11. What is bit stuffing?
Answer: CAN uses a Non-Return-to-Zero protocol, NRZ-5, with bit stuffing. The idea behind bit stuffing is to provide a guaranteed edge on the signal so the receiver can resynchronize with the transmitter before minor clock discrepancies between the two nodes can cause a problem. With NRZ-5 the transmitter transmits at most five consecutive bits with the same value. After five bits with the same value (zero or one), the transmitter inserts a stuff bit with the opposite state.
12. What is the use of bit stuffing?
Answer: Long NRZ messages cause problems in receivers
• Clock drift means that if there are no edges, receivers lose track of bits
• Periodic edges allow receiver to resynchronize to sender clock
13. What are the functions of CAN transceiver?
Answer: The transceiver provides differential transmit capability to the bus and differential receive capability to the CAN controller. Transceiver provides an advanced interface between the protocol controller and the physical bus in a Controller Area Network (CAN) node.
Typically, each node in a CAN system must have a device to convert the digital signals generated by a CAN controller to signals suitable for transmission over the bus cabling (differential output). It also provides a buffer between the CAN controller and the high-voltage spikes that can be generated on the CAN bus by outside sources (EMI, ESD, electrical transients, etc.).
The can transceiver is a device which detects the signal levels that are used on the CAN bus to the logical signal levels recognized by a microcontroller.
14. Functionality of Data link layer in CAN?
Answer: LLC (Logical Link Control) – Overload control, notification, Message filtering and Recovery management functions.
MAC (Medium Access Control) – Encapsulation/ de-capsulation, error detection and control, stuffing and de-stuffing and serialization/de-serialization.
15. What is meant by synchronization?
Answer: Synchronization is timekeeping which requires the coordination of events to operate a system in unison.
16. What is meant by Hard synchronization and soft synchronization?
Answer: Hard Synchronization to be performed at every edge from recessive-to-dominant edge during Bus Idle. Additionally, Hard Synchronization is required for each received SOF bit. An SOF bit can be received both during Bus Idle, and also during Suspend Transmission and at the end of Interframe Space. Any node disables Hard Synchronization if it samples an edge from recessive to dominant or if it starts to send the dominant SOF bit.
Two types of synchronization are supported:
– Hard synchronization is done with a falling edge on the bus while the bus is idle, which is interpreted as a Start of frame (SOF). It restarts the internal Bit Time Logic.
– Soft synchronization is used to lengthen or shorten a bit time while a CAN frame is received.
17. What is the difference between function and physical addressing?
Answer: Functional addressing is an addressing scheme that labels messages based upon their operation code or content. Physical addressing is an addressing scheme that labels messages based upon the physical address location of their source and/or destination(s).
18. What happens if I have to send more than 8-bytes of data?
Answer: The J1939 standard has defined a method of communicating more than 8 bytes of data by sending the data in packets as specified in the Transport Protocol (TP). There are two types of TP, one for broadcasting the data, and the other for sending it to a specific address.
DTC consists of 4 components – SPN, FMI, OC and CM.
A DTC is a combination of four independent fields: the Suspect Parameter Number (SPN) of the channel or feature that can have faults; a Failure Mode Identifier (FMI) of the specific fault; the occurrence count (OC) of the SPN/FMI combination; and the SPN conversion method (CM) which tells the receiving mode how to interpret the SPN. Together, the SPN, FMI, OC and CM form a number that a diagnostic tool can use to understand the failure that is being reported.
Figure 1 – Message format
19. What is KWP2000?
Answer: KWP 2000(ISO14230) is a Diagnostic communications standard. Specifies possible system configurations using the K & L lines. As 9141-2 but limited to the physical characteristics. Specifies possible system configurations using the K & L lines.
· 5 Baud wake up as 9141- 2
· New fast initialisation method
20. What is OBDII?
Answer: On-Board Diagnostics in an automotive context is a generic term referring to a vehicle’s self-diagnostic and reporting capability