EV3 communication over USB

[jaguilar]

First off, I want to say congratulations to the devs for getting ev3 baremetal working. It's an amazing piece of work and I'm tickled pink that it's been done.

I was going to try to implement one of the ev3 demo projects using pybricks and settled on doing the spinner. But I immediately ran into a slight problem: how do I command the second hub from the first one? In the building instructions, the EV3s are connected via a USB cable. Does the existing firmware allow input and output via USB? For example, if hub1 did print('foo\n'), would input() on hub2 be able to retrieve that string? If not, is any such API planned?

[laurensvalk]

Thanks @jaguilar !

how do I command the second hub from the first one?

We have two options, neither of which are currently implemented:

• Bluetooth communication between hubs
• USB connection between bricks (commonly known as EV3 daisy-chaining)

Our contributor @ArcaneNibble (who has made major contributions to make EV3 support possible) has been looking into both these to see what is possible.

Besides writing the driver, we'll also still need to settle on an API. For backwards compatibility, I think we'll want to implement the EV3 Mailboxes, but we should be able to do more creative things with Bluetooth data streams as well if we find the time.

[jaguilar]

Well, I took a crack at just getting it compiling with some ifdefs for bt classic and ble. What I found was that almost all of the existing Bluetooth code is le-specific (at least the stuff in bluetooth.c and bluetooth_btstack.c). It seems like we would be saving little by trying to interleave the classic and le state machines, so it seems like a better approach would be to create a bluetooth_classic.c file and just put a new state machine in there, patterned off the stuff in bluetooth.c.

My thinking, if we are trying to emulate mailboxes, is to keep it really simple, with just the rfcomm client and server stuff in C. The ev3 mailbox stuff can be written in python. Does that sound all right?

By the way, I don’t want to step on anyone’s toes if this is already being worked on. I don’t know how to check that though so I’m just going to proceed for now. If I can get something off the ground I’ll send some pull requests, but I don’t really know C all that well so we will see what can be done.

[laurensvalk]

Thanks for looking into this!

Agreed, this will be totally separate from the existing BLE implementations. These exist mainly to download and run programs from the web editor. This won't apply to EV3, at least not for now.

So starting with using Bluetooth classic to implement EV3-EV3 communication is a great start - and don't let any of our existing Bluetooth APIs hold you back.

Yes, we should be able to do much of the higher level stuff in Python. The EV3 has plenty of space. The other hubs did not, so we ended up doing pretty much everything in C, even if Python would have been more sensible at times.

It would be awesome to get a proof of concept up and running, and we can worry about bigger approaches when we get there.

[laurensvalk]

The other big use case here is connecting to Bluetooth gamepads like Playstation Controllers or other input devices like keyboards.

Since this involves only one side of the puzzle (rather than both a server and a client side), maybe this provides a slightly simpler starting point if that's any help.

[jaguilar]

My read of the peripheral library for the xbox was that y'all seemed to have found that the peripheral wouldn't connect properly without some LE-exclusive handshaking first. Particularly, these gatt functions are LE-only. Perhaps the xbox controllers have some classic-only protocol for pairing and connecting, but unless I'm misunderstanding something, it might be more work to figure it out than just getting the rfcomm stuff working.

For a proof of concept, I'll start with just the client code and have it talk to one of the example python programs on my computer. If we get that working, it should only be a small step further to get ev3-ev3 working.

[laurensvalk]

The Xbox Controller is LE only so it cannot be used with the EV3.

Playstation Controllers use Bluetooth classic, which have been popular with Pybricks 2.0 on ev3dev, which is why I brought it up. This was using Linux provided connectivity though.