I recently got some of these cables(see picture below) to control an old RCX motor. Some things to note before reading on. I tried both the medium and large motor blocks and played around with power, rotations, seconds, and even just on, but to no avail. If I program the EV3 to make the RCX motor spin counterclockwise, it does so on any setting without any issues. However, if I program the EV3 to spin the motor clockwise, it only spins for about a third of a second before the motor stops. The EV3 brick doesn’t shut down, just the motor. This happens on any setting. Why is this happening and how do I fix it? Is it some kind of over current or under current protection?

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Credit: Bartneck.de

Edit: Just tried the unregulated motor block and that didn’t fix it either.

1 Answer 1


These cable don't have anything connected to pins 5 or 6. This causes the software running on the EV3 that detects which type of motor is connected to think that there is nothing connected. Since it thinks nothing is connected, it periodically resets the output port which turns off the motor.

To work around this, the unregulated motor block needs to be called in a loop with two different speeds (e.g. 100 and 99) to keep the motor running. Each time the requested speed changes, it resets the device detection timer that would stop the motor.

Some people who don't mind cutting the cables have added resistors to the wires connected to pins 5 and 6 to simulate what a real EV3 motor looks like. This allows the unregulated motor block to able to be used as expected.

Here is an example of how to do that. (Although I would wire it up more like the actual EV3 motor schematic using 3.3kΩ resistors. Pin 5 to pin 4 via resistor and pin 6 to pin 4 via resistor. But if the other method is known to work...)

  • 1
    Thanks! Apparently you need to add two unregulated motor blocks with slightly different speeds to keep it on. Just one in a loop didn’t work for me. Sep 4, 2020 at 5:38
  • 1
    Right. I forgot about that part. Will update my answer. Sep 4, 2020 at 16:28

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