What is the difference between using VEXnet as opposed to Bluetooth and why is only VEXnet allowed on fields?
They both connect with the brain of your robot such that you can control using a controller. They are just 2 different methods of doing so. VEXnet must be used on the field because external sources may mess with Bluetooth. For instance, if your phone has Bluetooth and it is on, that signal could interfere with the connection of the robot. Hence, it is not allowed. This so not happen on VEXnet since it runs at a different frequency.
This is really not true at all. Both VexNET and Bluetooth operate on approximately 2.4 GHz.
VEXnet is just a custom protocol engineered by VEX with certain provisions, not present in Bluetooth, that theoretically make it a better choice for competition use.
I’m pretty sure that vexnet is what is needed for connecting to the competition control system, and is what the system uses to determine the state that the Robot should be in, (driver or auto) while on the field. Using Bluetooth doesn’t do this, and if you are plugged into the tower with Bluetooth on, then it won’t connect. At least it didn’t for me, I don’t know if that applies for everyone.
VEXnet and Bluetooth both use 2.4 GHz and both use some kind of channel hopping scheme. Likely both frequency and time diversity but that’s beyond the scope of this forum.
I don’t know the specifics but I did once take a crack at reversing the VEXnet transaction and I can tell you some amount of crypto is going on during the pairing process. It makes sense that a key exchange happens.
I can also tell you that the controller is directly communicating with the field control over an extremely simple process which does not involve the wireless communication to the robot. Its:
FC -> Controller <~~~> Robot
So there’s no real reason why any means including being tethered would be fine as far as the FC cares.
It’s my understanding that in general VEXnet improves the ability for multiple signal sources to work with one another at the same time, possibly as the protocol is likely simpler than BT.