88B-14V3 MR-413-X44-88B000$040.mif
MULTIPLEXING
Fault finding – Function88B
The multiplex network operates in a very similar way.
1. The transmitting computer formats the signal to be sent into a frame that can be identified by other computers.
2. The transmitting computer waits until the network is free, i.e. no other frames are being sent by another
computer.
3. The computer sends its frame.
4. All the other computers receive the transmitted frame. As the frame has already been formatted by the
transmitting computer, the receiving computer can determine whether the frame was meant for it or not.
5. The receiving computers acknowledge receipt of the frame. The transmitting computer will resend the frame if it
does not receive this acknowledgement.
Transmitting frames
The frames are transmitted on two twisted wires in order to avoid electromagnetic interference. These wires are
called multiplex line H and multiplex line L.
To reduce electrical interference when frames are sent, the two wires have differential voltages. In addition,
terminating resistors of 120 Ω are fitted at the ends of the network (in the airbag and injection computers) to
attenuate electrical signal reflections.
The electric signals are digital which means that there are only two levels of differential voltage known as recessive
state (corresponding to logic 1) and dominant state (corresponding to logic 0).
The status is recessive when the potential difference between multiplex line H and multiplex line L is zero (multiplex
line H = multiplex line L = 2.5 V). The status is dominant when the potential difference between multiplex line H and
multiplex line L is equal to approximately 2 V (multiplex line H = 3.5 V and multiplex line L = 1.5 V).
