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I'm trying to drive a motor control over the Raspberry Pi's GPIO serial pins (GPIO 14 and 15)

I've disabled the serial console, however, during bootup of the Raspberry PI, it still sends data over the serial pins. This sends seemingly random commands to the motor controller. Which is of course potentially dangerous and not what I want to happen.

My system:

  • Raspberry Pi 3 B+
  • Serial connection with the motor controller over GPIO 14 (it's one way, so GPIO 15 is not connected)
  • Ubuntu MATE 18.04.4 LTS

What I have already tried:

  • Use raspi-config to disable serial console (3 Interface Options > P5 1-wire > no (login shell) > yes (serial port) => this did not seem to work at all.
  • in /boot/firmware/nobtcmd.txt I have removed the "console" parameter => this correctly disabled the login shell

What is working:

  • When the Pi is booted, I can connect to the motor driver and send it commands. This works correctly

What is not working:

  • During boot I have to disconnect the serial connection to the motor driver, because data is transmitted over the connection

My question:

  • How can I completely disable all data written to the serial interface during boot?
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  • What data is being sent? What motor? What motor controller? – joan May 29 '20 at 8:17
  • The driver does not show what data it receives. Is there an easy way to figure that out? Can I connect another Raspberry Pi to read this? – Peter May 29 '20 at 8:19
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    Use Raspberry Pi OS. Ubuntu uses Canonical kernel and firmware. – Milliways May 29 '20 at 8:19
  • I'm not sure it matters, but it's the Sabertooth 2X5 in simplified serial mode with two DC motors connected. The driver is receiving data, when no data is expected over the serial connection. – Peter May 29 '20 at 8:20
  • I'm using Ubuntu MATE because that's what ROS prefers. I will set up Raspberry Pi OS on a separate card to see if it makes a difference. – Peter May 29 '20 at 8:22

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