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PCM in a BUS Network and BUS Topology!

1.1 PCM in a BUS Network

Quote:The abbreviation PCM stands for Power Control Module. PCM is the main automotive
computer used to achieve various purposes mostly oriented toward transmission, engine
management and performance. The PCM is one of the several on-board computers that
are interconnected in an automotive vehicle forming a network system called CAN BUS,
(CAN) for Controller Area Network, BUS is a form of network topology in which
different in board computers / modules on different locations are connected to the main
cable. BUS network topology is similar to passengers inside a bus sitting on seats that are
located on different locations but sharing the same passage. CAN network provide an
intercommunication system of various vehicle computers to make a CAN BUS system.
The CAN BUS system shares data to each computer connected to the data link. 

Quote:Each computer in the data link is sometimes called a node. The PCM which is the main
computer has the ultimate control of the Data BUS.
In brief the PCM’s primary function is to manage Powertrain. This includes the ignition
system, fuel system and emission system. PCM’s ultimate goal is to ensure that the
sensors work properly and that the car is non-polluting.

1.2 What is BUS topology?

Quote:In such type of topology, a long backbone cable (twisted pair cable in CAN BUS) is used
to link all the devices in the network. Wires are twisted to eliminate electrical noise.
Twisted pair cable is used to link all the devices in the network. Drop lines and taps are
used to connect different nodes/modules to this backbone. This topology allows only one
device to transmit at a time since all nodes on the network share a common BUS. The
node to transmit next is determined by distributed access protocol. This network topology
helps to eliminate redundant wiring to the modules / ECU ’ s that need the same sensor

1.2.1 Advantages of BUS topology

  1. Requires less cabling and therefore less expensive as compared to mesh, star and tree topologies.
  2. Easier to install than other topologies
  3. Weight reduction due to fewer components

1.2.2 Disadvantages of BUS topology

  1. It's difficult to add new devices
  2. It's difficult to do reconfiguration
  3. It's difficult to isolate faults in nodes
  4. Limited cable length required
  5. Limited number of nodes that can be connected
  6. The number of accessories inside automobile increased rapidly in the last three decades.
  7. A typical late model vehicle will have 10 or more modules. This therefore implies that the number of actuators, sensors, navigation, entertainment, climate control etc. and their corresponding electronic control modules increased as well. Since modern electronics is almost completely digital this therefore implies for the need for efficient communication that is immune to interruption and noise using defined protocol. 
  8. The communication between all on-board computers requires a network. This network allows all on-board computers to communicate with each other as well as scan tool communications. 
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