Image Processing Reference
In-Depth Information
The header of the frame that contains an identifier is broadcast by the master node and the slave
node that possesses this identifier inserts the data in the response ield. he “break” symbol is used
to signal the beginning of a frame. It contains at least dominant bits (logical value ) followed by
one recessive bit (logical value ) as a break delimiter. The rest of the frame is made of byte fields
delimited by one start bit (value ) and one stop bit (value ), thus resulting in a -bit stream per
byte. The “sync” byte has a fixed value (which corresponds to a bit stream of alternatively and ),
it allows slave nodes to detect the beginning of a new frame and be synchronized at the start of the
identifier field. The so-called “protected identifier” is composed of two subfields: the first bits are
used to encode the identifier and the last bits, the identifier parity. The data field can contain up
to bytes of data. A checksum is calculated over the protected identifier and the data field. Parity
bits and checksum enable the receiver of a frame to detect the bits that have been inverted during
transmission.
LIN defines five different frame types: unconditional, event-triggered, sporadic, diagnostic, and
user-defined. Frames of the latter type are assigned a specific identifier value and are intended to be
used in an application-specific way that is not described in the specification. The first three types
of frames are used to convey signals. Unconditional frames are the usual type of frames used in
the master-slave dialog and are always sent in their frame-slots. Sporadic frames are frames sent
by the master, only if at least one signal composing the frame has been updated. Usually, multi-
ple sporadic frames are assigned to the same frame-slot and the higher priority frame that has an
updatedsignalistransmitted.Anevent-triggeredframeisusedbythemasterwillingtoobtainalist
of several signals from different nodes. A slave will only answer the master if the signals it produces
have been updated, thus resulting in bandwidth savings if updates do not take place very often. If
more than one slave answers, a collision will occur. The master resolves the collision by requesting
all signals in the list one by one. A typical example of the use of the event-triggered transfer given
in Ref. [] is the doors' knob monitoring in a central locking system. As it is rare that multiple
passengers simultaneously press a knob, instead of polling each of the four doors, a single event-
triggered frame can be used. Of course, in the rare event when more than one slave responds, a
collision will occur. he master will then resolve the collision by sending one by one the individual
identifiers of the list during the successive frame-slots reserved for polling the list. Finally, diagnostic
frames have a fixed size of bytes, fixed value identifiers for both the master's request and the slave
answers and always contain diagnostic or configuration data whose interpretation is defined in the
specification.
It is also worth noting that LIN offers services to send nodes into a sleep mode (through a special
diagnostic frame termed “go-to-sleep-command”) and to wake them up, which is convenient as opti-
mizing energy consumption, especially when the engine is not running, is a real matter of concern
in the automotive context.
13.2.3.2 TTP/A Network
Like TTP/C, TTP/A [] was initially invented at the Vienna University of Technology. TTP/A pur-
sues the same aims and shares the main design principles as LIN and it offers, at the communication
controller level, some similar functionalities, in particular, in the areas of plug-and-play capabilities
and online diagnostics services. TTP/A implements the classic master-slave dialog, termed “master-
slaveround,”wheretheslaveanswersthemaster'srequestwithadataframehavingaixedlengthdata
payload of bytes. he “Multipartner” rounds enable several slaves to send up to an overall amount of
bytes of data after a single command frame. A “broadcast round” is a special master-slave round in
which the slaves do not send data; it is, for instance, used to implement sleep-wake-up services. he
data rate on a single-wire transmission support is, as for LIN, equal to kbit/s, but other transmis-
sion supports enabling higher data rates are possible. To our best knowledge, TTP/A is not currently
in use in production cars.
