uRemoteError (No data / Preamble mismatch) when initializing LineSensorUR inside an async wrapper or alongside task runners #3

[zoligyenge]

Hi. I am not sure this is the right place since I am not sure whether the problem is in Anton's LMS Line Sensor's firwmware, the pybricks helper functions or pybricks.

When using Anton's Mindstorms LMS Line Sensor Pybricks wrapper (line_sensor_pybricks.py) with uRemote, initializing the sensor instance at global scope while the script is structured for asynchronous execution (run_task()) results in a uRemoteError: Read error: No data. Is remote script running? or Preamble mismatch. However, an identical initialization script running completely synchronously works without issues.

Environment

Hub: LEGO SPIKE Prime / Inventor Hub

Pybricks Code App Version: v3.0.1

Pybricks Firmware Version: v4.0.1

Sensor: LMS Line Sensor (connected to Port C) [firmware_ch32_line_i2c_ur_v5.6.uf2]

Library: line_sensor_pybricks.py (v1.0 / uRemote protocol wrapper)

Code That Fails (Async Task Context)

from pybricks.hubs import InventorHub
from pybricks.parameters import Axis
from pybricks.tools import multitask, run_task, wait

from line_sensor_pybricks import LineSensorUR

# Set up.
ls = LineSensorUR('C')
inventor_hub = InventorHub(top_side=Axis.Z, front_side=Axis.X)

async def main():
    # upload line_sensor_pybricks.py into your pybricks code environment. Get it here:
    # https://github.com/AntonsMindstorms/lms-line-sensor/blob/main/micropython/line_sensor_pybricks.py
    print(ls.mode_calibrated())

run_task(main())
**Traceback / Error:

**Traceback (most recent call last):
File "main.py", line 16, in
File "main.py", line 13, in main
File "line_sensor_pybricks.py", line 111, in mode_calibrated
File "line_sensor_pybricks.py", line 301, in mode
File "line_sensor_pybricks.py", line 299, in call
uRemoteError: Read error: No data. Is remote script running?****

Code That Works (Synchronous Context)

from pybricks.hubs import InventorHub
from pybricks.parameters import Axis

from line_sensor_pybricks import LineSensorUR

# Set up.
ls = LineSensorUR('C')
inventor_hub = InventorHub(top_side=Axis.Z, front_side=Axis.X)

# The main program starts here.
# upload line_sensor_pybricks.py into your pybricks code environment. Get it here:
# https://github.com/AntonsMindstorms/lms-line-sensor/blob/main/micropython/line_sensor_pybricks.py
print(ls.mode_calibrated())

Steps to Reproduce

Connect the LMS Line Sensor to Port C on an Inventor Hub.

Upload line_sensor_pybricks.py to the hub.

Run the async script containing run_task(main()).

Observe the timeout / no data exception during the UART transaction.

Additional Context

Raw UARTDevice tests on the port pass successfully, proving the hardware and port connection are fully functional. The issue appears to be tied to how the uRemote RPC handshake initializes when the MicroPython cooperative async scheduler (run_task) is active under Pybricks firmware v4.0.1.

I tried with multiple HUB, Multiple LMS Line follower modules.

[dlech]

uRemoteError is not part stock Pybricks, so not sure we can help on the Pybricks

cc: @antonvh

[zoligyenge]

Thanks for your reply: I was just wondering whether the pybricks implementation of generic UART may have soemthing to do with the error. Espacially since calling the device fucntions works unless called from an async function. Which is strange...

[dlech]

Some methods need to be awaited when called in an async function. The Pybricks docs show this with await in front of the methods. If this mode_calibrated() method is calling such a method, then it would need to be re-written to include the async and await keywords in the appropriate places.

[antonvh]

Documentation about async uremote is here: https://docs.antonsmindstorms.com/en/latest/Software/uRemote/docs/index.html#client-async-call-async-cmd-args.

It might be that the linesensor has no async methods (yet). Uremote, underlying, has them.

[antonvh]

async is now part of uremote and lms-line-sensor libraries. Enjoy. And please share some example you make.

[zoligyenge]

Hi. Thanks for the quick reaction time - I truly appreciate it.

One question: is the calibration procedure documented somewhere? Like, how should the sensor be moved over black and white lines so that we can write a clibration movement routine?

[antonvh]

It is here: https://www.antonsmindstorms.com/docs/lms-line-sensor-1-0/#calibration

[zoligyenge]

Thanks @antonvh

BTW: Is there a way to switch between white line finding and black line finding within Pybricks. I have the following use case: The idea is to use the LMS sensors mounted longitudinally centereed about the driven axle axle to be able to stop precisely on a black line. Having several sensors is perfect for this task as the frontmost sensor can give an early signal to slow down and stop irrespecitve whether driving forward or backwards. This is as opposed to having a single downwards facing sensor between the driven wheels which means very sudden stops.

To make sure there is no false black reading, the idea is to first wait for a white line and only then wait for black. So the question is again: how to distinguish between gray and everything else and the white band around the black lines of a WRO mat.

[zoligyenge]

It is here: https://www.antonsmindstorms.com/docs/lms-line-sensor-1-0/#calibration

Sorry for having so many questions, but could you explain what you meant here "the LEDs show a flashing + then x pattern — the sensor walks you through sampling both extremes"