wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

LIN-Bus 45-9
45.5.8 Frame Types
There are three different ways of transmitting frames on the LIN bus: unconditional, event-triggered,
and sporadic frames. Diagnostic frames are also supported.
45.5.8.1 Unconditional Frame
Unconditional frame transfer signals may use the IDs 0x0–0x3B (0–59). Every frame can have only one
transmitter (publisher) but one or several receivers (subscribers). The setup of the frame is described
in the LIN description file. Unconditional frames can be transferred, as shown in the relationships of
Figure 45.4.
45.5.8.2 Event-Triggered Frame
The unconditional frame can be considered as the normal LIN frame. LIN2.0 also defines a so-called
event-triggered frame. Event-triggered frames improve the response time of the master–slave system by
mapping unconditional frames with rare-event data of multiple slaves into one event-triggered frame.
This reduces the cycle time of the time schedule table because the master does not have to poll the slave
nodes individually. Several frame IDs can be defined as “event triggered” within the network. If the
master requests such an “event-triggered” frame, all corresponding slaves will begin to transmit their
data. However, corresponding slave nodes only provide their frame response when the values in their
data fields have changed. But, multiple slaves can provide a frame response to a single event-triggered
frame. To enable the master to recognize which slave node has responded, the first byte of the associated
unconditional frames have to contain their frame ID.
As more than one slave can answer, by updating data in the frame, it is possible that a collision can
occur on the bus line. The master recognizes such a collision situation and resolves it by switching automatically
to the so-called collision resolving schedule table. The table will, at a minimum, contain the
associated unconditional frames and it will be a member of schedule table set.
45.5.8.3 Sporadic Frame
The sporadic frames were introduced in the LIN version 2.0 for better utilization of bandwidth. This
method provides some dynamic behavior to the otherwise static LIN protocol. They are in principle
placeholders in the time schedule table in which the master can send several different messages that
carry signals that seldom change, for example, commands. The master sends the message only if a signal
carried by this message is changed. Hence the time slot can remain empty. The assignment of the unconditional
frames to the sporadic frame takes place in the LIN description file. Figure 45.8 shows a simple
example. In slot three, the master can transfer either the unconditional frame “Master Command” or
“Outdoor Temperature” or it can leave the slot empty.
Slot 1 Slot 2 Slot 3 Slot 4
Master
command
Window
status
Mirror
status
Empty
slot
Keyboard
status
Outdoor
temperature
FIGURE 45.8
Principle of sporadic frames.
© 2011 by Taylor and Francis Group, LLC