Technic hub to hub bidirectional communication and multitasking

[vascolp]

Hi everybody!

I wanted to test multitasking and bidirectional TechnicHub hub to hub communication.
So I did a fancy test program that launches several threads and uses one hub button to change the state of the other hub.
The same program runs on each hub, the program detects that there is a motor connected to port A or B, to setup the communication channels.
Almost everything works fine, in fact it seems a funny demo program but:
The very first time I click on one of the hubs, the message is received by the other hub only after about 8s.
Then, everything seems to work fine in that direction. The same problem in the opposite direction.
I also tried with CityHubs and PrimeHubs and it seems to be a problem only related with TechnicHub message reception.

The program is below. The comments includes further details.
Is this a real issue?

Thank you!

#
# Bidirectional communication between two Technic Hubs, using tasks.
# Requires two Technic Hubs and two motors. One of the hubs connects motor to port A, the other to port B.
# Run this program on each hub.
#
# The program starts a task to blink the hub led, and and a task to move the motor. 
# There are three different types of moves and blinks (blinking green/cyan/white), as defined in the <status> global variable.
#
# If you click the hub's button, the other hub will change its status.
# Double click in one of the hubs should stop both hubs.
# Hub to hub communication is handled on its own task, so the program has 5 running tasks:
# -	<blink>: blink lights
# -	<spin>: spin motor
# -	<handle_buttons>: takes care of button clicks and keeps the state to send to the other hub
# -	<set_state_from_observations>: changes the state of the hub based on what was received from the other hub
# -	<comms.run>: takes care of hub to hub communications
#
# By: -vlp 2025-09-28 ([email protected])
#
# PROBLEM (bug?):
#   Version:  ('technichub', '3.6.1', 'ci-release-86-v3.6.1 on 2025-03-11')
# After starting both hubs (not connected to any computer), the first click on one of the hubs takes about 8 seconds to be received
# by the other hub.
# Hubs start blinking in green. 
# After starting both hubs, if you do a click (the first click on A) in hub A and wait about 8s, hub B will change to cyan.
# If you then click on hub B (the first click on B), hub A will take some time to change to Cyan (8s I guess, but sometimes it seems less).
# After this, clicking on each hub will change the state of the other hub very fast (it is very cool!).
# Double click on one of the hubs disconnects both of them.
# If the first click is a double click, the clicked hub disconnects immediately but the other hub does not disconnect. If, after sending the -1
# to the other hub, we change the wait to 8s (8000ms), then the other hub will disconnect.
# 
# I also tried the program with one Prime hub in one of the sides, and the problem only exists when the PrimeHub is broadcasting to the TechnicHub.
# When broadcasting form TechnicHub to PrimeHub, the PrimeHub receives the essage immediately.
# The same if instead of a PrimeHub I use a CityHub.
# It seems that there is some initial problems when receiving in a TechnicHub. 
#


from pybricks.iodevices import PUPDevice
from pybricks.hubs import ThisHub
from pybricks.pupdevices import Motor
from pybricks.parameters import Port, Direction, Stop, Color, Button
from pybricks.tools import wait, StopWatch, multitask, run_task
from pybricks import version

CLICK_COLOR=Color.RED
COLOR_PAIRS=((Color.GREEN, Color.GREEN*0.6), (Color.CYAN, Color.CYAN*0.6), (Color.WHITE, Color.WHITE*0.6))
status=0
direction=1

try:
    device = PUPDevice(Port.A)
    observe_ch=1
    broadcast_ch=2
    port=Port.A
except OSError as ex:
    observe_ch=2
    broadcast_ch=1
    port=Port.B


async def spin():
    global motor
    s1=100
    s2=200
    s3=300
    while True:
        if status in (0,1):
            await motor.run_time(speed=direction*s1, time=500, then=Stop.COAST, wait=True)
            await motor.run_time(speed=direction*s1, time=500, then=Stop.COAST, wait=True)
            await motor.run_time(speed=direction*s1, time=500, then=Stop.COAST, wait=True)
            await motor.run_time(speed=direction*s1, time=500, then=Stop.COAST, wait=True)
        else:
            await motor.run_time(speed= direction*s1, time=500, then=Stop.COAST, wait=True)
            await motor.run_time(speed=-direction*s1, time=500, then=Stop.COAST, wait=True)
        await wait(500)


async def blink():
    global hub
    while True:
        hub.light.on(COLOR_PAIRS[status][0])
        await wait(600)
        hub.light.on(COLOR_PAIRS[status][1])
        await wait(600)


class HandleComms:
    def __init__(self, hub, observe_ch):
        self.hub=hub
        self.observe_ch=observe_ch
        self.to_send=None
        self.received=None

    async def run(self):
        just_sent= 0

        while True:
            await wait(20)
            if self.to_send:
                self.hub.light.on(CLICK_COLOR)
                await self.hub.ble.broadcast(self.to_send)
                await wait(20) # To see the red blink
                just_sent = 3
                self.to_send=None
            elif just_sent > 0:
                just_sent -= 1

            self.received=self.hub.ble.observe(self.observe_ch)

    def do_send(self, value):
        self.to_send = value


    def check_received(self):
        return self.received


async def handle_buttons():
    global hub, comms
    other_side_status=0
    last_pressed=False
    sw=StopWatch()
    last_tick=None

    while True:
        this_tick= sw.time()
        pressed=hub.buttons.pressed()

        if Button.CENTER in pressed and not last_pressed:
            if last_tick and this_tick-last_tick < 300:   # Double click to exit
                comms.do_send((-1,)) # Exits the other side
                await wait(200) # So that the exit is sent to the other side
                #await wait(8000) # To show that a double click also takes longer if it is the first click of all
                break

            last_tick=this_tick
            last_pressed=True
            other_side_status = (other_side_status+1)%3
            comms.do_send((other_side_status,))
        elif Button.CENTER not in pressed and last_pressed:
            last_pressed=False
        await wait(20)


async def set_state_from_observations():
    global status, direction, comms

    while True:
        await wait(20)
        data = comms.check_received()

        if data is not None:
            data=data[0]
            if data not in (-1, 0, 1, 2):
                raise ValueError('Received wrong value: {}!'.format(data)) 
            if data == -1:
                break # Finish program!
            status = data
            direction = -1 if status else 1


hub = ThisHub(observe_channels=[observe_ch], broadcast_channel=broadcast_ch)
hub.system.set_stop_button(None)

comms = HandleComms(hub, observe_ch)
motor = Motor(port)

print('Voltage: ', hub.battery.voltage())
print('Version: ', version)

async def main():
    await multitask(comms.run(), spin(), blink(), handle_buttons(), set_state_from_observations(), race=True)

run_task(main())
motor.stop()

[laurensvalk]

Are you able to reproduce the same behavior in a smaller program?

[vascolp]

Apparently no. I tried two very simple programs, one receiver and one sender, with parts of the program above, but without the HandleComms object, and I could not replicate. Then I tried to put the HandleComms and it started to seem to have delays like the program above... but I quit, because the program above is much easier to debug than having two separate programs... One could think it is related with tasks, but with City and Prime everything works fine.

[vascolp]

Today I did some more tests with this setup, and for some reason, I went to a different room in my apartment. And there there was no problem! Coming back to the initial room and the problem is there! It seems this is related with the location where I test it! But with City and Prime I could not reproduce the problems…
Is there a big different in the TechnicHub BT hardware? Something that makes it more sensible?
So sorry and thanks for your help!

[laurensvalk]

Can you share the simplified program that reproduces the error in the bad room?

[vascolp]

I used the version above.... you see the version above has a simple exit mechanism (double click)... very useful for testing and retesting. maybe later today I can get some time for a simpler version...

[laurensvalk]

It would be good to have a test with just the hub, no motors or special setup, no multitasking. Making the test as small as possible rules out other issues.