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1、400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A.Tel: (724) 776-4841 Fax: (724) 776-5760 SAE TECHNICAL PAPER SERIES 1999-01-2840 Large Scale Application of J-1939 CAN Edward T. Heck, John Kitzerow and Tony Caravella HED (Hydro Electronic Devices, Inc.) International Off-Highway for con- struc
2、tion/agricultural, the communication media that is a twisted non-shielded quad (CAN_H, CAN_L, CAN_BAT, and CAN_GND). Figure 1. CAN cables. The 110 ohm termination resistors and twisting of the data pair make the data communication pair very EMI/ RFI tolerant. (Resistors should be sized for 400 mW) T
3、he use of the CAN_H and CAN_L concept for the com- munication pair also contributes to the robustness of the communication scheme. J-1939 also contains a message priority identifier in the first three bits of the 29 bit identifier field. A message of 000 is the highest priority. This priority featur
4、e allows the higher priority messages to displace “maintenance” messages. The J1939 protocol also includes collision detection and arbitration (CSMA/CD Carrier Sense, Mul- tiple Access with Collision Detection). Author:Gilligan-SID:1178-GUID:19160100-141.213.232.87 2 Figure 2. CAN pair showing O and
5、 1 states with maximum priority of “000”. With J-1939 there are five types of error detection: Bit error- bit received is not the value transmitted Stuff error- more than five consecutive bits of the same polarity CRC error- CRC is sent with data and is compared to calculated CRC Form error- fixed f
6、ormat fields are violated ACK error- no ACK is sent by receivers Fault confinement is embedded through the use of trans- mit and receive error counters. SAE J-1939 has sufficient capacity to handle navigation systems, radar or GPS. CAN 2.0B which has the added 29 bit identifier is the standard adopt
7、ed for J-1939. J-1939 systems are limited to 30 nodes per segment with a maximum backbone length of 40 meters per segment. The ECU stub must be less than 1 meter in length. Figure 3. J-1939 system layout. “REGISTERED” VERSUS “PROPRIETARY” J-1939 “Registered” addresses are described in SAE J-1939/71
8、VEHICLE APPLICATION LAYER and SAE J-1587 JOINT SAE/TMC ELECTRONIC DATA INTERCHANGE BETWEEN MICROCOMPUTER SYSTEMS IN HEAVY- DUTY VEHICLE APPLICATIONS. These addresses are the same for all systems. Many items on specific pur- pose heavy duty trucks and most Off-Highway vehicles have requirements not m
9、et by the “Registered” addresses. To address these requirements, the use of “Proprietary” addresses is adopted. To avoid conflict, it is strongly suggested that “Registered” and “Proprietary” codes not be used on the same segment. Vehicles with both types of code can accommodated through the use of
10、dual CAN systems. One system talks only to the “Registered” code nodes and the other system talks only to the “Proprietary” code nodes. Several J- 1939 CAN system manufacturers currently offer Bridges and/or ECUs that contain two J-1939 loops controlled by microprocessors that translate between the
11、two types of addresses. Figure 4. ECU with Registered and Proprietary capability. OTHER SAE INTERFACES, ETC. SAE J-1708 was issued in 1986. This 9.6 kbps CAN sys- tem is considered a Class B. It will support a minimum of 20 nodes over a 40 meter CAN backbone. This protocol was developed for heavy tr
12、ucks. SAE J-1850 was issued in 1988. The communication rates were either 10.4 kbps or 41.6 kbps. The standard was developed for Class B communication for cars. CAN KINGDOM is supported by the European based CAN Hydraulic Users Group. This variety of CAN is con- sidered to be a set of protocol primit
13、ives that are custom- ized by each user. CAN Kingdom reportedly allows J- 1939, Devicenet, and SDS on the same backbone. CAN 2.0B is formatted the same as J-1939 with a 29 bit identifier. Version B is backward compatible with J1850 and CAN 1.2. CAN 2.0B is run at speeds up to 1,000 kbps. There are a
14、lso supplier specific CAN systems such as Sauer-Sunstrands S-Net. These are designed to work with a specific manufacturers components. While this approach can be quite efficient for that manufacturer, addition of components from a different manufacturer is very difficult. Author:Gilligan-SID:1178-GU
15、ID:19160100-141.213.232.87 3 TYPICAL APPLICATIONS The J-1939 specification is initially being applied in vol- ume on trucks. The system is being used for engine, transmission, and ABS brake coordination. There is also a number of applications being introduced for Tractor to Trailer communications. F
16、igure 5. Tractor Trailer Application. Fire and rescue apparatus applications are a particularly appropriate application. The number of wires exiting a fire truck cab number in the hundreds. These wires can be eliminated through the use of J-1939. The wiring cost reduction and the reduction of troubl
17、e shooting time at the end of the line will pay for the cost of the J-1939 sys- tem. The additional capabilities of the J-1939 system adds significant benefits for the manufacturer, dealer, and truck owner. Figure 6. Typical Fire Aerial Ladder Application. Another application is the aerial work plat
18、form market. Operator stations both in the platform and on the ground as well as operation of the engine below rotation along with outriggers, etc. allows the CAN J-1939 system to reduce cost while improving operation characteristics. Figure 7. Aerial Work Platform Application. Road building equipme
19、nt is quite often offered in a multi- tude of variations as many options are added to the base machine. The use of J-1939 allows the pre-wiring of CAN and power for options and the addition of CAN mod- ules as need to implement the options. Often the soft- ware for all of the options will exist on t
20、he machine so that field additions do not require reprogramming. The use of node (module) identification through use of identification pins in the wiring harness activates the required soft- ware. Thus the modules can be the same part number and yet have different functions for specific locations. F
21、igure 8. Road Building Machine. Machines that maintain railroad tracks and beds require communications between cars as well as within the spe- cial purpose car. The number of nodes on a given car may be relatively low. However when the full work train is connected so that control of each car is coor
22、dinated with the main engine, the number of nodes can be quite signif- icant. The use of J-1939 allows cars to be added or dropped easily with automatic recognition of the trains new content. Author:Gilligan-SID:1178-GUID:19160100-141.213.232.87 4 Figure 9. Railroad Maintenance Machine. WHEN TO CAN
23、AND WHEN TO CAN CAN Today the hottest buzz word in Mobile Electrohydraulics is CAN. CAN is being considered in design where there is only a small advantage to using CAN at the best. If the number of nodes is three or less, consider other digital communication methods such as serial communi- cation o
24、ver a RS 232 or other system. If there will be a small number of systems built, the software development may not be justified. A typical example a low number of nodes is a crane with a LMI (Load Moment Indicator) where the LMI computer is mounted on the crane boom and the display is mounted on the t
25、ruck chassis mounted operators platform. The crane is a 380 degree design so that there were not any electrical slip rings. (If there were slip rings, the CAN system would provide superior signal transmission and a robust transmission error detection and checking.) An example of a project with a sma
26、ll number of machines would be a special purpose pipe handler that required 4 nodes but only two machines. WHERE IS CAN TODAY? SAE J-1939 is rapidly replacing J-1750 for communicat- ing with the registered components (engine, transmis- sion, and brakes). Because of the flexibility provided by “Propr
27、ietary” J-1939 it is the CAN of choice for Off-High- way machines for construction, material handling, for- estry, etc. Owner/operators as well as rental yards appreciate the additional features added to machines at a very nominal cost. The Heavy Duty On-Highway trucks are applying J1939 to communic
28、ations in the Tractor as well as for communi- cation with the trailer(s) where J-1939 is required for some ABS on trailers. Currently, J-1939 is moving down- ward to Medium Duty trucks used to mount equipment such as “Pin-on-Cranes”, etc. SAE J-1939 is the basis of several of the better recog- nized
29、 CAN communication protocols of large machinery manufacturers. Both the protocol sometimes referred to as “CAT CAN” and the protocol used by John Deere are like J-1939. Though, there is some discussion as to which came first. J-1939 is currently 250 kbps with several companies looking at the feasibi
30、lity of speeds up to 1,000 kbps for distances up to 40 meters. There has been little work to place CAN transmitting sen- sors on the CAN backbone because they often cost up to ten times more than existing analog sensors. Many of the sensor manufacturers have programs to change this over the next few
31、 years. Until then, sensors will be wired to nodes that will have a number of sensor inputs as well as other functions. Most CAN transmitting sensors avail- able today are either to allow the manufacturer to say “We make CAN compatible sensors.” Or, they are for market testing. MAKING J-1939 SYSTEMS
32、 SURVIVE IN THE REAL WORLD OF OFF-HIGHWAY EQUIPMENT The J-1939 backbone cable is probably the most reliable part of the machine. Because of the low design imped- ance, the high signal voltage levels, and the twisted pair, J-1939 is not very susceptible to interference from EMI/ RFI. Emissions from t
33、he CAN cable, however, are another matter. If the nodes are allowed to transmit messages with essentially vertical rise and fall of the pulse, the cable will perform as an efficient emitter. Addition of slew resistors to the transmit circuit will result in a modification to a trapezoidal waveform th
34、at has a much lower rise rate and thus much less RF emission from the CAN cable. Reverse voltage protection on the power input should be 1,000 volts with the same protection for each of the sourcing output drivers. This is needed to withstand Back EMF resulting from other components on the vehi- cle
35、. The module should be capable of surviving at least 60 VDC for 2 minutes at 25C to withstand cold weather jump-starts. The minimum operating voltage range is 9 to 32 VDC 0 to 5 VDC analog signal inputs should be capable of withstanding direct shorts to the battery. Digital inputs should be protecte
36、d if the connected to either + battery or battery. Outputs should be protected against shorts to + battery and battery. Negative spike protection to 1,000 VDC should be pro- vided on all outputs. For ease of field trouble shooting and training non-elec- tronic personnel, stick with sourcing outputs.
37、 While slightly higher cost, you will save far more dollars by not having to try to explain sinking outputs to someone over the telephone. Everyone can relate to the fact that if the trouble light is lit, the output is on. Most people have great difficulty in understanding that if the trouble light
38、at the output is on, the output is off and if the light is off, the output is on. Author:Gilligan-SID:1178-GUID:19160100-141.213.232.87 5 Figure 10.Sourcing and Sinking Circuits. While modules that operate in the temperature range of 40C to +85C with storage of 55C to +100C are com- monly available,
39、 the displays have a restricted tempera- ture range. The display electronics will operate in the same temperatures as any other modules. The actual display will be significantly restricted. Most manufactur- ers will add heaters to allow cold operation of the display. Only a few manufacturers will ad
40、d Peltier effect cooling modules to the display to provide higher operation tem- peratures. Without the cooling modules, the displays are limited to +50C to +70C maximums, depending on make. WHAT DOES CAN J-1939 MEAN TO YOU To the designer, J-1939 offers a proven fault tolerant communication system
41、that has the ability to incorporate modules from different manufacturers. Most module manufacturers have already developed the embedded code needed to have the inputs and outputs “do their thing”. For example, an output can be driven to operate as a PWM (Pulse Width Modulation) output with sophisti-
42、 cated control curves to limit “Jerk” (the derivative of acceleration) on swing applications. The designer may also have available to him software modules to use in creating the application specific software for his specific machine. As each CAN module manufacturer works on more and more different m
43、achines, more application soft- ware is placed in the module manufacturers library. As much as the designer may think that CAN J-1939 was created for him, the real winner at the equipment manu- facturer is the manufacturing department. The same module can be used in many places on many different mod
44、els. The common module takes on individual char- acteristics when pin identification is used. Pin identifica- tion is the use of on-off input pins wired to either battery or ground to tell the module what software it should use to operate in that particular position. The number of dif- ferent identi
45、fiable positions is limited by the number of input pins available. The number of different identifica- tions is 2 to the power of the number of available pins, less 1. Thus, eight pins yields 255 different positions. After the manufacturing manager gets over “one module does almost everything”, he c
46、an chalk up the savings resulting from the ease of installation. No longer must several hundred wires be pulled the length of the machine. In most cases, all but the CAN pair, power, and ground wires can be kept to an arms length. The heartbeat LED on the modules assures the trouble shooter that the
47、 module has power and is alive. A com- puter connected to the network can turn on or off any out- put and read the status of any input for trouble shooting. After determining that he is turning on the correct light, actual wiring trouble shooting is greatly simplified. The technician or assembler ca
48、n trace the short wire easily and check with a trouble light to be sure that power is in fact being supplied to the light socket as well as the out- put pin of the module. If there is power at the output pin and not at the light socket, replacing the wire is easy. The machine testing part of manufac
49、turing can be much simpler. The tester can actuate the inputs in order and the test computer connected to the CAN system can identify any failures or incorrect wiring. The computer then can activate each of the outputs and the tester can check to be sure that the function works. The Machine owner/operator wins big. The machine can have significantly increased automation and smarts. The computer used for CAN transmission will protect the machine from abuse and will notify the operator of required service intervals. The computer can notify the operator of faulty
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