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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)