DocID7510 Rev 2 7/18
L5951 Functional description
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2.6 Protocol description
The L5951 uses a Variable Pulse Width (VPW) modulated protocol. One frame consists of
an entire message not containing more than 12 bytes. The first bit of each byte will be the
most significant bit (MSB). A transmitted message begins with a SOF signal and ends with
the EOF signal.
The data to be transmitted has to be in a specific format as follows:
idle, SOF, DATA, CRC, EOD, NB, IFR, EOF, IFS, idle
Definitions below:
idle: Logic level low on communication bus
SOF: Start of Frame
DATA: Data Bytes
CRC: Cyclic Redundancy Check Error Detection Byte
EOD: End of DATA (only when IFR is used)
NB: Normalization Bit
IFR: In-Frame Response Byte(s)
EOF: End of Frame
IFS: Inter-Frame Separation
BRK: Break (can occur on network at any time)
Idle - Logic level low on bus any time after IFS.
Start of Frame (SOF) - The SOF signals the receiver that a new frame is beginning. SOF
signal is a logic level high pulse identified by a pulse width of about t = 200 µs.
DATA - Total number of bytes that can be transmitted (from SOF to EOF) is 12 bytes.
Cyclic Redundancy Check (CRC) - A method for determining if the message received is
the same as the message transmitted. If an invalid CRC number is detected, then an error
will be detected. The SOF signal is not used to determine the CRC. All bits in the CRC are
initially "ones" to avoid confusion with a data stream that are all "zeros".
End of Data (EOD) - Used to signal the receiver about the end of data transmission. If there
is a IRF signal, the sender of the frame will expect one or more bytes in the IFR following the
EOD. If there is no IFR used, then the bus would stay in a logic level low state resulting in a
EOF. EOD signal is recognized by a logic level low pulse for a duration of about 200 µs.
Normalization Bit (NB) - The sole reason for the NB is to define the start of the in-frame
response. The first bit the IFR is passive, therefore it is necessary to have a signal that
follows EOD. There are two forms to the NB. First of all, the NB is a logic level high pulse.
The two forms are distinguished by their pulse widths. The first form has a pulse width of
about 64 µs and indicates if the IFR contains a CRC or not. The second form has a longer
pulse width of about 128 µs and also indicates if there is a CRC in the IFR or not. The
manufacturer can manipulate the NB to any of the two methods.
In-Frame Response (IFR) - Response bytes are sent by the receiver of the transmission
and start after the EOD. If the IFR stays at a logic level low for a period of time then the
frame must be considered to be complete. IFR bytes can be used to send a signal back to
the originator indicating the correct CRC number to confirm the correct message was sent.
End of Frame (EOF) - Indicates the end of a frame. Once the last byte is transmitted, the
bus will be in a logic level low state for a period of time indicating the end of the frame. EOF
signal is recognized by a low pulse for a width of about 280 µs.
Inter-Frame Separation (IFS) - IFS is used to synchronize the receivers at various nodes.
Obsolete Product(s) - Obsolete Product(s)
