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CAN messages are transmitted digitally as a series of low or high values within a fixed structure known as a frame. The smallest data unit within these binary encoded messages is a bit, logically representing either 0 or 1. A message identifier follows the start of the frame. Various values, including the data payload, and a checksum follow the identifier.
When a control unit receives a message, it calculates a checksum from the data payload and compares it to the value broadcast within the message. If the two are equal, the message is valid. The receiving control unit confirms this by transmitting an acknowledgement during the penultimate bit of the message broadcast.
Therefore, the broadcaster will know if a control unit has received an invalid message. The application determines the bus speed.
CAN gateways connect buses of different speeds or types. For example, an IC might act as an interface between the powertrain and convenience CAN buses to provide, amongst other things, automatic door locking functionality; e.
The convenience control module would then know to lock the doors once a certain speed has been reached. Gateways can also control diagnostic access. When present, diagnostic testers must establish communication with the gateway via the DLC. The gateway then passes diagnostic messages between the tester and the other control units.
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. This has the benefit of creating a low-pass filter for the common-mode noise on the network. Or is there another method to accomplish this. Even 2x 62 ohms E24 would be fine.
Better to go slightly higher than nominal rather than sightly lower, since the receiver IC will have a couple of kiloohms of effective input differential impedance which will act in parallel with your termination network, lowering the combined impedance seen by the line.
Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. CAN bus split termination impedance - no 60 ohm resistors?
Ask Question. Asked 5 months ago. Active 5 months ago. Viewed times. TonyM 9, 1 1 gold badge 16 16 silver badges 34 34 bronze badges.
You could also use two ohm resistors in parallel. But I would just use the Just to make sure everyone understands what you are proposing? You can edit your question then add the schematic using the built-in schematic entry tool look for a button with schematic symbols on it. Short answer is that if 60 works, I assumed as much but just wanted to be sure because i couldn't find an answer to this here or elsewhere.
It is no big deal. Glad you got your answer. Active Oldest Votes. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.
The Overflow Blog. Podcast Programming tutorials can be a real drag. Featured on Meta. Community and Moderator guidelines for escalating issues via new response…. Feedback on Q2 Community Roadmap. Related 6. Hot Network Questions.It is very common in our company that, the engineers don't care about or don't know the termination resistors on a CAN-Bus.
They just simply put one Ohm resistor between two or several CAN-Devices mainly for testing purpose. I know this is not standard, and I don't like this workaround. I tried to figure out why such a workaround is bad, but failed. It seems that many engineers use such a workaround. I found a document saying this workaround is a mistake, but it does not provide why.
It is a common mistake to use one Ohm resistor instead of two Ohm resistors. This does not work correctly, however. In another words, when will such a workaround fail to work? So I am still struggle with idiotic things, and busy in urgent projects.
Having a terminating resistor matching the characteristic impedance is for AC functionality - making sure there are no signal bounces. For a very short cable, a single ohm resistor would normally work. But may not be needed depending on propagation delay compared to your transfer rate.
For short cables, you can often get by without any termination unless the baudrate is very high. How about your engineers verifies explicitly what reflections they have in the cable? No need to have extra loads if there aren't really any problems with reflections - less load is less power wasted.
Close, but not quite correct. Whether working without a terminator is possible at all depends primarily on which of several physical layers for CAN is in use; speed only comes into play because different PHYs have different ranges of allowed baud rates. Not to mention CAN-over-fiber. But when the debate is about ohm or 60 om termination, I think we can rule out your CAN-over-fiber or sing-ewire PHY from the debate. I don't think we know how to do this correctly.With a voltmeter handy, you can prevent garages from charging you expensive rates to fix this problem.
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The government has mandated to use CAN high speed data bus networking starting this year on all vehicles manufactured and imported in North America. Not only it is the fastest buss networking, it allows the use of more modules in the vehicle without major cost. Updates on the vehicle can be implemented by adding hardware to improve emission, safety and convenience. Again this diagram will show both GMLan low speed and high speed bus serial data wired together.
The low speed bus dot connections are shown on all the low speed data bus modules on the left with Red color connected to both splice pack 1 bar and splice pack 2 bar connectors.
Both splice bars are connected by way of the green dot connector on top which originates from DLC terminal pin 1 shown as green wire.
Notice how each high speed bus wires flow from the DLC terminals. Each of this bus wires end in a "termination resistors" shown as box R on bottom left. These resistors are used to give the network communication a cleaner signal. One resistor is connected remotely while the second one is inside the ECM module. In the Lo Speed, connection is called Linear because the modules are interconnected by a common bus link shown as splice bar.
If you disconnect the green dot at the top left S1, all DLC pin 1 bus data communication is cut off. This means, you can disconnect one of the 7 high speed modules on the right and bus communication continues with all the remaining 6 modules. These extra wires are used for ranking or prioritizing the 7 hi speed modules operation.
Troubleshooting GM truck bus network:. The modules are tested by disconnecting the blue dot connectors on each of them, one at a time. Any DLC pin 1 voltage reading higher or lower than 5 volts is the problem. Disconnecting the defective module will tell you which voltage would return the normal 5 volt reading.
Any reading should fluctuate between 1. Do this with ignition key off or removed.Can-bus Trouble
Expected reading is 60 ohms. Any reading higher or lower than this warrants further investigation. You will get the exact color of the bus wires and location of the bus circuit components if you have the actual bus wiring diagram.
This diagram is for demonstration only. Here below are sample fixes for GM bus no communication problems.The Solution" Automation Artisans Inc. CAN Bus. Search everywhere only in this topic. Advanced Search. Classic List Threaded. John cliffer. What i found that it started producing sound in connectivity test of Multimeter Even though CAT-5 cable are not short. John Dammeyer. Message Hi. First of all don't use the continuity test of the meter.
It has enough drive to see the 60R resistors as continuity. And even if it does it doesn't tell you if the signal is making it through the opto. David Harris. In reply to this post by John cliffer. In this case 60 ohms is close to zero. Try the resistance function instead. Your resistors should measure close to 60 ohms. I agree with John, disconnect the LEDs. Please suggest. Sorry ,i post wrong schematic it is HVD Continuity testers aren't magic. If you think about how they work, it will become clear.
They apply a voltage to the circuit under test, and measure the current. If the current is above some threshold then they give a sound.Welcome, Guest. Please login or register. Did you miss your activation email? This topic This board Entire forum Google Bing. Print Search. SparkFly Contributor Posts: 47 Country:. I've noted that whilst the CAN spec itself doesn't mention the physical layer, there are a few documents around from manufacturers which detail requirements set out by ISO What interests me the most is the requirement for a Ohm impedance along the bus.
Am I simply looking in the wrong place, and there are Ohm connectors, or is there something else going on? I would've thought that everything would need to be matched to avoid reflections Many thanks for your time, it is most appreciated.
The following users thanked this post: SparkFly. Quote from: SparkFly on December 12,am.
Diagnosing CANBUS Communication Problems
In a pinch I use CAT5e cable. Quote from: bktemp on December 12,am. Thanks very much for the quick replies. I've designed the Ohm resistors on the pcbs anyway, it is good to know the reasoning. Bktemp, I will endeavour to find some further sources on the matter! Thanks for your help once again. Because CAN is designed to operate in noisy environments industrial, automotive the cabling doesn't really matter that much, as long as it's a twisted pair ok, not 30 gauge, nor 5 gauge.
What does matter on the physical layer, is the impedance of the bus.