Title: CprE 458/558: Real-Time Systems
1CprE 458/558 Real-Time Systems
-
- Controller Area Network Overview
- (Updated by Ki-sung Koo, CprE 458/558 TA)
2Intra-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)
3Intra-vehicular communication
- An example of intra-vehicular communication.
4Intra-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
5How 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
6Fieldbus Networks basic motivation
Why use Fieldbus Networks? To avoid this
Figure 1 Traditional Wiring - two pairs of cables
can substitute all typical connections.
7Real-Time Communication Architecture
- Three different communication networks in
real-time application.
8Intra-vehicular communication
- A schematic diagram of a current in-vehicle
network
Smart Junction Box
9Fieldbus 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.
10Fieldbus 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.
11What constitutes a Fieldbus?
The specification of a Fieldbus should ideally
cover all of the seven layers of the OSI model.
12Fieldbus 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.
13What 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.
14Controller 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.
15CAN 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)
16CAN architecture
CAN Station 1
CAN Station 40 (max)
CS1
CS2
CS3
CS40
.
CAN Bus
17Working of the CAN network example
18Tradeoff 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.
19CAN message format
Supports only 11 bit identifier
20Extended CAN message format
Supports 29 bit identifier
21Physical CAN connection
ECU (Electrical Control Unit)
22Implicit 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.
23Implicit 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
24Other applications of CAN
- 1) Concrete State Monitor Control Ssytem
25Other applications of CAN
26Other applications of CAN
- 3) Tram Energy Recycle System
27References
- 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