CAN issues, multiple power supplies wiring advices
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Hello.
I have what seems to be a damaged 3HC CAN transceiver: CAN DIAG LED blinks 4 times however their is absolutely no software issue or 6HC board hardware issue (the printer works fine with replacement 3HC boards)
I have 4 questions intended for:
- a confirmation/correction of the diagnostics
- information to
- understand the origin of the issue
- solve the problem
- prevent improper ground management related issues
- curiosity
Below are more elements of context:
The 3D printer control system is built around one 6HC and one 3HC board.
Each board is powered by a separated 24V DC power supply due to the system total power requirement (large format printer).Power supplies has independent floating output ground. Per the manufacturer specifications, they are not meant to be plugged in parallel. Serial connection (for 48V) is OK per the manufacturer specification. Common ground is OK too (requires to carefully layout the electrical path to prevent running the supplies in parallel). A power diode can be added inline with each power supply output to build a "supply backup" configuration.
After about 3 or 4 month of "flawless" operation in this configuration, the printer suddenly notified being unable to find whichever device plugged on the 3HC board.
Long story short, I strongly suspect that the above described issue comes from a damaged 3HC board CAN transceiver (MCP2542WFD):
- the same firmware, bootloader, fuse bits configuration worked in the past and still worked once the 3HC is replaced
- below are the comparative "CANH <-> Vdd" and "CANL <-> Vss" measurement taken with a multimeter in Diode check mode (chip soldered, termination jumpers removed)
Those result seems to indicate faulty transceiver.
1) Is that a sound conclusion?The only connection point between the board is the non-shielded RJ11 cable. Which is the reason why I think that the problem comes from the use of independent floating ground supplies: power supply ground level balance is done through the sole CAN transceiver.
2) Is that a sound hypothesis? Do you think of other more probable cause of failure?If point 2 correct, then running a common ground should solve the issue. But I fear that:
- a) candidly connecting the grounds together could do create tricky ground loop issue through the CAN bus
- b) connect the board using shielded RJ11 cable would hardly solve any issue given the 100kOhm resistor between the shield and the ground
3) Are either of the concern a) or b) legitimate or not? Do you have wiring advice to prevent long term reliability issue?
I noticed that the CAN common mode choke part reference changed on the V1.01 3HC board (from "DR331-104AE" to "???").
4) What is the new common mode choke reference? Why is that change?Thank you in advance for your help.
Regards,
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@jp-douarvil-0 I think you are right, the CAN transceiver is most likely damaged.
As I understand it, you do not have a common ground connection between the power supplies. This is not a good idea. The CAN bus assumes that the connected systems have a common ground, although it is tolerant of quite a lot of noise between the grounds of the connected components.
When neither output terminal of a power supply is connected to ground, mains-borne transients can be coupled from the mains supply of the PSU through the capacitance of the transformer in the PSU to the output connections. I suspect that such a transient may have exceeded the transient voltage rating of the CAN transceiver, which is -58 to +58V continuous, with transients allowed up to -150V to +100V.
To avoid this, connect the GND sides of all the Duet main and expansion boards in the system together. Where boards have different power supplies, this will require the negative output terminals of the PSUs to be connected together; except that if you use two 24V supplies in series to get 48V, treat that as a single 48V supply and just connect the negative output of the "lower" one to the common ground. Using a common ground in this way does not create a ground loop via the CAN bus because the CAN bus itself does not have a ground wire, unless of coruse you use shielded CAN cable and ground the shields at both ends
Regarding the common mode choke, we changed it to the same part that we use on the tool board, other expansion boards and Duet 3 Mini. It is Wurth Electronics 744231121. However, we will be changing it again in the next revision. Its only function is to reduce EMI radiated by the CAN cable.
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@dc42 Thank you very much for your valuable answers. I will modify the wiring in accordance to your recommendations.
Regards,