Transcript HEAVY DUTY TRUCK SYSTEMS Chapter 12

Chapter 12
Multiplexing
Objectives (1 of 3)
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Describe a typical truck data bus.
List the key data bus hardware components.
Define the word multiplexing.
Describe how multiplexing can make data
exchange more efficient.
• Outline how a J1939/CAN 2.0 data bus
functions.
Objectives (2 of 3)
• Access J1587/1708 and J1939 data buses
using an ATA data connector.
• Explain how a “smart” ladder switch
operates.
• List the seven essential fields that make up a
data frame on a truck data bus transaction.
Objectives (3 of 3)
• Explain how FETs are used as relays to
effect data bus outcomes.
• Access a message identifier on a truck
chassis data bus with multiple networked
electronic systems.
• Outline the procedure required to access a
failure mode indicator (FMI) using electronic
service tools.
Multiplexing (1 of 3)
• Data bus communications make use of a
technology called multiplexing.
• Multiplexing means data-sharing between
multiple system control modules.
• Today, technicians regularly access the truck
data bus to troubleshoot, reprogram, and
read the systems within it.
Multiplexing (2 of 3)
• Common language: giving electronic subsystems a
common communication language
• Information highway: Using a data bus allows data
signals to take the place of hard wire.
• New trucks network all of the electronic controllers
in a chassis in a way that:
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Simplifies the hardware
Eliminates miles of hard wiring
Reduces the number of I/O (in-out) pins on modules
Optimizes vehicle operation
Multiplexing (3 of 3)
• So what do you really have to know about
multiplexing today?
– If you have ever used a 6-pin or 9-pin ATA
connector to connect a diagnostic reader or
shop computer to read the electronic systems
on a truck, you already have some experience
in accessing a truck data bus.
Multiplexing, Clients, and Servers
• Client: Anyone or anything that wants
something is referred to as a client.
• Server: The fulfillment of a client need is
provided by a server.
• Data backbone: It serves as a path for
connecting clients with servers.
• Packets: The messages are known as
packets.
Multiplexing
• Bandwidth
– The speed limit in telecommunications is
measured in baud and K-baud.
• Protocols
– They are rules and regulations to simplify
electronic transactions.
• Analog inputs are converted to digital signals
by the receiving processor.
Power Line Carrier
• A more primitive method of multiplexing
known as power line carrier has been in use
in trucks since 1985.
• Because all the wires on a standard SAE
J560, 7-pin connector between truck and
trailer were already dedicated, power line
carrier technology was used to convert a
communication signal to a radio frequency
signal and then superimpose it over the
12-volt auxiliary power wire.
Serial Data Transmission
• Controller area network (CAN)
– A data bus system developed by Robert Bosch and
Intel for vehicle applications.
– A serial data transmission network used for the
following applications in a vehicle:
• Electronic control module (ECM) networking
• Comfort and convenience electronics
• Mobile on-board and external communications
– CAN 2.0 is the basis for SAE J1939
• J1939 is about 25 times faster than J1587/1708.
Multiplexing
Bus Topology
• The word topology means an organized
structural system that remains unaffected
when subjected to a sequence of events.
• The term bus topology is used in multiplexing
to indicate that no single controller networked
to the bus has more priority or status than
another:
– No ECM/ECU is “in charge.”
CAN Message Format
Message Bit Encoding of a Data Packet
Message Bit Encoding
• How a freightliner data packet is sequenced
and the number of bits dedicated to each
segment
• Remember that there are 8 bits to a byte.
Shop Talk
• CAN H (high) and CAN L (low) on a
J1939 are not referenced to ground but to
each other.
• CAN H and CAN L are referred to by some
OEMs as CAN+ and CAN-.
Reasons for Twisting Data Wires
• Provide immunity to magnetic fields
• Provide consistent capacitance values
Comparison of Scope Patterns
Shop Talk
• When repairing low-bus, twisted wires, avoid
twisting the wires together prior to soldering.
• Lay the wires you wish to solder so they
contact each other, then apply tin solder.
– Twisting the wires together and applying a
large blob of solder can create unacceptably
high circuit resistance.
Data Bus Repairs
• Physical damage to any data bus described
as high bus (shielded) warrants replacement,
not repair.
• High bus lines are shielded.
• A J1939 high bus consists of:
– The twisted-wire pair (communication wires)
– A shield wire
– Often a couple of filler wires that function to
keep the communication wires separate
Shop Talk
• When troubleshooting, if you disconnect a
smart switch, a code will be logged
immediately.
• Always use the system self-diagnostics to
locate problems.
FET Operation
Shop Talk
• When adding loads to a multiplexed truck
chassis, always consult the OEM service
literature.
• Splicing into circuits can cause electrical
problems that become very difficult to
troubleshoot.
Caution
• Never splice into existing fuses in chassis power
distribution modules to source a battery voltage
requirement.
• In dealerships today, it is not uncommon to hear
horror stories that result when a truck driver splices
into a “hot” wire to power up his CB radio.
• Most OEMs provide non-dedicated terminals in their
power distribution module that can be used for
auxiliary electrical requirements such as CBs.
• Use the OEM literature and wiring schematics when
connecting auxiliary electrical equipment.
Summary (1 of 4)
• Multiplexing means the ability of electronic
components to exchange information by means of a
common data bus.
• Multiplexing can eliminate miles of chassis harness
wiring and duplication of hardware devices such as
throttle position sensors by giving electronic
subsystems a common communication language,
and by using a data bus or information highway
which allows data signals to take the place of hard
wire in the electronic input and output circuits.
Summary (2 of 4)
• The data bus acts as the “information
highway” in a multiplexed electronic truck
chassis.
• A “smart” ladder switch contains a ladder
of resistors.
– The processor that receives a signal from
the switch can interpret switch status data
by comparing ladder resistances with a
programmed library of resistance values
that identify the switch, its status, and
circuit integrity.
Summary (3 of 4)
• The fields that usually make up a data frame on a
truck data bus transaction are start of frame field,
arbitration field, control field, data field, cyclic
redundancy check field, ack field, and end of frame
field.
• Access to a truck data bus is obtained by using one
of two types of ATA connectors.
– A 6-pin Deutsch connector is used to access the
J1587/ 1708 data bus and a 9-pin Deutsch connector
can be used to access either the J1939 data bus or
the J1587/1708 data bus.
Summary (4 of 4)
• Current trucks equipped with a J1939 data
backbone are also fitted with a J1587/1708
bus.
• When accessing a specific ECM/ECM on a
truck data bus with multiple electronic
systems, you must first select the correct
message identifier (MID) if using genericreader software (such as ProLink).