Chapter 12 Bennet

8/10/2019 Chapter 12 Bennet

1/29

Chapter 12

Multiplexing


2/29

Objectives (1 of 3)

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.


3/29

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.


4/29

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 networkedelectronic systems.

Outline the procedure required to access a

failure mode indicator (FMI) using electronicservice tools.


5/29

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.


6/29


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:

Simplifies the hardware

Eliminates miles of hard wiring

Reduces the number of I/O (in-out) pins on modules

Optimizes vehicle operation


7/29


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 orshop computer to read the electronic systems

on a truck, you already have some experience

in accessing a truck data bus.


8/29

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 aspackets.


9/29

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 signalsby the receiving processor.


10/29

Power Line Carrier

A more primitive method of multiplexingknown as power line carrier has been in usein trucks since 1985.

Because all the wires on a standard SAEJ560, 7-pin connector between truck andtrailer were already dedicated, power linecarrier technology was used to convert a

communication signal to a radio frequencysignal and then superimpose it over the12-volt auxiliary power wire.


11/29

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 thefollowing 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.


12/29

Multiplexing


13/29

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 multiplexingto indicate that no single controller networked

to the bus has more priority or status than

another: No ECM/ECU is in charge.


14/29

CAN Message Format


15/29

Message Bit Encoding of a Data Packet


16/29


17/29

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 someOEMs as CAN+ and CAN-.


18/29

Reasons for Twisting Data Wires

Provide immunity to magnetic fields

Provide consistent capacitance values


19/29

Comparison of Scope Patterns


20/29

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.


21/29

Data Bus Repairs

Physical damage to any data bus describedas 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 tokeep the communication wires separate


22/29

Shop Talk

When troubleshooting, if you disconnect a

smart switch, a code will be logged

immediately.

Always use the system self-diagnostics tolocate problems.


23/29

FET Operation


24/29

Shop Talk

When adding loads to a multiplexed truck

chassis, always consult the OEM service

literature.

Splicing into circuits can cause electricalproblems that become very difficult to

troubleshoot.


25/29

Caution

Never splice into existing fuses in chassis powerdistribution modules to source a battery voltagerequirement.

In dealerships today, it is not uncommon to hear

horror stories that result when a truck driver splicesinto a hot wire to power up his CB radio.

Most OEMs provide non-dedicated terminals in theirpower 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.


26/29

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 harnesswiring 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 highwaywhich allows data signals to take the place of hard

wire in the electronic input and output circuits.


27/29

Summary (2 of 4)

The data bus acts as the informationhighway 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 aprogrammed library of resistance values

that identify the switch, its status, and

circuit integrity.


28/29

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 theJ1587/ 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.


29/29

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 atruck data bus with multiple electronic

systems, you must first select the correct

message identifier (MID) if using generic-reader software (such as ProLink).

Chapter 12 Bennet

Documents

Transcript of Chapter 12 Bennet