CITROEN XSARA Technical Training Manual page 18

Format of the communications message (frame): Only the rate of the Data line
is represented as the Data B line is the opposite.
A message in a multiplexed device is unambiguous. It is therefore not possible for
an error to occur regarding the recipient nor the nature of the command to be
performed.
A frame consists of 9 fields:
a start of message identifier used to locate the start of the message.
an arbitration identification field used to specify the recipient or recipients of
the message.
this field specifies whether an acknowledgement request from the receiver
is required, depending on whether it involves a request for or distribution of
information.
a field containing the data of the message.
a validity control field for the message in order to check the integrity of the
data upon arrival.
a field marking the end of the actual data to indicate that the message is
finished.
an acknowledgement field which allows the receiver to confirm that the
message has been correctly received.
an end of frame field.
a frame separator which allows the next frame to use the same sequence
of 9 fields.
The electrical signals conveyed by the two wires are square and follow an
information encoding sequence. Only the ECUs of the system can interpret them.
IMPORTANT: It is forbidden to connect a measuring device (multimeter, etc) to
Data and Data B wires can be repaired using Raychem connectors.
MULTIPLEXED BSI - XSARA PICASSO AND XSARA – PART 1
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the VAN network.
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Chapter 1