Chapter 12

1. Chapter 12 Multiplexing

2. 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. 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. 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.

5. 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. 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: • Simplifies the hardware • Eliminates miles of hard wiring • Reduces the number of I/O (in-out) pins on modules • Optimizes vehicle operation

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. Multiplexing

13. 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.”

14. CAN Message Format

15. Message Bit Encoding of a Data Packet

16. 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.

17. 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-.

18. Reasons for Twisting Data Wires • Provide immunity to magnetic fields • Provide consistent capacitance values

19. Comparison of Scope Patterns

20. 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. 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

22. 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.

23. FET Operation

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

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 a truck data bus with multiple electronic systems, you must first select the correct message identifier (MID) if using generic-reader software (such as ProLink).