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

45-6 Industrial Communication Systems
The duty cycle parameters are very important for the waveform of the LIN signal. They establish
limits that ensure the correct determination of the logical bit value for the serial communication interface
(SCI) of the receiver. They cover the issues of drift, oscillator tolerances, and asymmetries of
receiver propagation delay rising edge with respect to the falling edge [TR0363].
45.4.2 topology
The specification does not regulate the wire topology. One can use a line, tree, or star topology. However,
the length of the bus line must be limited to 40.m. The effective criterion of a bus line is the RC time
constant τ. It covers the capacitances and loads of all nodes and the bus line. The RC time constant must
be in the range of 1–5.μs.
The number of nodes in a LIN cluster should not exceed 16. Every additional node lowers the network
resistance by approximately 3%, because it integrates a parallel 30.kΩ resistor and can cause communication
errors under worst-case conditions.
However, although the LIN baud rate is slow, the large voltage swing can be a source of radiated emissions;
therefore, attention must be paid to physical wiring design and layout.
45.5 LIN Message Frames
The LIN message frame, as represented in Figure 45.7, is composed of a header and a message field,
the so-called response. The header field consists of the break field (Synch_Break), the synch byte field
(Synch_Field), and the identifier field, which is designated as the protected identifier (PID). The message
field (response) consists of the data field with one to eight data bytes and the checksum. All fields
except the break field are byte oriented and can be generated with the integrated serial interface of a
microcontroller.
Message activity is coordinated by the periodic execution of the LIN message schedule. For each
frame slot, the master task transmits a frame header with a frame identifier (ID). All slaves evaluate
the frame header, and a slave immediately transmits the frame response that is associated with the ID.
The LIN frame is the combined frame header and response.
45.5.1 Break Field
A synchronization mechanism, which permits slaves without stabilized clock frequencies to synchronize
themselves from the master clock, is implemented in the protocol. The break field informs the
slaves in the network when the data transmission of a frame starts. The master sends a zero, 13 bit
Frame
Frame slot
Idle
Break
field
Header
Sync field
Response
PID Data 1 Data 2 Data 3 Data N Checksum
Inter-frame
space
Idle
Break delimiter
Inter-byte
>=1 bit
space
(recommend 2 bits)
Response
space
Inter-byte
space
Inter-byte
space
Inter-byte
space
FIGURE 45.7
LIN message frame.
© 2011 by Taylor and Francis Group, LLC