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SAE TECHNICAL
PAPER SERIES
Dynamic Address Configuration in SAE J1939
Marvin L. Stone
Oklahoma State Univ.
The Engineering Society
For Advancing Mobility
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International Off-Highway &
Powerplant Congress & Exposition
Milwaukee, Wisconsin
September 8-10, 1997
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Fax:(412)776-5760
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Dynamic Address Configuration in SAE
Marvin L. Stone
Oklahoma State Univ.
industry initiated standards development for multiplexed
wiring in 1990 in conjunction with the establishment of
the
That working group has
focused on development of a standardized multiplex
communications protocol
11783) for agricultural and
forestry equipment.
ABSTRACT
SAE
is an emerging communications
protocol for on-board electronics in agricultural and
construction equipment as well as for heavy duty
highway equipment. The standard supports ECUs with
both statically and dynamically configured addresses.
This paper examines the dynamic addressing process,
the process for ECUs that select addresses during initial
power-up. A state diagram is proposed to implement the
process described in the standard. Simulation results of
systems with up to 100 dynamically configuredECUs are
presented. Use of a pseudo-random delay after a
power-ON self test was successful in minimizing bus-off
collisions and allowed initialization times of under 1
second for 30 ECU systems. Minimum bus-off states
during initialization of 30 ECU networks occurred with
pseudo-random delays having a range of to 15 ms.
An additional pressure to create a standardized
multiplex communications protocol for agricultural
equipment has been the recent market opportunities in
precision farming. Precision farming systems require
inter-operation of electronic components of agricultural
implements and tractors. This requirement makes a
standardized communications protocol between tractor
and implement critical.
Early in the standards development process, US
agricultural and construction equipment manufacturers
decided to pursue a joint standard (SAE
with the
heavy duty truck and bus industry. Parts of that standard
have become working documents for the international
11783).
standard effort by the industry
INTRODUCTION
Electronic components have become common
on agricultural and construction equipment as equipment
manufacturers have attempted to build more functionality
and efficiency into their products. A natural consequence
of the proliferation of electronic components has been
the development of multiplexed communications
networks on-board the equipment. These networks allow
communicationbetween multiple electronic control units
(ECUs) over multiplexed or shared communications
wiring. Multiplexingsimplifies wiring and lowers total
system costs while providing a means to share sensor
values and other data. On the other hand, multiplexing
increases the complexity of the electronics systems, as
some technique must be implemented for controlling
access by each of the ECUs to the shared wiring. A
further complication is the need for ECUs to have some
means of unique identity on the shared network.
SAE 51939
SAE
specifies a communications protocol
which is intended to allow communication between
electronic control units on-board a vehicle. Figure 1
shows a schematic of
agricultural application
of the network with a tractor and implement. SAE
(SAE, 1997) is based on CAN
(Bosch, 1991)
protocol with 29 bit identifiers. In this protocol, messages
are sent serially on a multiplexed data bus with a 29 bit
message identifier and including an 8 byte data field.
defines the structure of the identifier
SAE
and creates a physical addressing scheme.
Each ECU connected in a network must have a unique 8
bit address. CAN requires that identifiers used in
messages on the network be unique for proper
specifies that the address of
arbitration. The
the sending ECU (source address) be included in the
identifier of each message. The identification of the data
is included elsewhere in the identifier. This allows any
ECU to send any data. A consequence of the
requirement of an address for ECUs is that an address
must be assigned at manufacture or some means of
Efforts by agricultural and construction
equipment manufacturers to control product costs have
also affected development of multiplexed wiring systems.
Higher volumes of electronic components reduce cost
and have driven standardizationof multiplexed wiring
systems. The construction and agricultural equipment
I
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address assignment must be included in the network
protocol.
,
Self Confi urable
ECU A
Initialize
Messages
Time
Name
Address Claim
Figure
ECU B should select
a new address and
attempt to claim it
Tractor Implement Implementation of SAE
An initialization process is provided in
the Network Management document, to allow ECUs to
secure an address during initialization. The process
uses no centralized master and relies on each ECU to
manage its address. The process allows ECUs with hard
coded addresses to co-exist with ECUs which secure
their address dynamically during initialization.
ECUs require some form of unique identification
for the purposes of securing and arbitrating an address.
This identification is standardized as a NAME, an 8 byte
numeric quantity which includes a manufacturers code
and a manufacturer assigned identity number.
Manufacturers must assure that the NAME value is
unique within an interconnected system.
Figure 2 shows the message traffic and
sequence that occurs during an initialization process
when two ECUs attempt to secure the same address.
The resolution of the conflict is shown. After a power-on
self-test (POST), ECU A sends an address claim
message containing the address it is claiming and its
NAME. ECU B completes its POST at a later time and
attempts to claim the same address, not hearing the
original claim from ECU A. In this event, ECU A
compares the value of its NAME with the contender and
reclaims its address because it has a lower value (higher
priority) NAME. Since ECU B is self-configurable, it must
seek a new address and attempt to claim it. If ECU B is
unable to select or find a different address, it will send a
Cannot Claim Address message and be in a state of not
being able to claim an address.
Figure 2. Message sequence for a self-configuring ECU
(SAE
The CAN protocol in SAE
uses a
prioritized arbitration process to allow messages access
to the bus. When two messages are sent at the same
time, their identifiers are imposed bit-serially onto the
bus. The bus must be designed to allow dominant bits to
overwhelm recessive bits when both are applied by
different ECUs on the bus. This is described in SAE
the Physical Layer specification for
construction and agricultural applications. No conflict
occurs as long as the ECUs are sending the same bits,
but when one sends a recessive bit while the other sends
a dominant bit, the bus state is dominant. The ECU
sending the recessive bit must sense the bus is at a
dominant state when the bit was sent and must cease
transmitting the message at that time until the next time
the bus becomes idle. This strategy allows more
dominant identifiers, those with a lower value, to have a
higher priority on the bus. To allow this feature to work
properly, CAN synchronizes messages at the beginning
of each transmission to assure bits are aligned.
The prioritized message arbitration mechanism
built into CAN is only performed during the identifier
portion of the message. A consequence is that a
message error occurs when two messages are sent with
the same identifier at the same time. Both identifiers are
sent without conflict, but when the data are sent
immediately following the identifier, the data of both
together resulting in a garbled
messages are
transmission. This is detected by the receivers as a bad
checksum in the message, and signaled to the
transmitters via an error frame. The transmitter in most
protocol controllers automatically re-sends the message.
Unfortunately, both of the senders repeat this process.
The senders keep an error count and after repeating the
message 32 times with an error, both senders assume a
bus-OFF state and cease transmitting. In most protocol
controllers, the ECU must reset the protocol controller to
allow further transmission.
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The possibility exists in applying the SAE
specification for the bus-off condition to occur if
two self-configuring ECUs attempt to claim the same
address at the same time. If the ECUs have different
NAMES, they generate the same identifier, an address
claim for the same address followed by different data.
The data in the address claim message is the ECUs
NAME which should be unique. In this event, the
standard recommends each ECU retry the claim process
after a pseudo-randomdelay of to 256 message
lengths. This process would be expected to work well
when there are few ECUs attempting to claim the same
address at the same time.
INITIALIZATION STATE DIAGRAM
A state diagram can be constructed to describe
the address configurationprocess within a single
configuring ECU as shown in Figure 3. The state
diagram is entered after power-up. A power-ON self-test
is assumed, and the ECU then selects an address to
claim. This address should be the ECUs preferred
address
and may be a single fixed address as
or may be computed or selected in a
found in
different manner as specified for a particular application.
For agricultural and construction applications,
specifies application specific requirements including
computationof addresses for self-configuring ECUs.
Address
Selecting
'Bus-Off'
different from
Address Claim
received during the 250 ms period or afterward, the ECU
must compare its NAME with the contender. The ECU
with the lower valued NAME may re-claim its address,
while the ECU with the higher valued NAME must select
another address and attempt to claim that address. The
ability to select an alternate address and claim it is the
defining characteristic of a self-configurableECU. If the
ECU cannot find another address, the ECU sends a
cannot claim address message and waits.
This dynamic address configuration process is
likely to be used with agricultural implements.
Implements may be detached and moved from tractor to
tractor and multiple implements may be connected at
once. The nature of agricultural implements required
either some mutually exclusive addressing system or a
dynamic configuration process. The later process is now
being written into
783 and will be included in SAE
To improve the reliability of the process, the
current strategy proposed for
11783 is that ECUs
should use the last successfully claimed address as a
preferred address. The result of this constraint is that
address contentions may occur when a new
configuration of implements is connected, but should not
occur if no changes in configuration have been made.
There is some concern regarding the initialization
time required with a dynamic address configuration
process using the strategies described above. Some of
the implements for both agricultural equipment and for
construction equipment may consist of many nearly
identical ECUs. An example of this type of system might
be a planter where each row might be configured with a
single ECU. Current ECU costs probably prevent large
numbers of ECUs on an implement, but as ECU costs
drop, a 30
30 ECU planter would be possible. A
planter with this many ECUs initializing the could present
long initialization times and merits further study.
Without a mntending
INITIALIZATION SIMULATION
address
Contention
And Receiving
greater than mine
/AddressClaim
for Cannot
Cannot
Address
A message by message simulation was
developed to allow computation of initializationtime and
evaluation of the initializationprocess. The simulation
was based on the state diagram in Figure 3. A listing of
the time step subroutine of the simulation is included in
the appendix. The following assumptions were used:
1.
Time granularity was assumed to be one message
time. Message data lengths were assumed to be 8
bytes. This is consistent with
which
currently uses an 8 byte message for address
claim. Eight byte messages are approximately 125
bits long and at the 250 kbit per second rate used
require approximately 0.5 ms per
in SAE
message.
2.
The set of ECUs simulated were all self-configuring
ECUs.
Address
Figure 3. State diagram for Self-configuring ECUs
After address selection, the ECU sends an
address claim message and waits for any contending
claims for the same address. If none are received for a
period of 250 ms, the ECU may begin normal
communications on the network. If a contending claim is
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3.
4.
Only the power-ON self-test (POST) time and
initialization traffic was simulated. lnitialization
times will be longer as ECUs which have initialized
consume significant bandwidth with normal traffic.
This issue should not be a problem for the ECUs
involved in the claim process as they are required
to wait 250 ms before transmitting normal message
traffic after a claim. lnitialization time after the
POST was found to be less than 250 ms. (Total
times were under 550 ms including an assumed
300 ms POST.)
the POST time plus the 250 ms delay before normal
network traffic is begun.
A potential addition to the current proposed
requirements in the standard was investigated. That
addition would be to recommend that ECUs constructed
by the same manufacturer with nearly identical POST
times add a pseudo-random delay after POST before
issuing an address claim message. This delay would
minimize the probability that all of the ECUs issued the
address claim at the same time.
Bus-OFF times were computed as 32 message
times plus 9 bit times per message for error frames.
The Bosch CAN specification (Bosch, 1991)
specifies error frames will be 6 to 12 bit times in
length. An average number was used.
5.
ECUs claimed an address at random between 128
and 247 as a preferred address. This strategy has
been proposed at the
working group
11783. Later initializations of the
developing
system would use the addresses determined on an
early power-up. No address contention would
occur and the process would not require the long
initialization process simulated here.
6.
A 5 message time delay (2.5 ms) was assumed to
occur in an ECU before it attempts to claim a new
address after losing arbitration. This process is
shown in Figure 2. and is a conservative estimate
of the time between the first and second address
claim by the self-configurable ECU.
7.
ECUs do not use any address claims received
during pseudo-random delays to improve their
address selection by eliminating used addresses.
Logging of address claims by other ECUs during
pseudo-random delays while initializing could
reduce the chances of unnecessary arbitration.
The more conservative but less efficient path of
ignoring previous address claims was used here.
8.
Twenty five initialization cycles were simulated to
allow average times to be determined.
9.
The total average initializationtimes reported do
not include the 250 ms wait after claim to assure no
conflicting claim. This time must be added to the
total average initialization time for actual systems to
determine the time at which normal network traffic
can take place.
Figure 4 shows the effect of the number of
ECUs on the total time required for all ECUs to initialize.
In addition, the number of bus-OFF events was also
determined. For 10 or fewer ECUs, the average number
of bus-OFF events was low and initialization time was
essentially equal to the POST time plus the time to claim
and address. For 10 or more ECUs, the initialization
times become significantly longer. For 10 or fewer ECUs
the total initialization time for an actual system would be
Initialization Characteristics For SAE
Identical POST times
2
0
0
10
20
30
40
60
70
90
I
Number of ECUs
Figure 4. Effect of the number of ECUs on lnitialization
time
A potential method of reducing the number of
bus-OFF states and minimizing initialization time would
be to add a pseudo-random delay before transmitting the
initial address claim. This method reduces the probability
of simultaneous claims for the same address. The
simultaneous claim problem would not be a problem for
ECUs constructed by different manufacturers as the
variation of the POST time would be expected to be on
the order of 100 ms or larger. The problem occurs when
a set of nearly identical ECUs are connected from the
same manufacturer. In this case, the ECUs would be
expected to all have the same POST time and would all
attempt to claim an address at the same time. As the
number of ECUs increase, the probability that ECUs
claim the same address at the same time increases.
Figure 5 shows the result of varying the pseudorandom delay with a network of 30 ECUs. The pseudorandom delay is inserted after the POST. A pseudorandom delay after POST of 15
nearly eliminated the
bus-OFF states that occurred. The slight increase in
bus-OFF states at 35 ms was due to a single event and
would be eliminated by increasing the number of
simulations. Increasing the pseudo-random delay after
POST beyond 10 ms increased the initialization time
slightly. A prudent selection of pseudo-random delay
after POST would appear to be 15 ms as this would
nearly eliminate bus-OFF states and provide a short
initializationtime. The average number of Bus-OFF
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states at a pseudo-random delay after POST of 10 ms
was
or that 7% of the time, one bus-OFF state
could be expected, while this level was or did not occur
in 25 tests at the 15 ms pseudo-random delay.
Interestingly, this time is the amount that would be
required for one message for each self configuring ECU.
lnitialization Characteristics for
Randomly varied POST tlme
T
lnitialization Characteristics for
of POST Std.
T
4W
a
POST
20
30
40
50
60
Number of ECUs
0
20
30
40
50
60
0
100
70
Pseudo-Random Delay (Maximum)
Figure 5. Effect of pseudo-random delay after POST on
lnitialization time
lnitialization Characteristics for
Number of ECUs
0
m
5
....
...
200
...
POST
....
10
20
30
40
50
70
80
90
100
Number of ECUs
Figure 6. Effect of a 15 rns pseudo-random delay after
POST on lnitialization time
Figure 6 shows the effect of a 15 ms pseudorandom delay after POST on lnitialization time. The ideal
selection of pseudo-random delay can be seen to be a
function of the numbers of ECUs involved in the dynamic
address initializationprocess. A selection of 15 ms may
be appropriate for 30 ECUs, but a larger delay is
indicated for a much larger set of ECUs.
Figure 7 repeats Figure 6 except with 50 ms
pseudo-random delay. The 50 rns pseudo-random delay
appears adequate for 100 ECUs. Bus-OFF states are
maintained below 0.5 occurrences per initialization cycle.
The necessary amount of pseudo-random delay to
maintain bus-OFF states below 1 per initialization can be
computed approximately by multiplying the number of
identical self configuring ECUs in a system by 0.5 ms.
80
.:I
Figure 7. Effect of Number of ECUs on lnitialization time
for a 50 ms pseudo-random delay after POST.
SUMMARY
The dynamic address configuration process
was examined and a state
included in SAE
diagram presented describing the process. Using that
process, a simulation was developed to evaluate the
average initialization time and number of bus-OFF states
that occurred during initialization. The simulation results
demonstrate that a pseudo-randomdelay should be
inserted after the POST for ECUs which are
manufactured to have the same POST time, and will use
dynamic address configuration. This recommendation is
especially appropriate for construction and agricultural
equipment applications where multiple instances of the
same type of ECU are being connected in a system. An
example of this type of system would be planters where
each row might use a single ECU.
lnitialization times for 100 ECU systems were
found to be less than 500 rns including a 300 ms POST
for systems using a pseudo-random delay after the
POST. A pseudo-random delay after the POST of 0.5
ms per ECU in the initializing system was found to be
adequate with negligible numbers of bus-OFF states
during initialization. lnitialization times reported here do
not include the 250 ms delay that must be used before
starting normal network traffic.
REFERENCES
Bosch, Robert GmbH. 1991. CAN Specification.
50, D-7000
Version 2.0. Robert Bosch GmbH,
1, Germany.
SAE. 1997. SAE Recommended Practices
SAE, Warrendale, PA.
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APPENDIX
Single Time Step Segment of
InitializationSimulation
Address As lnteger
'Address of ECU
Time As Integer
'Time at which ECU attempts claim
CAN-Arbs As lnteger
'Number of CAN arbitrations
Claimed As Boolean
'Whether ECU has claimed
Bus-OFF As Boolean
'Whether the ECU is Bus OFF
Bus-Off-Time As lnteger 'Message cycles left in Bus-OFF
End Type
of 150 ECUs)
'Module wide variables
Dim
As a-node
Sub time-step()
Dim This As lnteger
Dim Other As lnteger
For This = 1 To ECUs
If
= t Then
'Claim (Check other nodes for same address to
arbitrate or same time and same address for
collision)
ClaimSuccess True
If
= False Then
For Other 1 To ECUs
If
=
Then
Other) Then
If (This
If
= True) Then
'Must Claim New Address
'Assume the other node has a lower name (one has
to reclaim and it does not matter which)
= 128 + 119
'Prepare to reclaim new address after hearing other
ECU reclaim. Assuming 4 msg lengths until reclaim
=t+5
ClaimSuccess False
=
Then
'ECUs will go Bus-OFF
Busoff-Count = Busoff-Count + 1
'Set up ECUs to transmit again next message cycle
=t+1
t+1
'Set ECUs to Bus-OFF
True
True
'Set ECUs Bus-OFF counter
32
= 32
ClaimSuccess = False
End If 'If
True)
End If 'If (This
Other) Then
=
Then
'No conflict but one message has higher priority and
the other is delayed by CAN arbitration
Then
If
=t+1
+1
Else
t+1
+1
ClairnSuccess = False
End If
=
End If 'If
Next Other
Bus-OFF
False
Else
'Decrement the Bus-OFF counter
Then
False
False
Compute the random message delay after Bus Off
1 (this is not the delay before initial
as per
claim)
= t + 255
= + 255
'Select a new address assuming the worst, that the
node does not listen during the delay for further
address claims
= 128 + 119
128 + 119
Else
'Set up for the next cycle
t+1
t+1
End If 'If
Bus-Off-Time
End If
= False
ClaimSuccess
Time t
End If 'If
Next This
End Sub