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

LIN-Bus 45-7
times, followed by a so-called delimiter that must be at least 1 bit time long (2 bits are recommended).
The slaves recognize this break after 11 bit times. This recognition can be carried out with the break
detection feature of a standard SCI. The master generates a break signal that is about two bit times longer
to ensure a safe synchronization to the master clock, for slaves without a crystal.
45.5.2 Sync Byte Field
The sync byte field follows the break field and supports the baud rate estimation. The master sends a
0x55 data byte on which the slaves synchronize themselves. The slaves measure the time period between
the falling edge of the start bit and those of the seventh data bit. The value is divided by 8, for example,
by simply shifting the measured value three bits to the right. Only the falling edges are allowed to be
used for the measurement of the baud rate because they are considerably steeper than rising ones and
therefore provide more exact results. The different edge steepness is due to the characteristics of the
physical layer, as the bus level is actively drawn to “zero” level by a transistor, while the “one” level is
raised by a pull-up resistor.
When the baud rate is adjusted on a standard SCI device, it must be ensured that it does not deviate
by more than the allowed 2% from the master clock.
45.5.3 Identifier
LIN uses a message ID in the third and final byte of the header. This byte consists of 6 bits for the ID and
two parity bits, and thus it is designated as the PID. The 64 possible IDs are shown in Table 45.1.
Up until version 1.3 of the LIN specification, bits four and five of the ID had a special meaning. When
the IDs 0–59 were used for the transmission of signals, bit four and five defined the number of data
bytes, as seen in Table 45.2. This table does not exist for LIN version 2.0 and later.
The implicit coding of the data length in the identifier is no longer obligatory since LIN version 1.2.
Now a field length of one to eight data bytes is possible for every ID. The field length must be indicated
at the message definition in the section “Frames” of the LDF.
Table 45.1 Partitioning of the Range of Values
of the “Protected Identifiers”
Identifier
0x0–0x3B (0–59)
0x3C (60)
0x3D (61)
0x3E/0x3F
62/63
Meaning
Transmission of signals
Master request frame (MRF) for the
configuration and diagnostics
Slave response frame (SRF) to ID 0x3C
Reserved for future expansions of the
protocol
Table 45.2 Meaning of Bit 4 and 5 of
the Identifiers (Mandatory until
Version 1.2; Optional from Version 1.3;
Invalid from Version 2.0)
ID5 ID4 Number of Data Bytes
0 0 2
0 1 2
1 0 4
1 1 8
© 2011 by Taylor and Francis Group, LLC