Toyota training overview of multiplexing

Technician Handbook

673 Electronic & Computer Controlled Systems

Section 2 Topics

Overview of Multiplex
Communication

Technical Training

• Why Use Multiplexing?
• How ECUs Communicate
• Communication Protocols
• Multiplex Topology
• Single Wire vs. Twisted Pair

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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.

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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.

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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.

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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.

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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

8 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 1 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 0 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.

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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.

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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.

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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.

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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.

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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.

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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).

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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

0 1 0

1

Data

0 1 0 1

Noise

Noise

0 1

0

?

Abnormality

Advantage of
Twisted-Pair Wiring

Cancel
Each
Other
0 1

0

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.

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