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I have a Rostock MAX, roughly v1. RAMBo v1.1b, now running firmware v0.92.2 downloaded from the SeeMeCNC Github.

I just upgraded the machine to use the HE280 and ball-end arms, and have had some problems with it, as follows:
1. As soon as the printer is turned on, all four fans turn on, and cannot be controlled from MatterControl. The blue LED under the HE280 logo is on, and the blue LED on the opposite side of the board is on. Notably, unplugging the PS-ON connector has no effect on the fans, but pulling the thermistor connector does shut them off (but not the LEDs).
2. If the I2C connector is attached to the board on boot, the machine performs a reboot loop. Console messages indicate read and write errors to the accelerometer.

When I extended the whip, I used a convenient piece of Cat5e. One I2C line (the red one on the whip) is on orange, the other (the black one on the whip) is on brown, and the corresponding mates in those pairs are grounded. The blue line for PS-ON is on blue, and the orange line for FAN0- is on white/blue. The thermistor is wired across the green pair. The main lines for HEAT0 are on a piece of AWG14 zip cord I had available, routed away from the other signals. I have had several people at my hackerspace check the harness over, and it rings out.

All the RAMBo-side connectors appear to be correct per the wiring diagram. However, I noticed that R12 appears to be missing from the HE280 board; is this expected?

1) Sounds like your whip wiring isn't quite right. The heat sink fan is supposed to be on all the time, but the three part fans only turn on when the rambo lights them up. If you see no RED leds on your hot end, you shouldn't have heat or running part fans. If the part fans are running and the heatsink fan isn't, that means they're switched where they plug in to the pcb. Otherwise there's a wiring problem with your whip. The PS ON connector is actually the signal wire for the accellerometer probe, so unplugging it won't do anything to the power. The fans and sensors in the hot end are grounded through one of the thermistor wires, that's why the fans shut down when you unplug it.

2) Also indicative of a wiring fault. Double check the whip. My favorite way to mess it up is to wire is backwards. If it looks right on both ends, please post pictures of each, maybe there's something else going on.

Not sure off the top of my head what the R12 you're talking about is. Can you give some more information?

Thanks for the quick reply. I will be at the hackerspace Friday evening, and will run through the suggested diagnostics, and report back.

Realize most of us (including me) don't have that version of the board to look at. I can find pictures to look at, but pictures from your end would probably be very helpful. There were significant layout changes between 1.1b and 1.2 on, although it's functionally near-identical.

Getting the thermistor lines swapped will screw things up, since one pin is used as the common ground for the HE280.

The I2C connector is rotated 90 degrees different on the 1.1b. Pin 1 is at the bottom, toward the stepper drivers. You may have it backwards. Another possible mistake is getting SDA and SCL lines swapped.

R12 (again from a picture) looks like it should be a 0 ohm resistor (That's a jumper wire in resistor format!) on an line going to the Y stepper. You should see identical examples for each of the other steppers. Show us a close-enough to read picture if you're not sure what you're seeing.

Go figure. As soon as I started taking images of my harness, I discovered I rolled two pins on a connector, connecting orange to black and vice versa. Guess I needed a rubber duck.

@Eric: I meant to be referring to the HE280 regarding R12, not the RAMBo board. Apparently that is intended to be non-populated; the printer is working flawlessly without it.

Thanks much, all!

Oops, my reading error on R12. Glad you found your problem!