Can Bus communication Protocol

CAN BUS
&
ALMA AMB

ALMA - ADE 1

General Information
•  History
The abbreviation CAN stands for Controller Area Network. This bus
system was developed by the Robert Bosch GmbH in the 1980s.
The original area of application was the automotive sector. The
reason for developing was the increasing share of controller and
automating technology (ABS, ASR) that required a communication
between different components. The foundations were laid by the
high quantities and a wide distribution. This caused that meanwhile
CAN is applied in wide areas that need a field bus.

11/06/2012 ALMA - ADE 2
Pedro Campana
Antenna Control Specialist

General Information
•  Basics
The Controller Area Network (CAN) is a serial communications protocol
which efficiently supports distributed real-time control with a very high level
of security. Its domain of application ranges from high speed networks to
low cost multiplex wiring. In automotive electronics, engine control units,
sensors, anti-skid-systems, etc. are connected using CAN with bitrates up
to 1 Mbit/s. At the same time it is cost effective to build into vehicle body
electronics, e.g. lamp clusters, electric windows etc. to replace the wiring
harness otherwise required.

Some characteristics:
• Minimization of wiring effort
• High error safety (fail-safe), robustness
• Small latency time (i.e. time between desired start of sending and actual start of
sending is as small as possible)
• Distributed systems, several receiver
• Good extensibility
• Priorization of messages
• Lower-cost

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Overview ISO / OSI Layer Model
Layer 7:
Execution of the field bus tasks

Layer 2:
Bus access, frame format and testing,addressing

Layer 1:
Definition of transmission medium and plug-andsocket
connection, level, coding, bit rate

Out of the 7 layers, only the blue colored ones ( 1, 2, and 7 ) are considered for field busses

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Overview ISO / OSI Layer Model

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Description of CAN
•  OSI Layer 1: “Physical Layer”

In principle, CAN is designed for serial data transmission in a bus topology.
But the CAN specification doesn’t determine the transmission medium. In
the ISO standards it was planned that the transmission medium for CAN is
an electrical differential two-wired line. However, in case of an error the
transmission can also take place with a one-wired line and the same
reference potential (ground). The advantage of the differential voltage
transmission on a two wired line compared to a one-wired line is the low
liability to potential differences and interferences on the line.

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Description of CAN
•  OSI Layer 2a: “MAC=Media Access Control”
The CAN bus is using CSMA/CA (Carrier Sense Multiple Access / Collision
Avoidance) as access method. It is working with the following scheme:
–  Carrier Sense
Each participant that wants to send listens if the bus is occupied ( carrier sense)
and starts the sending process when the bus was idle for a certain time.
–  Multiple Access/Collision Avoidance
There is the possibility that several participants start to send at the same time
( multiple access). The resolving of an eventually appearing collision is carried
out as part of the arbitration process. Hereby all the affected participants start by
putting their frames (starting with the identifier) bit by bit on the bus. After each bit
the sender checks if the actual level on the bus corresponds with the level that it
applied on the bus. If not it stops sending immediately. This results that message
with the highest priority gains the bus access without the destruction of the
message (collision avoidance).

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Description of CAN
•  OSI Layer 2b: “LLC-Logical Link Control”
The communication on the bus is based on four different frame formats:
1.  Data Frame
The data frame transfers data from a transmitter to one or more receivers on the data source’s
( transmitter) initiative.
2.  Remote Frame
A participant of the bus uses the remote frame to trigger the sending of a special data frame
with the same identifier by a data source. Data frame and remote frame only differ in a varying
RTR bit ( RTR bit of remote frame =1, RTR bit for data frame=0 ) and a missing data field of
the remote frame. This results that the data frame will prevail when there is a simultaneous
request and transmission of a frame.
3.  Error Frame
If a participant of the bus recognizes an erroneous frame ( e.g. wrong checksum in CRC field),
it reports this by sending an error frame. Therefore a sequence ( 6 bits with the same polarity)
that isn’t permissible during normal operation is switched on the bus. This implicates that all
the other ( fully functional node) switch themselves an error frame on the bus as the bit stuffing
rule was violated. An error frame is terminated with 8 recessive bits.
4.  Overload Frame
An overload frame is used to enlarge the spacing between two frames. It can only be sen dat
the beginning of the spacing. By switching of 6 dominant bits (overload flag) on the bus the
defined form of the spacing is destroyed and overload is signalized. As a result all the nodes
are also sending overload frames.

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Description of CAN
•  Frame 2.0 A Standard

Start of Frame: 1 dominant bit to identify
start
Arbitration field: 11 bit identifier + 1 bit
RTR (remote transmission request bit)
Control field: 2 bits reserved for extended
CAN, 4 bits for data length
Data field: 0-8 byte of data possible,
starting with MSB
CRC-field: 15 bit generator polynom
(x15+x14+x10+x8+x7+x4+x3+1) + 1
recessive CRC delimiter bit
ACK-field: 1 bit ACK-slot + 1 bit ACK
delimiter (recessive): transmitter sends
recessive bit in the slot; recipients that
received error free, send a dominant bit in
the slot.
End of Frame: 7 bit

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Description of CAN
•  Frame 2.0 B Extended Format (Used in ALMA)

SOF: Start of Frame
SRR: Substitute Remote Request BIT
IDE: Identifier Extension Bit
RTR: Remote Transmission Request BIT
r1: Reserved bit
r0: Reserved bit
DLC: Data Length Code

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Description of CAN
•  OSI Layer 7: Applications
While layer 1 and 2 are already intended for international standardizations (ISO-DIS
11 898 and ISO-DIS 11 519), there is not yet a norm for the application layer. The
introduction of a layer 7 the application process can be completely uncoupled from
the communication processes.

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Safety Mechanisms
In order to reduce the fault liability of the CAN bus the following safety
mechanisms are realized:

1.) Bit Monitoring Each transmitting network node monitors whether or not the bus level it has
transmitted is actually present on the bus. Otherwise a fault has appeared that has to be
dissolved or the sending process has to be terminated (arbitration phase).

2.) Frame Format Check As the frame format is fixed, every participant of the bus can check
whether this format is maintained ( e.g. recessive delimiter bits). Otherwise a fault is existent.

3.) Cyclic Redundancy Check The transmitted message is secured again by CRC. Thereby the
message is divided by a special polynomial generator and the result is deposit in the CRC
field. The message is divided again by this polynomial generator by the receiver and the result
is compared with the transmitted CRC field. If there are not identical a transmission error has
appeared. (polynomial generator x15+x14+x10+x8+x7+x4+x3+1)

4.) Bit-Stuffing The CAN protocol uses the NRZ bit coding ( non-return to zero). But this means
that there are longer phases without bit edges for synchronization. To avoid this an inverted
stuff bit is set after 5 bits of the same polarity. This bit permits the synchronization of every
node and is deleted when received.

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Bus Topology
•  Generalities
The cables can be parallel bus, twisted and / or shielded, depending on
requirements of the electromagnetic capacity. The topology of the wiring
should be as close as possible to structure a single line, to minimize
reflections. The segments of cable for connecting nodes in the bus should
be as short as possible, especially at higher bit rates.
The bus topology is with derivations short length. With loss of benefits in
terms of speed or maximum length structures can be taken into star. The
bus closes at the ends with load impedances (see Figure 3). The use of
differential voltages enables networks CAN work when one of the signal
lines are separate

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Bus Topology
•  Generalities

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Bus Topology
•  Signal Level
In the original specification of CAN, the physical layer was not defined, allowing different options
for choosing the means and levels of electric transmission. The characteristics of the electrical
signals on the bus were later established by the ISO 11898 standard

Absolute levels of bus lines with respect to ground, according to the ISO 11 898
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Bus Topology
•  CAN Conector

Male

Female

Connector used in ALMA
1 RST Global Slave Node Reset, Line A
2 CAN_L CAN_L bus line (Dominant Low)
3 CAN_GND CAN Ground
4 TE Time Event Signal A
5 CAN_SHIELD CAN Bus Shield
6 RST Global Slave Node Reset, Line B
7 CAN_H CAN_H bus line (Dominant High)

11/06/2012 ALMA - ADE 8 TE Time Event Signal B 16
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Antenna Control Specialist 9 N/A Not Used

ALMA Monitor and control System
•  Purpose

  This is the Networking specification for communicating with ALMA electronic
modules.
  Reading status (Monitoring)
  Sending commands and settings (Controlling)

Components and structure

“IFP0”
“DA41”
“IFP1”

“PM02”

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AMB: ALMA Monitor and Control BUS

It’s the networking specification for the communication with ALMA electronic
modules.

El BUS:
• CAN - Controller Area Network, Industrial communication standard. In our case wea are
mainly using the profile CAN 2.0B.
• Twisted pair cable 100 Ω, Terminator Resistor in both end of bus 120 Ω.
• Transition Speed 1 Mb/s
• Maximum Bus Length BUS 35 meters.
• Differential Signal

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•  ALMA node location in the BUS
Each device connected to the Bus must have an unique identifier, this identifier is a decimal number between 0
to 2047, for example in our system we have a standard number of ID in order to identify every device in the BUS:

Node Address (ID)
Device Name
Hex Dec
0x00 0 Antenna Control Unit ACU
0x29 & 0x2A 41 & 42 Production IF Processor (0x29 is Polarization 0)
0x50 – 0x53 80 - 83 DTS Transmitter Module DTX
0x40 – 0x43 64 - 67 LO2 synthesizer
0x60 & 0x61 96 & 97 BE antenna analog and digital power supply

http://edm.alma.cl/forums/alma/dispatch.cgi/icd/showFile/100149/d20070213045555/Yes/AMB+Node+allocations.pdf

11/06/2012 ALMA - ADE 20
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•  RCA (Relative CAN Address)

The address RCA is the memory address to which we access to read (Monitor Point) or write
(Command Point) in one device. RCA addresses are defined in the document ICD (Interface
Control Document) for each device.

For example:

In the ICD (ALMA-57.03.00.00-70.35.30.00-A-ICD) are defined the monitor and control points of
the power supply.
Then the monitor point:

GET_MID_1_Voltage the address RCA is = 0x00005 (hex) with a length of 2 bytes.

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•  Message identifier structure (Message ID) in the AMB (CAN)
In the ALMA project we are using the CAN Bus profile 2.0B. This means that the message ID has
a 29 bits of length with 8 bytes of data :

If we want to read the voltage value of PSD, we should make the following :

Message ID = ((Node Address + 1)* 2^18)+RCA convert to HEX

GET_MID_1_Voltage = ((97+1)*2^18)+5 = 25690117 0x0188 0005

Identifier A Identifier B

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0x0188 0x0005 DATA= 0 CRC

0x0188 0x0005 D0= 02 D1=D3 CRC

Reply Data from slave = 0x02D3 ( 2 Bytes)
in decimal = 723
According the ICD the reply value should be divided by 30.088
Then we have the final value:
GET_MID_1_VOLTAGE = 724 / 30.088 = 24.029514 Volts.

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•  Timing signal TE (Time Event)
The TE is an electrical signal distributed to most electronic devices in ALMA, as a reference for
the execution of commands so that they are synchronized in time. The pulse TE is delivered every
48 ms.
TE does not necessarily signal is generated by the Master of the CAN BUS, but it can be
generated by any other node. The signal generator contains a transmitter RS485 which will keep
the bus idle state with a logical "0" (FALSE), and takes the bus to logic state "1" (TRUE)
periodically with a duty cycle between 1% to 25% , Currently is 12.5%. The cycle is defined in 48
ms.
100 % of cycle
(48 ms)

12,5 % of cycle
(6 ms)

Example of TE signal. The voltages levels should be according the standard RS485

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•  Time Event
By default, every command transmitted to a device over the AMB is effective immediately upon receipt; and
every monitor request should return the most recent data available at the time the request is received.
However, under some circumstances the effective times may be different, as described in the following
paragraphs. These commands are sometimes referred to as “time critical”.
When specified in an Interface Control Document (ICD), a specific command may be considered effective at a
later time than its time of receipt. In such cases, the command is associated with the timing event (TE) that
immediately follows receipt of the command. See Figure 6. In exceptional circumstances a time-critical
command may be active at a TE later than the TE that immediately follows the receipt of a command.
Typically this is done to allow the slave extra time to execute the command. These exceptions must be noted
in the ICD.
The monitoring and control system begin transmission of a "Frame" CAN command associated with the TE
considering that the transfer must be completed no later than 24 ms. , That is no more than 50% of the cycle
of the TE.

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•  Time Event

The monitoring and control system can initiate the transmission of the response of request associated with the TE
not later than the last 24 ms, and the transfer must be completed no later than 4 ms before the next TE. This
means that any response to a request for monitoring should be passed during a window of 20ms in the latter half
of the range of 48 ms. Between each TE

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AMB: Using the Oscilloscope

Then we'll use the oscilloscope for analyzing the behavior of the CAN BUS.

For CAN Bus analyzing we need use the following instruments:

–  Tektronic Oscilloscope DPO 4034
–  Differential CAN probe Tektronic P6246
–  Connector to CAN BUS
–  Pen Drive or Memory Stick

In order to make the measurements is necessary connect the differential probe to the point 2 and 7 of DB9
connector, for this is necessary open the bus and connect the special connector in order to maintain the daisy
chain of the bus (see picture).

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n

AMB: Uso de Osciloscopio

•  Setting the Oscilloscope
• Select Chanel
• Connect differential probe to channel #1
• The impedance is 50 Ohms default
• Coupling = DC
• Invert = Off
• Band With = Full
• Vertical = 1.00 V/div
• Offset = 0 V (Press button MORE )
• Position = -2.5 div (Press button MORE)
• Hz. Scale = 1.0 ms

• Select button B1
• Source = B1(CAN)
• Threshold = 300 mV
• Bit rate = 1 Mbps

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AMB: Uso de Osciloscopio
Signal Type = Differential

Trigger = “Start of Frame
or

If you need to find a specific frame the you
can select “Identifier” and use your
message ID as a mask.
Always you should use the Extended
identifier.

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Bibliography and References
•  Paper “ANÁLISIS DE LA CAPA FÍSICA DEL BUS DE CAMPO CAN”, Ing. Hector Cashel y Ernesto Pinto
•  “Can Specifications”, Robert Bosch, BOSCH 1991
•  Lecturas ALMA, “Lectura 3”, Juan Pablo Caram, AIV EE.
•  ALMA Monitor and Control Interface Specification “ALMA-70.35.10.03-001-A-SPE”

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