Hub To PC communication issues

[ytraub]

Hello Dear Community

I was following this guide right here and it works great:
https://pybricks.com/projects/tutorials/wireless/hub-to-device/pc-communication/ (my sever is exactly like this + it sends inital pid gain values)

However do I now have the problem with this code that once i call the pid() function in my main loop nothing can be written to the standart output until there is an exception or i stop the program by hand:

from pybricks.hubs import PrimeHub
from pybricks.pupdevices import Motor
from pybricks.parameters import Port, Direction
from pybricks.tools import multitask, run_task, StopWatch, wait

from usys import stdin, stdout
import umath
import ustruct

hub = PrimeHub()

drive_left = Motor(Port.B, Direction.COUNTERCLOCKWISE)
drive_right = Motor(Port.E, Direction.CLOCKWISE)

t_dist = 1000
a_dist = 400
d_dist = 400

start_speed = 50
max_speed = 800

target = 0


async def brake():
    await drive_right.brake()
    await drive_right.brake()
    await drive_right.brake()
    await drive_left.brake()
    await drive_left.brake()
    await drive_left.brake()


def current_distance():
    return (drive_left.angle() + drive_right.angle()) / 2


def in_range(n, _x, _y):
    _max = float(max(_x, _y))
    _min = float(min(_x, _y))

    _n = float(n)

    return _n >= _min and _n <= _max


def f(x):
    return umath.sqrt(x)


def get_error(t=target):
    return t - hub.imu.heading()


async def reset():
    target = 180
    prev_error = 0

    while not in_range(get_error(target), 0.4, -0.4):
        error = get_error(target)
        derivative = error - prev_error

        correction = (error * 5) + (derivative * 0.05)

        drive_left.run(0 + correction)
        drive_right.run(0 - correction)

        prev_error = error


async def pid(kp, ki, kd):
    error = 0
    integral = 0
    derivative = 0
    last_error = 0

    drive_left.reset_angle(0)
    drive_right.reset_angle(0)
    hub.imu.reset_heading(0)

    while abs(current_distance()) <= t_dist:
        speed = start_speed

        if current_distance() < a_dist:
            c = (max_speed - start_speed) / f(a_dist)
            speed = c * f(current_distance()) + start_speed
        elif current_distance() < t_dist - d_dist:
            speed = max_speed
        else:
            try:
                c = (max_speed - start_speed) / f(d_dist)
                speed = c * f(t_dist - current_distance()) + start_speed
            except:
                speed = start_speed

        error = get_error()
        integral = integral + error
        derivative = error - last_error

        correction = (kp * error) + (ki * integral) + (kd * derivative)

        drive_left.run(speed + correction)
        drive_right.run(speed - correction)

        last_error = error

    brake()


async def main():
    watch = StopWatch()
    
    while True:
        stdout.buffer.write(b"rdy")

        """ cmd = ustruct.unpack("d", stdin.buffer.read(8))
        p = float(cmd[0])
        cmd = ustruct.unpack("d", stdin.buffer.read(8))
        i = float(cmd[0])
        cmd = ustruct.unpack("d", stdin.buffer.read(8))
        d = float(cmd[0]) """
        
        p = 1
        i = 0.002
        d = 0.1

        await pid(p, i, d)
        wait(500)
        
        error = get_error()

        # this is not written to stdout
        stdout.buffer.write(ustruct.pack("d", error))

        # this is then is executed but does not stop at the desired target
        await reset()


run_task(main())

Also there are two other problems I am encountering and I have no plan on what to do with these error messages:

handle: <Handle PybricksHub._pybricks_service_handler()()>
Traceback (most recent call last):
  File "C:\Users\yaron\AppData\Local\Programs\Python\Python313\Lib\asyncio\events.py", line 89, in _run
    self._context.run(self._callback, *self._args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Lib\site-packages\pybricksdev\connections\pybricks.py", line 264, in _pybricks_service_handler
    self._handle_line_data(payload)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Lib\site-packages\pybricksdev\connections\pybricks.py", line 229, in _handle_line_data
    self._line_handler(line)
    ~~~~~~~~~~~~~~~~~~^^^^^^
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Lib\site-packages\pybricksdev\connections\pybricks.py", line 197, in _line_handler
    line_str = line.decode()
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 8: invalid start byte

I mean the robot encodes the floating point value as a double, so where can this go wrong?
And also everytime I use pybricksdev download or I run it first with pybricksdev run and the progam ends is get:

  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Scripts\pybricksdev.exe\__main__.py", line 7, in <module>
    sys.exit(main())
             ~~~~^^
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Lib\site-packages\pybricksdev\cli\__init__.py", line 570, in main
    asyncio.run(subparsers.choices[args.tool].tool.run(args))
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\yaron\AppData\Local\Programs\Python\Python313\Lib\asyncio\runners.py", line 195, in run
    return runner.run(main)
           ~~~~~~~~~~^^^^^^
  File "C:\Users\yaron\AppData\Local\Programs\Python\Python313\Lib\asyncio\runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "C:\Users\yaron\AppData\Local\Programs\Python\Python313\Lib\asyncio\base_events.py", line 725, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Lib\site-packages\pybricksdev\cli\__init__.py", line 236, in run
    await hub.disconnect()
  File "C:\Users\yaron\Documents\source\projects\pid\.venv\Lib\site-packages\pybricksdev\connections\pybricks.py", line 305, in disconnect
    self.connection_state_observable.value == ConnectionState.DISCONNECTED
AssertionError

It doesn't seem to mess things up but it is very annoying to say the least.

Python version 3.13.5
pybricks==3.6.1
pybricksdev==1.2.0
bleak==1.0.1

on Windows 10 with lego Spike Prime via ble.

Thanks!
Kind Regards.

[BertLindeman]

Hello,

I am not good in multi programming, but in your pid function I do not see something like:

await wait(0)

Or any other await.
I assume that causes this function to lockup other tasks from running.

Might that be a cause?

Bert

[ytraub]

Yes thats true but I don't want anything to run concurrent. I want to wait for the pid function to finish before calling the next function.
The strange thing is that I can't write to standart output AFTER the pid() is finished it seems.

[ytraub]

(I just added these async/awaits because i initially thought they would resolve the problem but actually in this case the don't change anything which makes sense)

[ytraub]

I resolved this by using print instead of stdout.buffer.write

[dlech]

Yeah, if you are connecting stdout from the hub to a terminal on your computer, then you will want to use print() to ensure that it is something that can be printed in a terminal.

If you want to send binary data over stdout, then you can't connect it to a terminal. Generally, this means making your own host-side program using pybricksdev instead of using pybricksdev run and passing print_output=False, line_handler=False to PybricksHub.run().

[ytraub]

Are there docs for this?

[dlech]

Not really other than the source code. (There is https://docs.pybricks.com/projects/pybricksdev/en/latest/ but needs someone to actually add that part of the API to those docs - contributions welcome).

[ytraub]

Will look into that but print will do it for now.

But I have another problem which causes it to enter the REPL. What causes could this have or better said how can this happen?

Traceback (most rec
ent call last):

File "main.py", lin
e 138, in <module>

  File "main.py",
line 116, in main

KeyboardInterrupt:

Pybricks MicroPyt
hon ci-release-86-v
3.6.1 on 2025-03-11
; SPIKE Prime Hub w
ith STM32F413VG
Ty
pe "help()" for mor
e information.
>>>
 $?fQ@
Ucl^?


Traceback (most rec
ent call last):

File "<stdin>", lin
e 1
SyntaxError: i
nvalid syntax
>>>
?
?
tcn?
The program was sto
pped (SystemExit).

[laurensvalk]

A keyboard interrupt (CTRL+C) is equivalent to 0x03. This makes it exit the program and drop into the repl. During normal operation, this can be helpful (you can try it out by running a script and pressing CTRL+C in the output pane).

You can disable it.