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SAE TECHNICAL

PAPER SERIES970297

Body Electronics Area Network (BEAN)
Hiroshi Honda, Shigeru Uehara, Kazunori Sakai, and Takao Akatsuka
Toyota Motor Corp.

Susumu Akiyama
Denso Corp.

Reprinted from: Multiplexing
(SP-1224)

International Congress
Detroit, & Exposition
Michigan February

24-27, 1997 1997

400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A.

Tel: (412)776-4841

Fax:(412)776-5760

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970297

Body Electronics Area Network (BEAN)
Hiroshi Honda, Shigeru Uehara, Kazunori Sakai, and Takao Akatsuka
Toyota Motor Corp.

Susumu Akiyama
Denso Corp.
Copyright 1997 Society of Automotive Engineers, Inc.

incorporates a jam protection control feature as well
as remote power window control. Moreover, the door

ABSTRACT

lock control system is linked to the airbag system for
unlock control. Figure 1 shows the data to be
multiplexed in a body control system built into a
passenger car. Switch data has been the main data

This paper describes the multiplex
communication protocol, BEAN (Body Electronics

Area Network), developed for body control system on

of the conventional body system, but the use of

passenger cars which in recent years has increased
the scope of multiplex communication. BEAN is
based on a protocol developed in 1992 (SAE920231)
but expands upon the performance in areas, such as
the suitability of the ID system for increase of ECUs ,
the variable data length enabling the transmission of
diagnostic data, and the transmission rate, while
keeping the cost and radiation noise level low. The
software size of BEAN is compact enough to be
implemented by general purpose 8bit MCUs which
have recently seen improvements in performance.

control data, like vehicle speed, cooling system fluid
temperature and ambient temperature are also
increasing in order to reduce the wire harness size

and to share data among different ECUs. Though
the data is not transmitted frequently, it is still several
bytes in length.

The BEAN communication devices are available

corresponding to the scale of the application and
configuration of the ECU taking into account the
software capability. This protocol was evaluated
using simulation with the body control system on
luxury passenger cars.
INTRODUCTION

In 1992, we developed a simple and low cost

multiplex communication protocol for the body
control system. This system was aimed mainly at
door control systems such as the power window and
door lock control and

has been used

in mass

Figure 2 shows an example of a large scale

production on the LEXUS LS400 since 1995.
Recently, the number of functions available

body control system. For the servicing of such large
systems, diagnostic capability is required.
However, each ECU implementing the diagnostic
interface circuit (IS09141) would result in an
increase in cost of the system. To solve this
problem, it is more efficient that one ECU has the

in the electronic system for the body control has been
increasing, and it has become necessary to enlarge
the area with multiplexing to reduce wire harness
volume. For example, the power window function

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interface circuit and function as a gateway for all
multiplex transmission to other ECUs via the
multiplex bus. Therefore, it is necessary to be able to

4.Over twenty nodes with two hundred possible
messages.

5.Protocol is compact enough to be realized by a
general purpose 8bit MCU.
6.Power saving function to reduce power

handle such complex data.
Under these circumstances, the amount and
variety of the data to be transmitted is also
increasing. In the body electronics area, optional

consumption when the ignition key is off.

7.Ability to transmit diagnostic messages.

functions frequently change and cost is a strong
consideration.

The protocol developed for the LS400 does
not have enough expandability, but is able to achieve
reduction in cost. Since the body control system is
severely limited by cost and ECU size, we

We believe that the selection of an

optimal network system is needed, and so, we

initiated the development of a protocol with both high
performance and flexibility that keeps cost at a

determined

minimum.

that

it

was

best

to

enhance

the

communication ability of the Toyota protocol to attain
both low cost and expandability.

This paper summarizes the requirements of
the protocol and describes the specifications of the
protocol that was developed for the body electronics
system (BEAN). Next, we present some actual
examples of the data communication devices and
tools necessary for the development of the system.
Then, using the LEXUS LS400 as a luxury car

SPECIFICATIONS FOR BEAN PROTOCOL

In this section, the physical layer and data
link layer, the primary features of BEAN, are
discussed.

example, a case study of the BEAN application is
conducted. Finally, the future issues of building an
in-vehicle network for the body system are

PHYSICAL LAYER - The specifications
taking the adaptability to passenger cars into
consideration for the physical layer of BEAN are as

discussed.

follows:

1.

Low electromagnetic radiation noise

2.CSMA/CD method

3.Non-shielded single wire
4.Resistance to ground offset among ECUs
5.Resistance against surge noise
6.Larger clock tolerance
Low Electromagnetic Radiation Noise Rec ntly,
the use of the glass printed antenna for the radio
has been increasing. Because this antenna, however,
is easily influenced by noise as compared with
the conventional rod antenna, it is vital that we limit
the electromagnetic radiation noise generated from
electronic components. In the bus interface circuit
adopted for LS400, the wave form output to the
communication line was controlled in the form of a

trapezoid to reduce the noise. The bus interface circuit
is shown in Figure 3. 20

REQUIREMENTS FOR THE NETWORK OF BODY
ELECTRONICS SYSTEM

The requirements for the network of body
electronics system are summarized as follows:
1. Maximum transmission rate using a single wire.
2.Data transmission interface which is designed at
a low cost.

3.Low noise level that does not affect in-vehicle

systems.

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Now, we confirmed the radiation noise level

with a similar control method by changing the
transmission rate to 10kbps. (We chose NRZ as the
bit encoding method and adopted bit stuffing to
increase the clock tolerance. ) However, as a
result, the noise

increased largely because the

slope of the wave form edge had to be increased in
accordance with the transmission rate. (Figure 4)
Accordingly, we newly developed a new
current control type driver which could achieve a low

noise level with a transmission rate of 10kbps. The
details of this driver will be explained in the next
section.

The terminating circuit consists of a constant
current source working as current limiting circuit.

When the output current of the driver begins to drain
current greater than the limit of the terminating
circuit, the voltage of the bus is high, and when it
begins to drain a current less than the limit of the

terminating circuit, the voltage of the bus is low.

Current Control Type Bus Interface Circuit

The block diagram of Figure 5 shows the structure of
the driver/receiver corresponding to a non-shielded
single wire, transmission rate of 10kbps and NRZ
encoding method.
The driver/receiver gradually outputs current
to

the bus in order to reduce the radiation noise.

The voltage wave form is quickly changed to stably
receive the data. The functions of the circuit are
described as follows and the wave form is shown in

Figure 6.
The driver consists of an integrator circuit
and voltage-current conversion circuit.
The terminating circuit functions as a constant
current source.

When the output (VTx) of the communication
IC shown in Figure 5 changes, the integrator circuit
changes the output (VIC) voltage gradually. The
output signal of integrator circuit is connected to the
voltage-current conversion circuit. The voltage
current conversion circuit gradually changes the
output current (Ivc) according to the output wave of
the integrator circuit. In this way, the driver outputs
current to the BUS. The voltage of the BUS (VBUS)
is shown in Figure 6.

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A bus interface circuit that meets the above

nodes, also make it advntageous in the body

specifications is shown in Figure 7. Although this

control system. Also,

circuit has a single wire construction, the
electromagnetic radiation noise has been reduced.

to ef iciently transmit the various application
signals (i.e. switch signals, diagnostic data),
it is neces ary to make the length of data area variable.

The wave form and the noise level is shown in

Figure 8.

In

a multiplex system, external noise may cause
communication

errors. In

this protocol, 9 types

of error check methods, such as checking of the
frame format in ad it on to the error check by CRC,
are defined to ensure the reliability of the data. If
an error occurs, the data is automaticaly resent up to
three times to prevent the omission of data. Outline
of data link layer -The outline of data link

layer for the newly developed in-vehicle network is
described. Table 1 shows the primary specifications.
Figure 9 shows the communication frame
format. The

DATA LINK LAYER - The data link layer is a
very important section that plays a large part in
determining the system performance and greatly
affects

the

cost

of

the

communication

IC.

Optimization of the data link layer of the protocol is
studied by taking into consideration the requirements
described in the previous section and the scale of the
communication device.

Optimization Of The Data Link Layer In
the in-vehicle network of the body control system, major

communication data is trig ered by the operation
of a driver. Therefore, the most efficient transmission

method is the CSMA/CD method, suited
to signals with fewer periodical properties. The
system's drawback, namely its inability to
ensure the data delay time except for messages which
have the highest priority, may be neglected in a

body control system. It

is improbable that two or more

transmission requests originate at the same time,
because the probability that a driver and a pas enger

operate switches at the exact same time is
considered to be an extremely rare case (most of the

signals are triggered by switch operations). Features
of

the CSMA/CD,

such

as

better

expandability

bit stuf ing rule is employed - an inverted bit

and capability of easily adding or deleting

is inserted when 5 consecutive bits have the same value

in the period from SOF to CRC. Broadcast type 22

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messages are available in addition to the point-to
point type messages, and the communication
method is switched by the DST-ID. When the
DST-ID is FFH, the communication is set to the
broadcasting method. If the DST-ID contains a
value other than FFH, that value represents the
destination ID. The length of the data area is
variable up to 11 bytes which allows the packaging of
other protocol messages into the data area. (It is
possible to include 3bytes of header and 8bytes of
data.)

BODY CONTROL - The body control microcomputer

Wake-up/Sleep Control - Some ECUs of the

high speed, low power microcomputer designed for

EXAMPLE USING ORIGINAL MCU FOR

(B05-microcomputer) used for BEAN is an 8-bit

body control system are required to function even
when the ignition key is set at OFF. To prevent
draining the battery, a function to enter the power
saving mode when operation of the ECU is not
needed and to return the normal operation by
detecting voltage changes on the transmission line is
provided.

in-vehicle use, with built-in communication hardware

that

describes a

the

BEAN

protocol

using

Using an original dual-task MCU, each task

functions

alternately based on the time sharing,

and processes 2 instructions at 0.5 ìs intervals when

the external frequency is set at 8 MHz (internal
frequency is 4 MHz). This enables high speed

EXAMPLE OF BEAN DEVICE
This section

implements

customized communication software.

processing because 96% of the available 48
instructions are executed in 1 cycle. The
communication software is processed only in the A

communication

task (one task of the dual-task) and the L task (the

device (data link controller) which enables
communication with the BEAN protocol explained
in the previous sections.

other task) is not influenced by the communication.
Therefore, the L task can be programmed for the I/O

controller such as the input/output process or error

EXAMPLE USING GENERAL MCU - Using
a general single chip microprocessor, all
communication is controlled by the software. This
is applicable for small scale ECUs and is a small load
on control programs.
Advantages of this system are shortened
development time and ease of modification for

process, without worrying about the communication
process overhead(Figure 11). Features are as
follows:

1.The application program can be created easily,
independent of the BEAN protocol
2.Simplification of peripheral circuits (Figure 12)
3.Built-in malfunction prevention (I.e. watch dog
timer)
4.Low power consumption

changes in the protocol. For example, the load on the

MCU is 61% on average using assembly language
when BEAN is built using a Hitachi H8 MCU. (Where
the clock frequency is 8MHz.)
EXAMPLE USING CUSTOM

COMMUNICATION IC - Wired logic circuits
are
achieved by using the custom communication IC

independent of the MCU. This system can be used by
any MCU without limitation. The advantage of this
system is that the system reduces the load on the
host MCU. Figure 10 shows a photo of the IC chip.
Table 2 shows the primary specifications.

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BEAN DEVELOPMENT TOOLS

The multiplex transmission protocol has the
advantages of simplified wire harnesses. However,
as the scale of the system becomes larger,
development and evaluation becomes more difficult.
The development tools corresponding to each
development phase - such as the simulation prior to

CASE STUDY USING LS400

available.

This section describes the BEAN case study
using the LEXUS LS400 which has a complex large
scale body electronic system. Figure 2 shows the
configuration of the system. The number of nodes
connected to the body control network is 15.

The primary functions of the development
tools are explained below:

the structure of the network is shown.

development, prototype modeling

of each ECU,

functional checks of all ECUs, and confirmation of
functions an actual vehicle - are needed.

For these

purposes, development tools called "LAN testers" are

STRUCTURE OF THE NETWORK - First,

Collection, recording, and display of

2.
3.
4.

Detection and display of errors.
Specified data sent at a specified timing.
Specified collection data such as switch data
output from ports in real time.
Analog signals (i.e. the vehicle speed, water
temperature) output in real time.

communication bus data.

5.

fail-safe into consideration.

Multiplex Transmission Signals - Multiplex
transmission signals are selected as follows by taking
into
consideration
the
reliability
of the
communication:

Signals necessary for the control of a node,
even during bus failure, are inputted directly by the

Functions 3 to 5 are useful for the functional

check of a single ECU or used as an alternative ECU.

node and then transmitted to the other nodes.

Functions 4 and 5 are useful for monitoring the signal

Figure 13 shows a custom LAN tester. Figure 14
card(PCMCIA

type2)

For

example, the vehicle speed signal is calculated by
the combination meter ECU and the ignition key start

of sensors and switches on an actual vehicle.

shows the PC
development tool.

Functions

restricted by the protocol, such as fault tolerance are
performed within the respective application taking

1.

position is detected by the engine ECU and then

type

both signals are transmitted through the network by
the respective ECU.
In the body system, there are many signals
suitable for asynchronous event triggering. If an error
occurs in the message, some of the data will be lost.
To solve this problem, signals like the warning signal
to the combination meter ECU is not only sent by
event transmission, but also by periodical
transmission.

Communication Method - Each node uses

messages corresponding to the vehicle mode - the
ignition key is set at OFF, Accessory, ON, or
Diagnosis. Each node basically sends the data
received from sensors and switches to the network.

For example, when all sensor data necessary for
control is read by an ECU, (i.e. engine ECU), Data

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such as the RPM of the engine and water
temperature is sent to the network during the
periodical transmission only when the ignition key is
set in the ON position even though the amount of the

performance of the CSMA/CD system, the interval of
transmission is optimized based on a average bus
utilization target of 40% or less. In order to verify the
performance of the network, the following

data received from the body control system is small.

simulations were performed.

To efficiently design the wire harness system, signals

Simulation System - The configuration of the
simulation system is shown in Figure 15. The
details of the simulation system are contained in the
SAE paper 910463 and will not be described in this

around the nodes are inputted to the ECU to send
them to the network, even though they may not be
needed for control. For example, the engine ECU

reads the hydraulic pressure switch of the engine and

paper. The simulation system was designed to be

sends the data to the instrument cluster ECU.

capable of evaluating both the entire communication

Bus Reliability - Since this protocol's goal is
reduce the cost of the physical layer, the following is

line as well as processing of individual signals.
Items which can be evaluated by the system are as

taken into consideration on the vehicle side to

follows:

improve the bus reliability. The bus is connected In

daisy

chain

configuration

to

ensure

1.

Signal base evaluation: Number of data
originally produced, number of collisions,
number of data discarded, delay time.
2.Node base evaluation: Number of data sent,
Amount of data sent, Average data sending

safe

communication even if a wire in the system is

broken. Multiple bus terminating circuits are mounted
in the network to prevent the entire system from
malfunctioning even if a terminating circuit fails.

time.

Gateway For Diagnosis Data - In Figure 2,

3.Communication system base evaluation: Total
size of messages, Total data discarded.

the engine ECU and body ECU are nodes that use
the IS09141

interface for communicating with

diagnostic tools. The other nodes are connected to
the diagnostic tools through BEAN. The messages
specially designed for diagnosis is set on each node
beforehand and is sent only when In the diagnosis
mode.

The diagnostic message is placed into the
DATA area of BEAN by the body ECU. Since this

message is sent via the broadcast method, other
ECUs automatically read the message and confirm
that the diagnosis request has been sent. If there is
diagnostic data, the ECU returns to the body ECU a
message to transmit the diagnostic data to the tool.
When the body ECU determines that the MES-ID is

for diagnostic data, the body ECU puts the DATA

Result Of Evaluation - According to the

area into a diagnostic message to be sent to the tool.
Thus, the body ECU is able to transmit the diagnosis

results of the

message from other nodes without reading its
CAPACITY

and

simulation,

the

verified as follows:

contents, resulting in a reduction in the load on the
body ECU.
COMMUNICATION

evaluation

performance of the communication protocol was
1.Delay time: Fig. 16 shows the delay time
distribution of all passenger seat window UP/DOWN
signals which have the highest priority and the

AND

SIMULATION - Using the model shown in Figure 2,

vehicle status signal which has the lowest priority.
The delay time refers to the interval between

the volume of communication data shown in Table 3
is needed.

occurrence

of the

transmission

data

and

the

completion of its reception. For the highest priority,
the delay time is 20 ms or less.
2.Bus utilization: The bus utilization is an index

representing the rate of time when a message

(including headers and error check codes) is on the

communication line.
The bus utilization
calculated based on the formula below:

Since short interval periodic data may greatly

affect the communication capacity, to insure the

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Table4

shows

the

results

of

the

communication line evaluation, including bus
utilization. According to this table, BEAN has
sufficient performance for the body communication

FUTURE ISSUE OF THE BODY CONTROL
NETWORK

system, even at a transmission rate of 10 kbps.

The following subjects will arise as the use of
in-vehicle body control network increases In the
future.
EXPANSION

OF

COMMUNICATION

CAPACITY - It will be necessary to consider the
expansion of the communication capacity as the
communication items and communication frequency
increase.

Verification - As explained above, according

One method to meet this is to increase

the transmission rate. However, to achieve this, it will

to the simulation, we confirmed that the proposed
communication protocol satisfies the requirements

be necessary to develop faster communication
devices to avoid increasing the cost of the ECU.

for the body communications system and shows

Another method

sufficient performance with regard to timing and
reliability.

is to

reduce the

amount of

transmission data for each block by dividing the bus.
For this case, it will be necessary to reduce the
burden on the gateway by dividing the bus at points
where the amount of transmission data is small.

IMPROVEMENT OF FAILSAFE - Currently,
the body control system network handles only signals
that do not affect control even if communication were

to fail. As the number of systems increases, the
signal communication range is enlarged while

improving the redundancy of the communication
lines. Additionally, depending on the type of
vehicle, the wire harnesses may need to be run
through an area where it may be exposed to a higher
noise level. To solve this, it may be necessary to
use wire harnesses which have a higher noise

resistance.

As a result, various physical layers,

including a faster transmission rate, will be needed in

the future.

CONNECTION TO OTHER LAN - In the

near future,

several

types

of

LAN

will

be

implemented in vehicles. Examples are shown
below:

1.High speed control system bus for the power
train and chassis control.

2.Low cost, low speed, flexible body
system bus.
3.High speed, large capacity multimedia
bus to transmit visual images and voice
4.Standard diagnostic bus for use with

control
system
data.
service

tools.
Since each LAN has different features and

each vehicle has different equipment, we don't
believe it is a good idea to integrated all in-vehicle
LANs into a single LAN. The in-vehicle network will

be built by using several LANs that take advantage of

the benefits of each LAN.

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CONCLUSION

We have developed a new protocol that is
applicable for large scale network systems, that can
be implemented by the software program of a
general single chip microprocessor. So engineers are
free to select communication devices according to

their specific applications. We also developed the
physical layer for a high data transmission rate and
low radiation noise. Case studies using the LS400

were conducted and excellent results concerning the
bus utilization and data delay time were obtained.
Additionally, the effect of wire harness reduction was
calculated, and we found that approximately 15% of
the wire harness could be reduced. As the body
control system continues to change and evolve ,

while there is increasing pressure to reduce cost , the
ability to select an optimal system that can meet all
our design needs will be a vital issue to be tackled in
the future.
ACKNOWLEDGMENTS
The authors would like to thank the members

of the BEAN developing activity for their valuable
assistance and cooperation.
REFERENCES

[1] Seiji Nakamura, Toshiaki lsobe, Yuuji,
Hirabayasi, "The High-Speed In-Vehicle Network of
Integrated Control System for Vehicle Dynamics",
SAE Paper 910463

[2] Toshiaki lsobe, Hiroshi Honda, Shigeru
Uehara, Susumu Akiyama, "A Low-Speed In-Vehicle
Network for Body Electronics", SAE Paper 920231

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