CprE 458/558: Real-Time Systems

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CprE 458/558 Real-Time Systems

• Controller Area Network Overview
• (Updated by Ki-sung Koo, CprE 458/558 TA)

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Intra-vehicular communication

• A typical vehicle has a large number of
  electronic control systems
• The growth of automotive electronics is a result
  of
• Customers wish for better comfort and better
  safety.
• Government requirements for improved emission
  control
• Reduced fuel consumption
• Some of such control systems
• Engine timing
• Gearbox and carburetor throttle control
• Anti-block systems (ABS)
• Acceleration skid control (ASC)

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Intra-vehicular communication

• An example of intra-vehicular communication.

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Intra-vehicular communication

• The complexity of these functions implemented by
  these electronic control systems necessitates an
  efficient communication between them.
• In addition, a number of systems are being
  developed which will cover more than one device.
• For example
• ASC requires the interplay of the engine timing
  and carburetor control in order to reduce torque
  when drive wheel slippage occurs.
• In the electronic gearbox control, the ease of
  gear changing can be improved by a brief
  adjustment to ignition timing

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How do we connect these control devices?

• With conventional systems, data is exchanged by
  means of dedicated signal lines or wires.
• But this is becoming increasingly difficult and
  expensive as control functions become ever more
  complex.
• In the case of complex control systems in
  particular, the number of connections cannot be
  increased much further.
• Solution Use Fieldbus networks for connecting
  the control devices

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Fieldbus Networks basic motivation
Why use Fieldbus Networks? To avoid this
Figure 1 Traditional Wiring - two pairs of cables
can substitute all typical connections.
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Real-Time Communication Architecture

• Three different communication networks in
  real-time application.

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Intra-vehicular communication

• A schematic diagram of a current in-vehicle
  network

Smart Junction Box
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Fieldbus Networks

• Fieldbuses are communication technologies and
  products used in vehicular, automation and
  process control industries.
• 1) Proprietary Fieldbuses (Closed Fileldbuses)
• Proprietary Fieldbuses are an intellectual
  property of a particular company or body.
• 2) Open Fieldbuses
• For a Fieldbus to be Open, it must satisfy the
  following criteria.
• a) The full Fieldbus Specification must be
  published and available at a reasonable price.
• b) Critical ASIC components must be available,
  also at a reasonable price.
• c) Well defined validation process, open to all
  of the Fieldbus users.

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Fieldbus Advantages

• 1) Reduces the complexity of the control system
  in terms of hardware outlay.
• 2) Resulting in the reduced complexity of the
  control system, project design engineering is
  made simpler, more efficient and conversely less
  expensive.
• 3) By selecting a recognized and well established
  system, this will make the Fieldbus equipment in
  you plant or plants interchangeable between
  suppliers.
• 4) The need to be concerned about connections,
  compatibility and other potential problems is
  eradicated.

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What constitutes a Fieldbus?
The specification of a Fieldbus should ideally
cover all of the seven layers of the OSI model.
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Fieldbus OSI layer details

• Physical Layer 1 What types of signals are
  present, levels, representation of 1's and 0's,
  what type of media connects, etc.
• Link Layer 2 Techniques for establishing links
  between communicating parties.
• Network Layer 3 Method of selecting the node of
  interest, method of routing data.
• Transport Layer 4 Ensuring what was sent
  arrives at the receiver correcting any
  correctable problems.
• Session Layer 5 Not applicable to Fieldbuses.
• Presentation Layer 6 Not applicable to
  Fieldbuses.
• Application Layer 7 Meaning of data.
• The best way of covering layer 7 is to define
  standard profiles for standard devices.

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What Fieldbus Networks are currently on the
market?

• some of the Fieldbus technologies currently on
  the market
• AS-Interface (Europe)
• CAN (German, Bosch, we will discuss in detail)
• Interbus (German, Phoenix Contract)
• ModBus (America, Modicon)
• Profibus (German, Siemens)
• EtherNet (America, AB)
• Controlnet (America, AB)
• Etc.

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Controller Area Network (CAN)

• Controller Area Network (CAN) is a fast serial
  bus that is designed to provide
• an efficient,
• Reliable and
• very economical link between sensors and
  actuators.
• CAN uses a twisted pair cable (dual-wire) to
  communicate at speeds up to 1Mbit/s (max) with up
  to 40 devices.
• It originally developed to simplify the wiring in
  automobiles.
• CAN (fieldbuse) are now used in machine and
  factory automation products as well.

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CAN features

• 1) Any node can access the bus when the bus is
  quiet.
• 2) Non-destructive bit-wise arbitration to allow
  100 use of the bandwidth without loss of data
  (example)
• 3) Variable message priority based on 11-bit / 29
  bit packet identifier
• 3) Peer-to-peer and multi-cast reception
• 4) Automatic error detection, signaling and
  retries
• 5) Data packets 8 bytes long
• 6) Asynchronous communication (Even Triggered)

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CAN architecture
CAN Station 1
CAN Station 40 (max)
CS1
CS2
CS3
CS40
.
CAN Bus
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Working of the CAN network example
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Tradeoff CAN bus versus point-to-point
connections

• By introducing one single bus as the only means
  of communication as opposed to the point-to-point
  network, we traded off the channel access
  simplicity for the circuit simplicity
• Since two devices might want to transmit
  simultaneously, we need to have a MAC protocol to
  handle the situation.
• CAN manages MAC issues by using a unique
  identifier for each of the outgoing messages
• Identifier of a message represents its priority.

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CAN message format
Supports only 11 bit identifier
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Extended CAN message format
Supports 29 bit identifier
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Physical CAN connection
ECU (Electrical Control Unit)
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Implicit collision handling in the CAN bus

• If two messages are simultaneously sent over the
  CAN bus, the bus takes the logical AND of all
  them
• Hence, the messages identifiers with the lowest
  binary number gets the highest priority
• Every device listens on the channel and backs off
  when it notices a mismatch between the buss bit
  and its identifiers bit.

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Implicit collision handling in the CAN bus
example
Node B notices a mismatch in bit 3 on the
bus. Therefore, it stops transmitting thereafter
1
1
1
BUS
0
0
0
0
0
0
1
1
1
Node As message-ID
0
0
0
0
0
0
1
1
1
1
Node Bs message-ID
0
0
0
0
0
Unlike the MAC protocols we learnt, in CAN a
collision does not result in wastage of
bandwidth. Hence, CAN achieves 100 bandwidth
utilization
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Other applications of CAN

• 1) Concrete State Monitor Control Ssytem

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Other applications of CAN

• 2 ) MRI Cooling System

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Other applications of CAN

• 3) Tram Energy Recycle System

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References

• http//www.fieldbus.com.au/techinfo.htmTop
• http//www.esd-electronics.com/german/PDF-file/CAN
  /Englisch/intro-e.pdf
• http//www.eng.man.ac.uk/mech/merg/FieldbusTeam/Fi
  eldbus20Introduction.htm_Toc487265349
• In-Vehicle Network Architecture for the
  Next-Generation Vehicles Syed Masud Mahmud,
  Sheran Alles
• http//www.can-cia.de/
• http//www.icpdas.com/products/Remote_IO/can_bus/a
  pplication.htm