SECTION 15: MESSAGE DATA LINK CONTROLLER
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MOTOROLA
Page 117
15.5.3.1
Logic "0"
A logic zero is defined as either an active to passive transition followed by a passive period
64
μ
s in length, or a passive to active transition followed by an active period 128
μ
s in length
(Figure 15-11(a)).
15.5.3.2
Logic "1"
A logic one is defined as either an active to passive transition followed by a passive period
128
μ
s in length, or a passive to active transition followed by an active period 64
μ
s in length
(Figure 15-11(b)).
15.5.3.3
SOF - Start of Frame Symbol
The SOF symbol is defined as a passive to active transition followed by an active period
200
μ
s in length (Figure 15-11(c)). This allows the data bytes that follow the SOF symbol
to begin with a passive bit, regardless of whether it is a logic one or a logic zero.
15.5.3.4
EOD - End of Data Symbol
The EOD symbol is defined as an active to passive transition followed by a passive period
200
μ
s in length (Figure 15-11(d)).
15.5.3.5
EOF - End of Frame Symbol
The EOF symbol is defined as an active to passive transition followed by a passive period
of at least 280
μ
s in length (Figure 15-11(e)). If there is no IFR byte transmitted after an
EOD symbol is transmitted, after another 80
μ
s the EOD becomes an EOF, indicating the
completion of the message.
15.5.3.6
IFS - Inter-Frame Separation Symbol
The IFS symbol is defined as a passive period 300
μ
s in length. The IFS symbol contains
no transition, since when used it always follows an EOF symbol.
15.5.3.7
BREAK - Break Signal
The BREAK signal is defined as a passive to active transition followed by an active period
of at least 280
μ
s (Figure 15-11(f)).
15.5.4
J1850 VPW VALID/INVALID BITS & SYMBOLS
The timing tolerances for receiving data bits and symbols from the J1850 bus have been
defined to allow for variations in oscillator frequencies. In many cases the maximum time
allowed to define a data bit or symbol is equal to the minimum time allowed to define
another data bit or symbol.
Since the minimum resolution of the MDLC for determining what symbol is being received
is equal to a single period of the MUX Interface clock (t
MDLC
), an apparent separation in
these maximum time/minimum time concurrences equal to one cycle of t
MDLC
occurs.
This one clock resolution allows the MDLC to properly differentiate between the different
bits and symbols, without reducing the valid window for receiving bits and symbols from
transmitters onto the J1850 bus having varying oscillator frequencies.
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