Scrambled GLCD on Startup with OctoPi connected

I recently converted my Wanhao D6 (Monoprice IIIP) to Smoothie board as the original controller would crash intermittently.

All is fine and the machine works better than it ever has the entire time I have owned it.

However on occasion the display on the GLCD will scramble with two bands of hash across the screen if the OctoPi is connected to the USB port. If I start it with the USB cord not connected it never happens.

Thinking electrical issues I taped over the 5V+ pin in the USB connector to no avail. It is kind of a PITA to have to plug in the USB cord after powering up the Printer and OctoPi.

Any suggestions?

Bill V

What kind of LCD is it? If it’s controlled via SPI, occasional scrambles are to be expected from electrical noise if the cable is longer than a few centimeters.

It is the white genuine GLCD. I had a previous run of issues with the clones.

As a note, if the Pi is not plugged into the USB port of the Smoothie, this never happens.

Other than this one annoying issue, the conversion went well and the machine works amazingly well.

Bill V

Does it help to use ferrite beads?

sounds like a ground loop. are the smoothieboard and raspberry pi powered from the same 5V psu? How is each powered exactly?

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Pi is using a Wall Wart designed specifically for it. The Smoothie is powered by a Large PS built into the bottom of the machine.

As mentioned in the OP, I modified a cord that eliminates the 5V power circuit in the USB cable.

This is not a huge deal, but having to plug in the Pi to the machine each time is a mild PITA.


Where is each 5V psu plugged in, mains voltage side? Same power strip? Cable lengths on each side of each PSU?

Pi’s wall wart is plugged into the output of a UPS. Cord, maybe 4’ long.

PS inside the base of the unit is also plugged into the UPS. Wire length between the PS and the Smoothie is about 6 inches and fairly heavy gauge:

I have the UPS because we had one of our occasional voltage fluctuations and it took out the original controller of the printer when I was running the OEM Wanhao board.


how long between the smoothie power supply and the mains? if the mains->psu cables are too long you can have a ground loop, which would cause what you describe. I would try reducing all cable lengths even if just as a test. Also, measure voltage diff between raspi gnd and smoothie gnd, as well as between the 5v gnd of each psu and report.

The cord that exits the printer housing is maybe 18"

Will go dig out my scope this evening.