Mitsubishi Outlander XL. Manual — part 1340

｠

When data frames conflict with one another (when
plural ECUs transmit signals simultaneously), data
is prioritized for transmission by mediation,
therefore, plural data frames are not sent
simultaneously. For further details, consult the
mediation section below.

｠

Data is transmitted not by the conventional voltage-
using method but by voltage potential difference.

For further details, consult the section on CAN bus
voltage transformation.

｠

Reliability of each ECU transmitting signals via CAN
communication is secured by several error
detection and recovery processes. For further
details, consult the sections on error detection and
system recovery.

｠

For major communication signals (transmitting
signals) among ECUs.

MEDIATION

Because each ECU transmits data independently on
the CAN bus, there are cases of data collision when
multiple data frames that ECUs attempt to transmit
simultaneously (if multiple ECUs transmit at nearly the
same moment). At this moment, processing of the
ECUs attempting transmission is performed in the
following way.

1.

Data frame with high priority is transmitted first
according to ID codes memorized in data frames.

2.

Transmission of low-priority data (data frames) is
suspended by the issuing ECUs until the bus
clears (when no transmission data exists on the
CAN bus).

NOTE:

If the suspended state continues for a

specific time, new data (data frame content) is
created and sent.

3.

ECU containing suspended data frames transmits
the data when the bus becomes available.

NOTE:

There is enough capacity on the CAN bus,

which never prevents data frames from being sent.

VOLTAGE TRANSFORMATION ON THE CAN-B BUS and CAN-C BUS

CAN_H

3.5 V

2.5 V

1.5 V

0

1

5 V

4 V

1 V

0 V

0

1

ZC5017690000

CAN_L

<CAN-C>

<CAN-B>

Voltage
transformation
in CAN_L

State:

Logic value:

Recessive

Recessive

Dominant

Voltage
transformation
in CAN_H

Recessive

Recessive

Dominant

The voltage transformation (output signal) when the
data frame is sent to the CAN-C bus line has a
distinctive CAN-C profile. The transmitting ECU
through the CAN_H and CAN_L bus lines sends 2.5
to 3.5 volts signals to the CAN_H side and 2.5 to 1.5
volts signals to the CAN_L side. The receiving ECU
reads the data from the CAN_H and CAN_L potential
difference. "Recessive" refers to the state where both
CAN_H and CAN_L are under the 2.5 volts state, and
"Dominant" refers to the state where CAN_H is under

CONTROLLER AREA NETWORK (CAN)

54D-5

SYSTEM OPERATION

the 3.5 volts state and CAN_L is under the 1.5 volts
state. On the other hand, the voltage transformation
(output signal) when the data frame is sent to the
CAN-B bus line has a distinctive CAN-B profile. The
transmitting ECU through the CAN_H and CAN_L bus
lines sends 0 to 4 volts signals to the CAN_H side, and
1 to 5 volts signals to the CAN_L side. "Recessive" is
a state in which CAN_H is at 0 volt and CAN_L is at 5
volts, and "Dominant" is a state in which CAN_H is at

4 volts and CAN_L is at 1 volt. By transformation
mainly to 2.5 volts, even in cases when voltage is
rendered 0 from faulty grounding or the like (causing
a problem of an approximate 0.5 volt increase on the
communications line), communication can be
continued

uninterrupted.

Employing

dual

communications lines improves reliability to prevent
the presence of noise, compared to the conventional
communication method.

DATA FRAMES

ZC5017700000

SOF

EOF

ID

region

CRC

region

ACK

region

Control
region

Data region

Data frames

SOF (Start of Frame)

｠

Indicates the start of the frame

ID (Identifier) region

｠

Identifies the data content while specifying priority
rank in case of mediation

Control region

｠

Specifies the frame type, data length, etc.

Data region

｠

Values used for data control, etc.

Cyclic Redundancy Check (CRC) region

｠

Region where to check for errors in sent data. The
transmitting ECU calculates data regions by
applying prescribed operations and stores the
results. The receiving ECU detects erroneous

communication by comparing the CRC region with
the data region.

ACK (Acknowledge) region

｠

Region where to conform the reception of sent data

EOF (End of Frame)

｠

Indicates the end of the frame

ERROR DETECTION AND RECOVERY
CAN protocol secures its reliability of communication
by providing several error detection function such as
CRC shown in data frame, and the recovery function
(recovery is performed by resending, from abnormal
state such as transmission errors). If an error is
detected but it is not resolved even after recovery,
communication is stopped. This state is called "Bus
Off."

SELF-DIAGNOSIS

M25402000004USA0000010000

｠

CAN self-diagnosis is performed by each ECU
connected to the CAN bus.

｠

Diagnosis codes related to communication are
named with the capital letter U, and are called "U-
codes."

｠

A summary of the CAN self-diagnosis system is
presented below. For further details on each
diagnostic trouble code stored by the ECUs.

TIME-OUT
Each ECU transmits data frames periodically. If the
data frame is not received within the specified period,
the intended receiving ECU transmits a diagnostic
trouble code indicating communication time-out for
the ECU that failed to transmit.

54D-6

CONTROLLER AREA NETWORK (CAN)

SELF-DIAGNOSIS

BUS OFF
Related to a communication error that persists even
after the transmitting ECU attempts recovery for a

specified number of attempts or that persists for a
specified period after recovery. Communication is
stopped and this diagnostic code is issued.

CAN BUS DIAGNOSTICS

M25402000005USA0000010000

As ECUs are connected via CAN bus (including scan
tool), always diagnose CAN bus to confirm its
normality when inspecting. Simply by performing scan
tool screen operations, the following inspections can
be performed automatically, and the result can be
used to verify the CAN bus status.

｠

Confirmation of diagnostic trouble code of ETACS-
ECU

｠

Confirmation of communication of all ECUs

CONTROLLER AREA NETWORK (CAN)

54D-7

CAN BUS DIAGNOSTICS

MEMO

54D-8

CONTROLLER AREA NETWORK (CAN)