isolates part of the multiplex circuit and sets a DTC Communication lines (bus) Two Opens in a Ring Network When one ECU sends data to another, the receiving ECU typically sends back a message that it received the data When a ring network experiences two open wires in the ring, one or more of the ECUs in the network become isolated from the others An isolated ECU does not receive messages and cannot acknowledge them The lack of response from an ECU may cause a diagnostic trouble code (DTC) to be set By studying the network topology and identifying the location of the unresponsive ECUs, you can determine which legs of the circuit contain the open wires Technical Training 31 673 Electronic & Computer Controlled Systems Technician Handbook Star Topology A single open in a star network isolates only one component Master ECU Open in a Star Network 32 In a star network, the master ECU has a separate communication line to each of the other ECUs An open in any connection affects only one ECU and does not affect the entire network Technical Training 673 Electronic & Computer Controlled Systems Technician Handbook Bus Topology The effect of an open in a bus network depends on the location An open on the main bus line isolates part of the network An open on a sub bus (branch line) isolates only the component on that branch Open in a Bus Network In a bus network, each ECU is connected to a common communication line called the main bus An open in the main bus divides the network into two segments The ECUs that are still connected together in one segment can communicate among themselves but cannot communicate with ECUs in the other segment The connection between an ECU and the main bus is called a sub bus (or branch line) An open in the sub bus isolates only the ECU on that branch Technical Training 33 Technician Handbook 673 Electronic & Computer Controlled Systems Single Wire vs Twisted Pair Communication Wire Feature Twisted-pair Wire This communication line is a pair of twisted wires Communication occurs by applying Hi or positive (+) and Lo or negative (-) voltages to the two lines in order to send a signal (Differential Voltage Drive) AV Single Wire This communication wire is thin and lightweight compared with the Twisted-pair Wire Voltage is applied to this line in order to drive the communication (Single Wire Voltage Drive) for BEAN, LIN, etc Differential Voltage Drive Hi Single Wire Voltage Drive Hi ECU ECU Lo Single Wire vs Twisted-Pair ECU ECU Lo Communication over a multiplex line consists of a series of voltage pulses that form a pattern of bits interpreted as data by the receiving ECU In a typical multiplex system, the voltage pulses are carried over a single wire In some multiplex systems (CAN and AVC-LAN for example), a pair of twisted wires carry matching pulses—one positive and one negative This method reduces electromagnetic interference or noise and is more reliable in circuits requiring a greater degree of transmission reliability For additional reliability and protection from voltage being induced by nearby wiring, some systems use twisted-pair wires with added shielding (AVCLAN, for example) 34 Technical Training Technician Handbook 673 Electronic & Computer Controlled Systems Advantage of Twisted-Pair Wiring Single Wire Voltage Drive Differential Voltage Drive 3.5 V 4.0 V 2.5 V 0V 1.5 V Data 1 Data 1 Noise Noise ? Abnormality Advantage of Twisted-Pair Wiring Cancel Each Other 1 Electromagnetic interference from nearby wiring can induce unexpected voltage spikes (noise) in a multiplex communication line which alters the coded data being transmitted The receiving ECU has a way of detecting the data has been altered, but it then has to send a request to the sending ECU to retransmit the data This slows down communication between the ECUs To keep high-speed networks operating at high speed, twisted-pair wires provide protection from induced noise When a network that is wired with twisted pair wiring experiences noise, the abnormality affects each wire in the same way, so the effect of the interference is cancelled out Technical Training 35 673 Electronic & Computer Controlled Systems Technician Handbook This Page Intentionally Left Blank 36 Technical Training ... diagnostics Applications of Multiplexing Multiplexing (or MPX) is a method for communicating between multiple components over a single one-wire or two-wire communication line Without multiplexing, inter-module... displays, and audio systems Benefits of Multiplexing In-vehicle networking provides a number of benefits: • Each function requires fewer dedicated wires, reducing the size of the wiring harness This yields... can be added through software changes in the ECU Without multiplexing, systems require an additional module or additional terminals for each function added 24 Technical Training Technician Handbook