21.     Why Diagnostic Standards?

Answer:  As systems got more complex the link between cause and symptom became less obvious. This meant that electronic systems had to have some level of self diagnosis and to communicate to the outside world. Initially many systems used their own protocols which meant that garages had to have a large number of tools – even to diagnose a single vehicle.

 22.     What is meant by verification and validation??

Answer: Verification and Validation (V&V) is the process of checking that a software system meets specifications and that it fulfills its intended purpose. It is normally part of the software testing process of a project.

According to the Capability Maturity Model (CMMI-SW v1.1),

·         Verification: The process of evaluating software to determine whether the products of a given development phase satisfy the conditions imposed at the start of that phase.

·         Validation: The process of evaluating software during or at the end of the development process to determine whether it satisfies specified requirements.

·         Verification shows conformance with specification; validation shows that the program meets the customer’s needs

 23.  Can you have two transmitters using the same exact header field?

Answer:  No – that would produce a bus conflict

• Unless you have middleware that ensures only one node can transmit at a time

– For example use a low priority message as a token to emulate token-passing

24.   CAN physical layer voltage levels

25. CAN bit timing:

According to the CAN specification, the bit time is divided into four segments. The Synchronization Segment, the Propagation Time Segment, the Phase Buffer Segment 1, and the Phase Buffer Segment 2. Each segment consists of a specific, programmable number of time quanta (see Table 1). The length of the time quantum (tq), which is the basic time unit of the bit time, is defined by the CAN controller’s system clock fsys and the Baud Rate Prescaler (BRP) :tq = BRP / fsys. Typical system clocks are : fsys = fosc or fsys = fosc/2.

The Synchronization Segment Sync_Seg is that part of the bit time where edges of the CAN bus level are expected to occur; the distance between an edge that occurs outside of Sync_Seg and the Sync_Seg is called the phase error of that edge. The Propagation Time Segment Prop_Seg is intended to compensate for the physical delay times within the CAN network. The Phase Buffer Segments Phase_Seg1 and Phase_Seg2 surround the Sample Point. The (Re-)Synchronization Jump Width (SJW) defines how far a resynchronization may move the Sample Point inside the limits defined by the Phase Buffer Segments to compensate for edge phase errors.

Two types of synchronization exist : Hard Synchronization and Resynchronization. A Hard Synchronization is done once at the start of a frame; inside a frame only Resynchronizations occur.

• Hard Synchronization After a hard synchronization, the bit time is restarted with the end of Sync_Seg, regardless of the edge phase error. Thus hard synchronization forces the edge which has caused the hard synchronization to lie within the synchronization segment of the restarted bit time.

• Bit Resynchronization Resynchronization leads to a shortening or lengthening of the bit time such that the position of the sample point is shifted with regard to the edge.

26. Formula for Baudrate calculation?

 The baud rate is calculated as:

baud rate (bits per second) = 18.432 x 10^6 / BRP / (1 + TSEG1 + TSEG2)

27. What happen when two CAN nodes are sending same identifier at a same time?

Two nodes on the network are not allowed to send messages with the same id. If two nodes try to send a message with the same id at the same time arbitration will not work. Instead, one of the transmitting nodes will detect that his message is distorted outside of the arbitration field. The nodes will then use the error handling of CAN, which in this case ultimately will lead to one of the transmitting node being switched off (bus-off mode).

28. what is the difference between Bit Rate and Baud Rate?

 The difference between Bit and Baud rate is complicated and intertwining. Both are dependent and inter-related. But the simplest explanation is that a Bit Rate is how many data bits are transmitted per second. A baud Rate is the number of times per second a signal in a communications channel changes.Bit rates measure the number of data bits (that is 0′s and 1′s) transmitted in one second in a communication channel. A figure of 2400 bits per second means 2400 zeros or ones can be transmitted in one second, hence the abbreviation “bps.” Individual characters (for example letters or numbers) that are also referred to as bytes are composed of several bits.A baud rate is the number of times a signal in a communications channel changes state or varies. For example, a 2400 baud rate means that the channel can change states up to 2400 times per second. The term “change state” means that it can change from 0 to 1 or from 1 to 0 up to X (in this case, 2400) times per second. It also refers to the actual state of the connection, such as voltage, frequency, or phase level).