"A sensor or motor is not connected to the specified port" happens randomly

[lebenski]

Hello,

I'm on the latest version of the Pybricks firmware and around 1/3 times I start up my City Hub I have the following:

Traceback (most recent call last):
File "Train_AllInOne.py", line 17, in
OSError: [Errno 19] ENODEV:

A sensor or motor is not connected to the specified port:
--> Check the cables to each motor and sensor.
--> Check the port settings in your script.
--> Check the line in your script that matches
the line number given in the 'Traceback' above.

With this being the offending line:

motor = Motor(port=Port.A,gears=[24,16])

I restart the hub and 9 times out of 10 it works then. Sometimes I have to restart a third time. The error appears random and I'm convinced it's not a loose connection.

Any ideas?

Cheers,
Ben

[BertLindeman]

Hi Ben,

Not sure if this is even possible on your current model, but here goes:

hardware:

The obvious things:

• batteries
• try another port with the same motor
• try another motor on the same port

A step deeper:

• the same program on another hub (if you happen to have one or another hub type)

firmware:

• can you share the result of

from pybricks import version

print(version)

program:

Is your program large?
Can you share the complete program?
Or better a short program that shows the problem?

Just some thoughts,

Bert

[lebenski]

Thanks Bert. I'll try swapping out the hub and motors as you suggest and report back. If I can consistently recreate the behaviour on a second hub with a short program i'll post it here.

[laurensvalk]

What version of the firmware are you on? It's hard to say what 'latest' means.

To find out, you can just open the REPL. (Click >>> next to the stop button). It starts by printing the version.

[lebenski]

Thank you!

Pybricks MicroPython v1.20.0-23-g6c633a8dd on 2024-04-11; Powered Up City Hub with STM32F030RC

[lebenski]

I had only assumed latest because I was recently asked by Pybricks Code to update the firmware. Please let me know if I'm still out of date! Cheers, Ben

[laurensvalk]

Thanks. Which motor are you using? Did this problem happen before with the same setup?

It would also be great to know if you want to test it with https://beta.pybricks.com/ (and install the firmware from there as well.)

[lebenski]

Well after following Bert's suggestion, I changed for fresh batteries, tried a different hub then put the original configuration back together and am no longer able to recreate the problem.

So apologies for wasting both of your time! Seems like it was in the end a hardware issue of some sort.

Your help is much appreciated

[BertLindeman]

Hi Ben,

Thanks for reporting back.

We really like reports like yours (Hm, I should perhaps say "I")
People with similar problems might find this issue and find a "fix".

So, Ben, no time wasted.

Happy pybricking,

Bert

[lebenski]

Well the plot thickens a bit here guys... I'm now more confused than I was before. So I've been using my setup all day and it came time to change the batteries so I replaced them with some freshly re-charged Energizer batteries (these ones: https://www.amazon.co.uk/dp/B004HUYOQ0). I fire up the hub again and no matter how many times I restart it I'm getting either this:

The requested operation is not valid in the current state:
--> Check the documentation for required conditions.
--> Check the line in your script that matches
the line number given in the 'Traceback' above.

On this line:

motor.run(1000)

OR I'm getting the original error in this thread about the motor not being connected.

I restart the thing, replug in the motor and nothing works. Either one of these two errors.

Then I decide to just run a really cutdown program:

from pybricks.hubs import CityHub
from pybricks.pupdevices import Motor, Light
from pybricks.parameters import Button, Color, Direction, Port, Side, Stop
from pybricks.robotics import DriveBase
from pybricks.tools import wait, StopWatch

hub = CityHub()

motor = Motor(port=Port.A)

while True:
    motor.run(1000)

THIS WORKS!

I then reinstall my original program and it's working again.

I've got no idea what's going on here but figured it was worth adding to this conversation.

[lebenski]

backticks added! Yeah a weird one...

[dlech]

This error means that the UART communication to the motor is not working as it should. Did you ever say specifically which type of motor this is?

[lebenski]

I’m using the Large Technic Motor. This one:

https://www.lego.com/en-gb/product/technic-large-motor-88013?gQT=1

I have no idea if this is related but given you mentioned UART, I am using bluetooth for pc to hub (and also hub to hub) communication as described here:

https://pybricks.com/projects/tutorials/wireless/hub-to-device/pc-communication/

Oh and to answer Bert’s earlier question i’m using the original stable firmware, not the beta.

[laurensvalk]

Could the motor cable be slightly damaged? If one of the contacts will occasionally work but not always, this could explain the intermittent nature of the issue.

Do you have another motor of the same kind to try it with?

Alternately, try a program like the following. If it works at first, does the error reproduce if you wiggle the cable or connector around a bit?

from pybricks.hubs import CityHub
from pybricks.pupdevices import Motor, Light
from pybricks.parameters import Button, Color, Direction, Port, Side, Stop
from pybricks.robotics import DriveBase
from pybricks.tools import wait, StopWatch

hub = CityHub()

motor = Motor(port=Port.A)

while True:
    motor.run_angle(500, 90) # this is slightly different from your original

[lebenski]

Motor damage is not impossible for sure. When this happens again I will try wiggling the cable to rule out a hardware issue. I suppose all things considered that still seems most likely. I also wonder whether it's related to the way I'm stopping the program. Here is a video of my machine:

https://www.dropbox.com/scl/fi/hkmhtri2f7svx7g47ra13/IMG_1974.MOV?rlkey=vcr84mugx5v3ovhnc8lamjlsf&st=4e879a5g&dl=0

It scans and organises Magic The Gathering cards into train carriages by price (over 50p = green, over 15p = blue, under 15p = red, errors = yellow). I have two lego hubs, one to control the movement of the train and a second to control dispensing the cards into the carriages with a button press. Prior to all of that the cards are scanned and image recognition is performed to identify the card.

As such there are lots of moving parts (figuratively and literally!) and sometimes I need to just shut it off in a hurry because something has gone wrong. I do that by hitting the button on the hub that controls the train.

I wonder is there any world in which if the train is in the middle of an operation related to the motor when it is shut off that it is then somehow left in an "invalid" state when I restart the program? I think it's unlikely that there is somehow a "memory" between restarts of the program but I'm just thinking out loud.

Either way I figured it would be instructive to share a video of the machine so you can see what I'm working with.

I'll report back with the results of the wiggle if this happens again!