Question 1

What is each data 'message' known as in a multiplexed truck system?

Accepted Answer

A packet is a unit of data that is sent over a network. In a multiplexed truck system, each data message is known as a packet.

Question 2

How many terminating resistors are required on a J1939 data backbone?

Accepted Answer

Two terminating resistors are required on a J1939 data backbone, one at each end of the main bus line, to prevent signal reflections that can cause data errors.

Question 3

Technician A says that the physical distance between the two wires on a twisted wire pair determines their capacitance,with capacitance decreasing as the wires become further apart,accounting for the fact that replacement is usually preferred over repair.Technician B says that the insulation wrap over a twisted wire pair also defines the capacitance which explains why it should never be replaced with ordinary wire.Who is correct?

Accepted Answer

Both technicians are correct. Technician A is correct in stating that the physical distance between two wires in a twisted pair affects their capacitance, with greater distance resulting in lower capacitance. This is a fundamental principle of capacitance, which is influenced by the distance between conductors. Technician B is correct in stating that the insulation wrap over a twisted wire pair affects its capacitance. The type of insulation and its thickness can influence the capacitance between the wires, which is why specific types of insulation are used for twisted pair cables to achieve desired electrical properties. Replacing it with ordinary wire could alter these properties, affecting the cable's performance.

Question 4

What is the maximum permissible,physical length of a J1939 data bus?

Accepted Answer

The maximum permissible physical length of a J1939 data bus is 130 feet (40 meters), allowing for extensive vehicle network coverage.

Question 5

Technician A says that CAN L (-)and (chassis)ground are of equal voltage potential.Technician B says that modules networked to a J1939 bus are designed to identify the difference between CAN H (+)and CAN L (-)in message packet without reference to chassis ground.Who is correct?

Accepted Answer

Technician B is correct because in a J1939 bus (and other CAN bus systems), the modules indeed identify messages based on the voltage difference between CAN H (High) and CAN L (Low) without needing a reference to the chassis ground. This differential signaling allows for more reliable communication in the presence of electrical noise.

Question 6

Technician A says that a high bus backbone consists of a twisted wire pair wound with bare copper wire and wrapped with a foil shield.Technician B says that the bare copper wire used in a high bus backbone is known as the drain.Who is correct?

Accepted Answer

Both technicians are correct. A high bus backbone indeed consists of a twisted wire pair, often wound with a bare copper wire and wrapped with a foil shield for electromagnetic interference protection. The bare copper wire is known as the drain wire, which helps to ground and shield the cable.

Question 7

What is the maximum permitted length of an stub extending from a J1939 data bus?

Accepted Answer

The answer of What is the maximum permitted length of...

Question 8

What term is used to describe the volume of data that can be handled on a data bus?

Accepted Answer

Bandwidth is the term used to describe the volume of data that can be handled on a data bus, referring to the data transfer rate or the amount of data that can be transmitted in a fixed amount of time.

Question 9

What is the usual voltage differential used on the J1939 data backbones to transmit message packets?

Accepted Answer

The answer of What is the usual voltage differential used...

Question 10

What is used to rate the transaction speed of a multiplexed data backbone?

Accepted Answer

K-baud is used to rate the transaction speed of a multiplexed data backbone, as it measures the number of signal changes (modulation events) per second, which is directly related to the data transmission speed.

Question 11

Technician A says that some OEMs permit physical repairs on high bus wiring providing guidelines in their service literature are observed.Technician B says that low bus wiring can never be repaired once damaged.Who is correct?

Accepted Answer

The answer of Technician A says that some OEMs permit...

Question 12

Technician A says that processors networked to J1939 data bus must have clock speeds of at least 16 MHz.Technician B says that a frequency of 16 MHz translates into a capability of making 16,000 binary decisions per second.Who is correct?

Accepted Answer

The answer of Technician A says that processors networked to...

Question 13

A twisted wire pair requires a full cycle twist of the wires for every ____ distance of travel.

Accepted Answer

The answer of A twisted wire pair requires a full...

Question 14

Which companies were responsible for developing the CAN data bus standards that are now used in both automotive and truck multiplex systems?

Accepted Answer

Bosch, in collaboration with Intel, developed the CAN (Controller Area Network) data bus standards, which are widely used in automotive and truck multiplex systems for facilitating communication between various electronic control units (ECUs) without a central computer.

Question 15

When several SAs/MIDs attempt to broadcast messages over a truck data bus simultaneously,what process ensures that the message with the highest priority gets broadcast first?

Accepted Answer

Bus arbitration is the process that ensures that when multiple system components attempt to communicate over a bus simultaneously, the message with the highest priority is sent first. This is crucial in systems like vehicle data buses, where timely delivery of high-priority messages is essential for safety and performance.

Question 16

Technician A says that there can be up to 2 open Tees on a J1939 data backbone.Technician B says that most truck data backbones use electrical pressures of around 5 volts.Who is correct?

Accepted Answer

The J1939 data backbone does not allow for open tees; it requires a specific structure for its network to ensure data integrity and minimize reflections. Additionally, J1939 networks typically operate at 250k bits per second with a 24V electrical system in heavy-duty vehicles, not around 5 volts.

Question 17

What is the resistance of a terminating resistor used on a J1939 data backbone?

Accepted Answer

The resistance of a terminating resistor used on a J1939 data backbone is 120 ohms. This is standard for CAN (Controller Area Network) bus systems to ensure signal integrity by reducing reflections.

Question 18

What is the reason for using a twisted wire pair for the data backbone on J1939 buses?

Accepted Answer

Twisted wire pairs are used to reduce the surface area on which electromagnetic interference (EMI) can act, thereby minimizing the impact of EMI on the data signals transmitted through the wires. This is crucial for maintaining the integrity of data communication in environments with potential electrical noise.

Question 19

Technician A says that a disadvantage of multiplexing is that it increases the amount of wiring on a vehicle chassis.Technician B says that a Navistar Diamond Plus multiplex dash has just 7 wires as opposed to over 60 wires on earlier Navistar dash.Who is correct?

Accepted Answer

Multiplexing actually reduces the amount of wiring needed in a vehicle by allowing multiple signals to travel over the same wire, which is why Technician B is correct in stating that a Navistar Diamond Plus multiplex dash has significantly fewer wires compared to earlier versions. Technician A is incorrect because the primary advantage of multiplexing is to decrease wiring complexity and weight.

Question 20

Which of the following messages broadcast to a truck data bus would probably be assigned the highest priority?

Accepted Answer

Panic brake application would likely be assigned the highest priority because it is directly related to the immediate safety and control of the vehicle. Quick response to such a message is crucial to prevent accidents.

Quiz 12: Multiplexing