The J1939 protocol plays a crucial role in ensuring seamless communication between various electronic control units (ECUs) in vehicles, making it a widely used communication standard in the automotive industry. Whether you’re a seasoned professional or a budding engineer, mastering the J1939 protocol is essential. To help you prepare for interviews in this field, we’ve compiled a comprehensive list of J1939 protocol interview questions with detailed answers and examples. You can also check our comprehensive guide on “CAN Protocol Interview Questions“, “LIN Protocol Interview Questions“, and “UDS Protocol Interview Questions“.

Most commonly asked “J1939 Protocol Interview Questions and Answers”

Q1) What is the J1939 protocol?

The J1939 protocol facilitates communication between ECUs in heavy-duty vehicles, serving as a standardized communication protocol. It defines the exchange of information, ensuring compatibility and interoperability among different vehicle components.

Example: In a truck, the engine control module (ECM) communicates with the transmission control module (TCM) using J1939 to optimize performance.

Q2) How does J1939 differ from other communication protocols?

J1939 stands out due to its specific focus on the automotive industry. Unlike other protocols, it standardizes communication parameters, message formats, and network management, ensuring a seamless exchange of data in heavy-duty vehicles.

Example: While Controller Area Network (CAN) serves as a general-purpose protocol, J1939 is specifically tailored to meet the unique requirements of commercial vehicles.

Q3) Explain the CAN (Controller Area Network) and its relation to J1939.

J1939 is built upon the CAN protocol, utilizing its data-link layer and physical layer. CAN forms the backbone, providing a reliable and efficient means of communication between ECUs in the J1939 network.

Example: The CAN bus facilitates real-time data exchange between the engine and transmission controllers in a J1939-enabled truck.

Q4) What is the significance of the Parameter Group Number (PGN) in J1939?

PGN is a vital element in J1939 messages, uniquely identifying different types of data. It helps receivers interpret incoming messages, ensuring they understand the content and respond accordingly.

Example: ECUs monitor and adjust engine parameters based on temperature readings associated with PGN 65265, which is linked to Engine Temperature.

Q5) How do Address Claiming and Request for Address Claiming work in J1939?

Address Claiming is the process through which ECUs on the network obtain unique addresses. An ECU wanting to join the network initiates the Request for Address Claiming, ensuring proper assignment of addresses to prevent conflicts.

Example: A new fuel injector ECU will send a Request for Address Claiming to acquire a unique address within the J1939 network.

Q6) Explain the SPN (Suspect Parameter Number) in J1939.

SPN identifies specific parameters within a PGN, providing a standardized way of referencing different types of data. It enhances the readability and interoperability of J1939 messages.

Example: In PGN 65262 (High-Resolution Vehicle Distance), SPN 84 represents the total vehicle distance traveled.

Q7) How does Transport Protocol work in J1939?

J1939 employs a Transport Protocol to handle large messages that cannot fit into a single frame. It breaks down these messages into smaller packets, ensuring reliable and complete data transmission.

Example: When sending a lengthy diagnostic message, the Transport Protocol divides the message into segments, ensuring that the receiving ECU receives and assembles all parts correctly.

Q8) Describe the Address Claiming Process in J1939 in detail.

Address Claiming is a critical procedure in J1939, ensuring that each Electronic Control Unit (ECU) on the network has a unique identifier. This process starts when a new ECU joins the network or when the existing network configuration changes. During Address Claiming, the ECU sends a Request for Address Claiming (RAC) message to the network.

Example: Imagine a scenario where a new anti-lock braking system (ABS) ECU is added to a truck’s network. The ABS ECU initiates the Address Claiming process by sending an RAC message, requesting a unique address from the network.

Q9) What role does the DM1 message play in J1939 diagnostics?

The DM1 (Diagnostic Message 1) is a special message in J1939 that provides a snapshot of the vehicle’s current diagnostic status. It includes information about active diagnostic trouble codes (DTCs), helping technicians and ECUs identify and address potential issues promptly.

Example: If the engine encounters a fault, the ECM will generate a DM1 message containing relevant diagnostic information, such as the specific DTC associated with the fault and its severity level.

Q10) Explain the concept of Suspect Parameter Number (SPN) in J1939 and provide a real-world application.

SPN is a standardized numbering system in J1939 that allows for consistent identification of parameters within a Parameter Group (PGN). It simplifies communication by providing a common reference point for specific data.

Example: In PGN 65262 (High-Resolution Vehicle Distance), SPN 84 consistently refers to the total vehicle distance traveled. This standardization ensures that different ECUs interpret and utilize this parameter uniformly.

Q11) How does the J1939 Transport Protocol ensure reliable data transmission?

When a message is too large to fit into a single frame, J1939 employs the Transport Protocol. It divides the message into smaller packets, known as transport frames, and then transmits them over the network. The receiving ECU reassembles these frames to reconstruct the complete message.

Example: When sending a firmware update to the engine control module (ECM), the Transport Protocol breaks down the update into smaller frames, ensuring that each frame reaches the ECM intact and in the correct order.

Q12) Discuss the role of NAME and ADDR fields in the J1939 Address Claiming process.

In the J1939 Address Claiming process, the NAME field contains information about the ECU, such as its function and manufacturer code. The ADDR field, on the other hand, holds the requested address during the Address Claiming process.

Example: If a transmission control module (TCM) initiates Address Claiming, its NAME field might indicate “Transmission Control Module” with a manufacturer code, and the ADDR field would hold the address it is requesting from the network.

Q13) Can you explain the difference between Destination Address and Source Address in a J1939 message?

In a J1939 message, the Destination Address (DA) represents the intended recipient of the message, while the Source Address (SA) identifies the sender. This addressing scheme ensures that messages are directed to the appropriate ECUs within the network.

Example: If an engine control module (ECM) wants to communicate with a transmission control module (TCM), the ECM’s SA would be its own address, and the DA would be the address of the TCM.

Q14) How does the J1939 protocol handle message priority?

J1939 uses a priority-based messaging system, where messages with lower numerical values have higher priority. Lower priority messages are preempted by higher-priority messages, ensuring that critical information is processed without delay.

Example: A message related to engine failure (higher priority) would interrupt a lower-priority message about current fuel levels to address the more urgent issue first.

Q15) Explain the significance of the NAME field in J1939 messages.

The NAME field in J1939 messages contains information about the ECU, including its function and manufacturer code. This standardized field enhances interoperability by providing a clear identification of the sender or recipient.

Example: If a brake control module sends a message, its NAME field might indicate “Brake Control Module” along with a manufacturer code, aiding in the proper interpretation of the message by other ECUs on the network.

Q16) Discuss the role of the PDU Format (PF) and PDU Specific (PS) fields in J1939 messages.

The PDU Format (PF) and PDU Specific (PS) fields in a J1939 message jointly identify the Parameter Group Number (PGN) and, consequently, the type of data being transmitted. PF identifies the broad category, while PS further refines the information within that category.

Example: In a message with PF=61444 and PS=1, the PGN is 61444, and the specific data relates to a predefined category, such as Electronic Engine Controller 1.

Q17) How does J1939 ensure data integrity during transmission?

J1939 employs a combination of checksums and cyclical redundancy checks (CRC) to ensure data integrity. These checks allow receiving ECUs to verify that the data received matches the data sent, reducing the risk of errors during transmission.

Example: If an ECU sends a message with a checksum value, the receiving ECU recalculates the checksum upon receipt and compares it with the transmitted value to verify the integrity of the data.

Q18) What protocol does J1939 use?

J1939 uses the Controller Area Network (CAN) protocol. It defines the communication rules and parameters specific to heavy-duty vehicles, ensuring standardized and efficient data exchange between Electronic Control Units (ECUs).

Q19) What is the difference between CAN and J1939?

CAN (Controller Area Network) serves as a general-purpose protocol, while J1939, built on top of CAN, operates as an application layer protocol. J1939 specifically organizes and exchanges information for the heavy-duty automotive industry, introducing an additional layer of standardization to CAN.

Q20) Is J1939 used in cars?

Heavy-duty vehicles, such as trucks, buses, and agricultural machinery, primarily use J1939. Standard passenger cars, which typically employ other protocols like OBD-II, do not commonly use J1939.

Q21) What is the data format for J1939?

The data format for J1939 messages follows the format specified by the Controller Area Network (CAN) protocol. It includes an identifier, control bits, data bytes, and a cyclic redundancy check (CRC) for error checking.

Q22) Why J1939 protocol?

Designed to standardize communication in the heavy-duty automotive industry, the J1939 protocol promotes interoperability and compatibility among vehicles from different manufacturers. It ensures seamless data exchange, leading to improved diagnostics, efficiency, and overall performance.

Q23) What is a J1939 port?

A J1939 port refers to the physical or logical connection point on an Electronic Control Unit (ECU) that facilitates communication using the J1939 protocol. It’s the interface through which data is sent and received on the J1939 network.

Q24) What is the maximum bus load for J1939?

The maximum bus load for J1939 is typically specified at 30%. This ensures that the network can handle the communication demands without compromising performance or causing delays.

Q25) What is the maximum length of J1939?

The maximum length of a J1939 network depends on factors such as the type of cabling and the baud rate used. In practical applications, the length can extend to several hundred meters.

Q26) What layer is CAN bus?

CAN bus operates at the Data Link Layer (Layer 2) and the Physical Layer (Layer 1) of the OSI model. It provides a reliable and efficient method for communication between devices on a network.

Q27) What is the voltage range of J1939?

The voltage range for J1939 conforms to the specifications of the Controller Area Network (CAN) protocol, which typically operates with voltage levels between 2.5V and 3.5V.

Q28) What is the baud rate of J1939?

The standard baud rate for J1939 communication is 250 kbps (kilobits per second). However, other baud rates such as 500 kbps are also used in specific applications.

Q29) What is the priority of J1939?

J1939 uses an 8-bit priority field in its messages, ranging from 0 to 7. A lower numerical value indicates a higher priority. This allows critical messages to take precedence over lower-priority ones.

Q30) What is J1939 failure?

J1939 failure refers to a situation where there is a breakdown or disruption in the communication between Electronic Control Units (ECUs) using the J1939 protocol. This can result from issues such as network faults, hardware failures, or software errors.

Q31) Where is the J1939 resistor located?

The J1939 resistor, also known as the termination resistor, is typically located at each end of the J1939 network. It helps terminate the communication lines and prevent signal reflections that could degrade the network’s performance.

Q32) What color is J1939 high and low?

The standard color coding for J1939 high and low wires is green for high and yellow for low. However, it’s essential to refer to specific vehicle or equipment documentation, as color codes can vary.

Q33) What is the source address in J1939?

The source address in J1939 identifies the ECU that is transmitting a message. It is an 8-bit value that allows other ECUs on the network to determine the sender of the message.

Q34) How many nodes are in a CAN bus?

A CAN bus can support up to 64 nodes (devices or ECUs) in a network. However, the practical limit may vary depending on factors such as the network topology and communication requirements.

Q35) How data is transferred in CAN bus?

In a CAN bus, devices transfer data through a message-oriented communication approach. Devices on the network both send and receive messages containing data packets, and the bus is designed to accommodate simultaneous communication among multiple devices.

Q36) How data is transmitted in CAN bus?

CAN bus uses a non-destructive bitwise arbitration method for data transmission. Devices transmit messages, and the one with the highest priority, determined by the message identifier, gets to send its data. This ensures an efficient and collision-free communication process.

Q37) How do I test a J1939 connector?

Testing a J1939 connector involves using diagnostic tools that can communicate with the network, checking for proper voltage levels, and verifying the integrity of the wiring. Specialized J1939 diagnostic tools can provide detailed information about the network’s health.

Q38) Who invented J1939?

The Society of Automotive Engineers (SAE) developed J1939 as a set of standards collaboratively created by industry experts. Its purpose is to establish a uniform communication protocol specifically tailored for heavy-duty vehicles.

Q39) How long is J1939 data?

The length of J1939 data can vary depending on the specific message being transmitted. J1939 messages can carry different amounts of data, and the structure allows for flexibility in accommodating various types of information.

Q40) What is J1939 network 1?

J1939 network 1 typically refers to the primary communication network in a vehicle or system. It’s the main network where ECUs communicate with each other using the J1939 protocol.

Q41) What is DM3 in J1939?

DM3 in J1939 refers to a specific diagnostic message format used to transmit more extensive diagnostic information. It allows for the exchange of detailed diagnostic trouble code (DTC) information between ECUs on the network.

Q42) What is J1939 backbone?

The J1939 backbone is the core communication network within a vehicle or system. It serves as the primary channel for high-priority and critical messages, facilitating essential functions such as engine control and diagnostics.

Q43) What is code 639 on J1939?

Code 639 in J1939 typically refers to a specific diagnostic trouble code (DTC). Each DTC has a unique identifier, and code 639 would correspond to a specific issue or fault in the vehicle’s system.

Q44) How many ohms should J1939 have?

The resistance (ohms) of a J1939 network depends on factors such as the length of the network and the impedance of the cables used. A common practice is to have termination resistors at each end of the network to match the cable impedance.

Q45) How many ohms is a J1939 data link?

The specific resistance of a J1939 data link can vary, but it commonly falls within the range of 60 to 120 ohms. This resistance helps ensure proper termination and signal integrity on the network.

Q46) What is a J1939 end-of-line resistor?

Placing the J1939 end-of-line resistor, also referred to as the termination resistor, at the end of the network prevents signal reflections. It matches the impedance of the network, optimizing signal quality.

Q47) What color is used to identify the J1939 data connector?

The color used to identify the J1939 data connector can vary. However, connectors typically have green housings or labels, and users rely on the actual wiring color codes (such as green for high and yellow for low) for easy identification.

Q48) What is J1939 simplified?

J1939 is a standardized communication protocol developed by the Society of Automotive Engineers (SAE) for heavy-duty vehicles. It defines how Electronic Control Units (ECUs) communicate over a Controller Area Network (CAN), enabling data exchange for efficient vehicle operation and diagnostics.

Q49) What is source and destination IP?

Source and destination IP refer to the Internet Protocol (IP) addresses associated with the origin and destination of data packets in network communication. They are numerical labels assigned to devices to facilitate proper routing and delivery of data across the internet.

Q50) What is source and destination address?

Source and destination addresses in the context of J1939 refer to the unique identifiers assigned to Electronic Control Units (ECUs) on the network. The source address indicates the sender of a J1939 message, while the destination address specifies the intended recipient.

Q51) What is the source IP address field?

The source IP address field is a part of the IP header in network communication. It contains the numerical label assigned to the device originating the data packet. It helps routers and switches determine the source of the communication.

Q52) Does J1939 have a fuse?

J1939 itself does not have a fuse. However, in a vehicle equipped with J1939 communication, individual components or devices connected to the J1939 network may have fuses to protect them from electrical faults.

Q53) What is the difference between J1939 and NMEA?

J1939 and NMEA (National Marine Electronics Association) are both communication protocols, but they serve different industries. Heavy-duty vehicles use J1939, whereas marine applications commonly employ NMEA for communication between marine electronic devices.

Q54) How is J1939 different from CAN?

J1939 is an application layer protocol built on top of the Controller Area Network (CAN) protocol. While CAN is a general-purpose communication standard, J1939 specifically addresses the needs of the heavy-duty automotive industry, defining message formats and parameters for standardized communication.

Q55) What are the message types in J1939?

J1939 supports various message types, including Request/Command (RQST), Data (PGN-specific), Acknowledgment (ACK), and others. These messages serve different purposes within the protocol, such as requesting data, transmitting data, or confirming receipt.

Q56) What are the benefits of J1939?

The benefits of J1939 include standardized communication in heavy-duty vehicles, improved diagnostics, enhanced interoperability between different manufacturers’ components, optimized vehicle performance, and efficient data exchange among Electronic Control Units (ECUs).

Q57) What is the data type of J1939?

J1939 supports various data types depending on the Parameter Group Number (PGN) being used. It can handle numeric values, ASCII strings, binary data, and more, allowing for flexibility in representing different types of information.

Q58) What is the OSI model of J1939?

J1939 does not strictly adhere to the OSI model but can be loosely mapped onto it. It operates mainly at the higher layers, including the Application Layer, where it defines its specific message formats and communication rules.

Q59) What is the length of J1939 message?

Depending on the Parameter Group Number (PGN) and the specific data being transmitted, a J1939 message varies in length. Messages can range from a few bytes to several dozen bytes.

Q60) Why termination resistors?

Termination resistors in a J1939 network prevent signal reflections and ensure a clean and stable communication environment. They match the impedance of the network, reducing signal distortion and potential communication errors.

Q61) Why is termination resistor needed?

To prevent signal reflections caused by the impedance mismatch between the network cable and the connected devices, a J1939 network requires a termination resistor. This ensures reliable and accurate data transmission.

Q62) How many ohms is an end-of-line resistor?

The typical value for an end-of-line resistor in a J1939 network is 120 ohms. This value matches the characteristic impedance of the network, optimizing signal integrity.

Q63) What is the voltage of J1939?

The voltage of a J1939 network conforms to the specifications of the Controller Area Network (CAN) protocol, typically operating between 2.5V and 3.5V.

Q64) What is the speed of J1939?

The standard speed for J1939 communication is 250 kbps (kilobits per second). Specific applications may use other speeds, such as 500 kbps.

Q65) What is J1939 communication?

J1939 communication involves the exchange of data between Electronic Control Units (ECUs) in heavy-duty vehicles over a Controller Area Network (CAN). It enables real-time monitoring, control, and diagnostics for optimal vehicle performance.

Q66) What is the bandwidth of 9600 baud?

Baud rate is a measure of signal changes per second, not bandwidth. Determine the bandwidth required for a baud rate of 9600 bps (bits per second) by analyzing the frequency components of the signal.

Q67) What is meant by baud rate?

Baud rate refers to the number of signal changes per second in digital communication. It demonstrates the speed at which data can transmit over a communication channel.

Q68) Where is J1939 used?

Heavy-duty vehicles, such as trucks, buses, agricultural machinery, and construction equipment, primarily use J1939. It facilitates communication between various Electronic Control Units (ECUs) within these vehicles.

Q69) What is J1939 protocol address?

In J1939, the protocol address refers to the unique source address assigned to each Electronic Control Unit (ECU) on the network. This address helps identify the sender of a J1939 message.

Q70) What is the formula for baud rate?

The formula for the baud rate is:

Baud Rate=(1/Time per bit)

​It represents the number of signal changes per second in digital communication.

These concise answers aim to provide clarity on various aspects related to J1939, including its simplified definition, addressing, differences from other protocols, and technical details. If you have any further questions or need additional information, feel free to ask in the comment section!