[Bug] Technichub sometimes gets locked-up on a connect of a remote.

[BertLindeman]

Describe the bug
A connect to a remote sometimes makes the hub "hang" and a long press of the button lets the hub rappidly blink blue. The hub is irresponsive to UI buttons. Batteries need to be taken out to power-off the hub.

To reproduce
Steps to reproduce the behavior:

1. Go to beta.pybricks.com
2. Upload and run the test program (see below)
3. Wait a bit and then activate a remote
4. If all goes well you'll see Disconnected 2 DONE
5. If not and you see no timeout for the remote, then the hub is locked-up
6. long press the hub button to power-off
7. If no power-off occurs, you'll see rapid blue blinking and need to remove the batteries to power-off

Expected behavior
A simple remote connect or timeout

Screenshots
There is a saying that a picture is worth a 1000 words. Screenshots really help to identify and solve problems.

Extra info
Currently have this problem with V4.0.0:
('technichub', '4.0.0', 'ci-build-4848-v4.0.0 on 2026-06-08')
Had similar problems with older V4 beta firmware versions,
but I could not make a reproducible testcase.
With this test program I have about 50% chance to hit the problem.

test program

Output example of good run:

('technichub', '4.0.0', 'ci-build-4848-v4.0.0 on 2026-06-08')
Got   remote name 1 Handset left
Disconnected 1
start remote 2
Got   remote name 2 Handset left
Disconnected 2 DONE

Hang example:

('technichub', '4.0.0', 'ci-build-4848-v4.0.0 on 2026-06-08')
Got   remote name 1 Handset right
Disconnected 1
start remote 2

and although there is a remote blinking to get connected, the connect does not complete.
CTRL-C on the terminal pane does not react,
long press on hub button gets a rapid blue blink.

from pybricks.pupdevices import Remote
from pybricks.tools import wait, StopWatch
from pybricks import version
print(version)

remote = Remote(timeout=10000)
print("Got   remote name 1", remote.name())
remote.disconnect()
print("Disconnected 1")

wait(2000)
print("start remote 2")
remote = Remote(timeout=None)
print("Got   remote name 2", remote.name())
remote.disconnect()
print("Disconnected 2 DONE\n")

[laurensvalk]

Thanks for reporting! I can't seem to reproduce this unfortunately.

[laurensvalk]

What was the exact name of your program and remote? In cases like these, having the same length of file name and devices can help.

Did you still have a debug cable for one of the ports? It's the same as you made for EV3, just with a different plug. I suppose we could try building a debug firmware to see where it hangs. Although I suspect this is one of those things that goes away when we turn debugging on.

[BertLindeman]

What was the exact name of your program and remote? In cases like these, having the same length of file name and devices can help.

I have two remotes I use randomly.
In the "good" run in the original post I see "Handset left"
In the "hang" run "Handset right"

Did you still have a debug cable for one of the ports? It's the same as you made for EV3, just with a different plug. I suppose we could try building a debug firmware to see where it hangs. Although I suspect this is one of those things that goes away when we turn debugging on.

I do have the EV3 debug cable but not one with the powered-up plug.

The same problem I had / have with a program not doing the connect twice in the program.
The feeling is if I do not connect fast after the hub is ready for it, this situation can occur.

That TechnicHub has screws to lock the backplane :-) in the model (sigh)

[BertLindeman]

Ah, forgot (and needed to search) program name: test_remote_keep_connection.py