Keith Browning Posted July 7, 2012 Share Posted July 7, 2012 What exactly is the purpose of the 120 ohm termination resisters in a J1939 network? Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted July 7, 2012 Share Posted July 7, 2012 To load the circuit and pull the voltage from 5v to zero. There is not enough time for nature to do this since pulses may be as short as 100 microseconds (1/10,000 of a second). They are not needed in a J1708 bus because it runs at a much slower speed. Quote Link to comment Share on other sites More sharing options...
Brad Clayton Posted July 7, 2012 Share Posted July 7, 2012 We use these in the ski industry on the lift towers for the communication setup from the top lift shack to the bottom lift shack. Quote Link to comment Share on other sites More sharing options...
jimmy57 Posted July 8, 2012 Share Posted July 8, 2012 When I had CAN training 15 years ago as prep for training a new car launch they listed termination resistor prime function as a load for suppression reasons. Routing of CAN pairs near some higher amp pulsed signal wiring could create induced signals but the stray signal strength is almost always below the 60 ohm connection between the CAN pairs. Error handling strategies can deal with odd irregular crap but continuous spurious signals will take down the communication. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted July 8, 2012 Share Posted July 8, 2012 I believe you are correct, and there are several reasons for terminating resistors: From an IH tech paper by Joe Bell: Terminating Resistor Another difference between J1708 and J1939 is the necessity to terminate each end of the J1939 backbone with a resistor. Each terminating resistor has a value of 120 Ohms. This is a “real” resistance, which can be measured using an Ohmmeter unlike the imaginary characteristic impedance mentioned earlier. One end of the backbone is located inside the cab near the fuseblock. The terminating resistor inside the cab is located in a connector cap and sealed with heat-shrink. This resistor is the same as that used on previous models of International medium and heavy vehicles equipped with J1939. The other end of the backbone is located near the engine controller on manual transmission vehicles or near the transmission controller on automatic transmission vehicles. The terminating resistor outside of the cab is contained in a cylinder shaped connector cap on vehicles equipped with shielded J1939/11 cable. This connector cap is directly attached to the J1939 Y-splice near the engine controller (with manual transmissions) or near the automatic transmission controller on vehicles with automatic transmissions. On vehicles with non-shielded J1939/15 cable, the terminating resistor outside the cab is identical to that used inside the cab and is located near the engine controller (with manual transmissions) or near the automatic transmission controller. Terminating resistors are used on J1939 to minimize a phenomenon called standing waves. Standing waves can be thought of as reflections or echoes. These reflections can cause major problems on a communications system. Like an echo, a standing wave occurs in time after the original transmission. Trying to communicate on a system with standing waves present is similar to being at an outdoor event with several loud speakers spaced far apart such as those at a racetrack. It is sometimes difficult to understand the announcer because what is currently being broadcast from the speaker closest to you is mixed with the delayed sound waves from loud speakers farther away. What ends up at your ears is a garbled, unintelligible mess of sounds. If standing waves are present on the data link, devices can’t tell what is a reflection and what is the next piece of information. Terminating resistors cause the signal energy to be absorbed leaving no energy for reflections. To a high frequency source, each 120-Ohm terminating resistor looks like an infinitely long data link cable with a characteristic impedance of 120 Ohms. In addition to minimizing standing waves, the terminating resistors also provide a relatively low resistance path for current to flow between CAN_H and CAN_L. This permits capacitance in the system to discharge rapidly. The length of time for a capacitor to discharge is directly proportional to the amount of resistance the capacitor is discharging through. The higher the level of resistance the capacitor is discharging through, the longer it takes for a capacitor to discharge. If the system capacitance cannot rapidly discharge when a device is trying to transmit a low level, the voltage differential between CAN_H and CAN_L will remain at a high level. This false high level may be interpreted by the device that is currently transmitting as a “stop transmitting, I’ve got something more important to say” command from another controller. This process is called arbitration and is covered in more detail in a later section. Experience has shown that if only one J1939 terminating resistor is missing, the vehicle will likely not exhibit any observable symptoms. The shape of the waveform changes slightly due to longer capacitance discharge times and increased signal reflections. However, if both terminating resistors are missing, no communication is possible. Figure 5 shows what the waveform for one logic-high bit on J1939 looks like with both termination resistors present. Figure 6 shows the same J1939 data link with both termination resistors missing. When both termination resistors are missing, the length of time for the differential voltage between the conductors to decay interferes with the arbitration process, which causes all communications on the J1939 data link to cease. If anyone wants the whole paper it's 140 pages of interesting reading on how an IH multiplexing system works from one of the engineers who designed the system. I can e-mail it to you. Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted July 8, 2012 Author Share Posted July 8, 2012 Terminating resistors are used on J1939 to minimize a phenomenon called standing waves. Standing waves can be thought of as reflections or echoes. These reflections can cause major problems on a communications system. Like an echo, a standing wave occurs in time after the original transmission. Trying to communicate on a system with standing waves present is similar to being at an outdoor event with several loud speakers spaced far apart such as those at a racetrack. It is sometimes difficult to understand the announcer because what is currently being broadcast from the speaker closest to you is mixed with the delayed sound waves from loud speakers farther away. What ends up at your ears is a garbled, unintelligible mess of sounds. If standing waves are present on the data link, devices can’t tell what is a reflection and what is the next piece of information. Terminating resistors cause the signal energy to be absorbed leaving no energy for reflections. This is what I was looking for. I read something that simply mentioned "signal reflections" and the above statement clarifies it pretty well! Thanks Bruce! Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted July 8, 2012 Share Posted July 8, 2012 I'm not sure that "imaginary" was a good word to use to describe impedance. It conjures up a mental image of "magic" (pun intended)... which it most certainly isn't. mechanic@agt.net should get a copy to me, if you please. Thanx. Quote Link to comment Share on other sites More sharing options...
GregH Posted July 9, 2012 Share Posted July 9, 2012 If anyone wants the whole paper it's 140 pages of interesting reading on how an IH multiplexing system works from one of the engineers who designed the system. I can e-mail it to you. May I please have a copy? holekampg@bellsouth.net Thanks! Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted July 9, 2012 Author Share Posted July 9, 2012 You have my address Mr. Amacker! Please include me in your sharing sir? Quote Link to comment Share on other sites More sharing options...
Brad Clayton Posted July 9, 2012 Share Posted July 9, 2012 ditto, thank you very much. Quote Link to comment Share on other sites More sharing options...
Jeff Clyde Posted July 10, 2012 Share Posted July 10, 2012 me too please ! jclyde69@msn.com Quote Link to comment Share on other sites More sharing options...
mchan68 Posted July 21, 2012 Share Posted July 21, 2012 Ditto for me too please. michael_chan_1228@yahoo.ca Quote Link to comment Share on other sites More sharing options...
ktmlew Posted July 22, 2012 Share Posted July 22, 2012 ktmlew @ yahoo.com please? Quote Link to comment Share on other sites More sharing options...
robp823 Posted September 12, 2012 Share Posted September 12, 2012 bobp823@aol.com please please! Quote Link to comment Share on other sites More sharing options...
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