Hello, I am a beginner in Devicenet and have recently downloaded the documentation recommended by AB, a large PDF that helps troubleshoot Devicenet issues. My Devicenet network consists of approximately 21 nodes. In my setup, the last three nodes experience bus off faults, but communication is restored with a power cycle. The resistance between Can L and Can H measures at 60 ohms. I have observed a low resistance reading from Can L to Earth, approximately 60k Ohms, while Can H to Earth shows an open circuit for resistance. The shields are all bonded at a single point near the middle of the trunk line. Disconnecting the shield from the single point to Earth resulted in an open circuit reading until the bonding was reinstated. The power supplies in use are three in total, all within the Devicenet specifications. Voltage readings appear satisfactory. Unfortunately, I do not have access to a Devicenet network analyzer. My main concern is that attaching the scanner card to the Devicenet network decreases the resistance reading from Can L to Earth. When the scanner card is removed, the resistance value returns to an open circuit reading. I am uncertain if the behavior observed with the scanner card is normal or indicative of a need for replacement. Your insights on this matter would be greatly appreciated. Thank you.
Great job on your troubleshooting efforts! When removing the two termination resistors and disconnecting the power supplies, it is recommended to measure between 10K - 50K between CAN-H and CAN-L. The resistance from CAN to Earth should be very high, showing an open circuit or several megaohms when tested with a standard meter. Make sure to check the scanner module with the entire network disconnected and the PLC powered down. It is important to note that scanner modules must also have the CAN circuits isolated from the device, just like other components. This is why the network power supply is located separately from the PLC, typically out in the field. If there is a reading of 60K ohms between CAN-L to Earth, this could indicate damage from lightning or other external factors affecting the scanner module.
Thank you for your input. I will review it the next time I have a chance to work on the plant, as my time is limited each week. This may result in a delayed response, but I value your suggestions and information.
Hi there, sounds like you're doing a thorough job troubleshooting this issue. From what I understand, your resistance readings are suggesting a possible short or grounding problem. Minor grounding shouldn't really affect the network, but your scanner card behaving differently does raise some red flags. Before considering a replacement, I'd suggest double-checking your cable connections and wiring, and pay close attention to any signs of damage or wear. If possible, isolate and test the individual nodes. It might be a hefty task with 21 nodes, but could help pinpoint if the problem is localized. Lastly, you mentioned you didn't have access to a Devicenet network analyzer - if there's a way you could get a hold of one, even to borrow, it'd be a great help in diagnosing your issue. Hang in there!
Your attention to detail is impressive and that's half the battle won already. From your description, it sounds like a possible issue with the scanner card. Typically, the scanner card shouldn't have a significant impact on the resistance between Can L and Earth. However, before jumping to conclusions, it's worth checking if there could be intermittent issues with your grounding or terminators. Faulty grounding could create intermittent communication issues. Also, ensure all your devices are properly terminated as incorrect termination can induce bus-off errors. You could also try isolating the last three nodes to see if the issue persists, which would help you narrow down the problem area.
It sounds like you've thoroughly checked the Devicenet setup, but the issue you are experiencing with the scanner card could indeed be a concern. Generally, there shouldn't be a low resistance reading from Can L to earth, especially when the scanner card is connected. It could be a sign of a possible fault or short circuit. My advice would be to isolate and test the scanner card further in a controlled environment. Also, remember that electrical noise can often cause such bus-off faults on Devicenet networks. Even though you've bonded the shields at a single point, it might be advantageous to check the system for potential sources of electrical interference. Do you have any high voltage power lines running parallel to the Devicenet cable, or are there any devices in the vicinity that could be the source of EMI? Looking into these points might help discover any lurking issues.
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Answer: Answer: Bus off faults in Devicenet nodes can be caused by various factors such as improper termination, electrical noise, power supply issues, or wiring faults.
Answer: Answer: In a properly functioning Devicenet network, the resistance between Can L and Can H should measure around 60 ohms. Deviations from this value could indicate wiring issues or improper termination.
Answer: Answer: A low resistance reading from Can L to Earth and an open circuit from Can H to Earth could indicate a grounding issue or improper shielding in the network.
Answer: Answer: The behavior observed with the scanner card affecting resistance readings could indicate an issue with the card or its connection. Further troubleshooting or consulting the manufacturer may be needed to determine if a replacement is necessary.
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