Investigate why TechnicMediumHub responds with LedLight when VccPortControlOn/Off is sent

[tthiery]

Current Behavior
hub.VccPortControlOnAsync() or hub.VccPortControlOffAsync seems to trigger a detach on all ports with an attached device and then a registration of a LedLight (which as a device is currently not implemented).

Question

• Why is that?
• What is that for?
• Maybe VccPortControlOnAsync means to switch of the wire communication protocol and instead just supply energy? Useful for more dumb devices? However, maybe this command is used to preserve energy on the hub, but why is then a LedLight showing up.

[tthiery]

See pybricks/support#71

[tthiery]

And pin out on https://bricks.stackexchange.com/questions/10364/lego-power-function-pf-2-0-specification

[tthiery]

Considering @dlech reply in pybricks question, this feature more sounds like disabling the energy supply to all ports (external/internal?).

According to this post, LED lights are not using VCC at all. Maybe that is the reason why the hub defaults to led lights when switching of power.
https://bricks.stackexchange.com/questions/10373/which-resistor-value-is-used-for-which-motor-in-power-functions-2-0

@dlech You seem to be a hardware genius ... thoughts on this?

[dlech]

This turns power to pin 4 on the I/O ports on and off. I'm not sure what any practical purpose there would be for this though. Maybe it was used during product development to reboot the sensors or something like that?

[tthiery]

Personal guess: Something like a red button "emergency switch" to immediately halt all devies. But that does not explain the LEDs.

[dlech]

It is probably just a coincidence that with the power off the electrical circuit looks like the LED lights and so the automatic device detection algorithm detects that instead of no device connected.

[dlech]

to immediately halt all devices

It would not stop motors since those use direct battery power. 😉

[dlech]

On second thought, maybe it would since it would think that the motor is detached.