Bluetooth Hub to Hub communication

[bartneck]

I just started working with Pybricks and I hope that my question is not completely silly. I am trying to communicate between a city hub and a technic hub to control my train. The train sends a message "switch" to the track switch. This does, however, not happen all the time. How can I ensure that the message is sent and received? Moreover, the message should only be processed once. Right now, I pause the program on the technic hub (track switch) for a longer time after it received a message to ensure it does not process the same message twice. The train broadcasts the message for 800ms right now. I hope that is long enough.

I made a little YouTube video to show the behaviour of the system: https://youtu.be/Z5ptezsdyBY

Here is the program for the technic hub (track switch):

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from pybricks.hubs import TechnicHub
from pybricks.parameters import Axis, Color, Direction, Port
from pybricks.pupdevices import DCMotor
from pybricks.tools import wait

# Set up all devices.
technic_hub = TechnicHub(top_side=Axis.Z, front_side=Axis.X, broadcast_channel=1, observe_channels=[2])
switch_a = DCMotor(Port.A, Direction.COUNTERCLOCKWISE)

# Initialize variables.
switch_a_position = 0
message = 0


# The main program starts here.
print('Hello, Technic Hub')
switch_a_position = 'straight'
technic_hub.light.on(Color.GREEN)
while True:
    message = technic_hub.ble.observe(2)
    if message == 'switch':
        if switch_a_position == 'straight':
            print('switch to turn')
            switch_a.dc(50)
            wait(1000)
            switch_a.brake()
            switch_a_position = 'turn'
            technic_hub.light.on(Color.RED)
        elif switch_a_position == 'turn':
            print('switch to staight')
            switch_a.dc(-50)
            wait(1000)
            switch_a.brake()
            switch_a_position = 'straight'
            technic_hub.light.on(Color.GREEN)
        else:
            print('no position')
        # Wait for BL broadcast to stop
        wait(2000)
    elif message == None:
        print('none message')
        wait(100)
    else:
        print('not switch message')

and here is the program for the train:

# pybricks blocks 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from pybricks.hubs import CityHub
from pybricks.parameters import Color, Direction, Port
from pybricks.pupdevices import ColorDistanceSensor, DCMotor
from pybricks.tools import wait

# Set up custom colors.
Color.RED_TILE = Color(358, 95, 78)
Color.GREEN_TILE = Color(138, 91, 49)
Color.BLUE_TILE = Color(219, 93, 70)
Color.TRACK = Color(173, 40, 27)
Color.FLOOR = Color(9, 30, 45)

# Set up all devices.
orange_train = CityHub(broadcast_channel=2)
color_sensor = ColorDistanceSensor(Port.B)
color_sensor.detectable_colors((Color.RED_TILE, Color.GREEN_TILE, Color.BLUE_TILE, Color.TRACK, Color.FLOOR))
train_motor = DCMotor(Port.A, Direction.CLOCKWISE)


# The main program starts here.
print('Hello, train!')
train_motor.dc(50)
while True:
    if color_sensor.color() == Color.BLUE_TILE:
        train_motor.stop()
        orange_train.light.on(Color.BLUE)
        orange_train.ble.broadcast('switch')
        wait(800)
        print('sent')
        orange_train.ble.broadcast(None)
        wait(2000)
        train_motor.dc(50)
    elif color_sensor.color() == Color.GREEN_TILE:
        train_motor.stop()
        orange_train.light.on(Color.GREEN)
        wait(2000)
        train_motor.dc(50)
    elif color_sensor.color() == Color.RED_TILE:
        train_motor.stop()
        orange_train.light.on(Color.RED)
        wait(2000)
        train_motor.dc(50)
    else:
        pass

[laurensvalk]

This communication method works best for broadcasting continuous 'states' rather than sending 'messages'.

As a reasonable analogy, think of you and a friend on distant rooftops where you can see but not hear each other. Instead of blinking a light briefly to indicate a switch (which would be easy to miss) you might set the light continously green for straight and red for turning. Your friend would always know how to set the switch by observing the signal, no matter when they look or no matter the initial position.

In this example, instead of broadcasting 'switch now' for some time in the hope that someone will hear it, it is better to broadcast 'straight' or 'turn' as applicable. This will be correct even if you miss any messages. You also don't need any delays this way. The hub continues broadcasting your last value until you change it - no need to repeat it yourself.

On the receiving end, you could make the motor move the switch if the motor is not already in the correct position.

As a side note, it works even better if your broadcasts all have the same length, such as 'trn' and 'str'. Or simply 1 and 2. (This isn't the issue here, but it can be a factor if you want to send more data signals at once)

[bartneck]

Thanks for this information. That clarifies it. I will change the code to work with the messaging paradigm. Maybe an event-based messaging system could be useful for some applications.

[laurensvalk]

Glad it helped! An event-based messaging system would require establishing connections between the hubs, but the options for that are limited unfortunately since it also has to connect to your PC and the Bluetooth chip can't do too many things at once.

I should probably also add that observing works best when the receiving hub is disconnected from your computer (since its Bluetooth chip is less busy).