Does controller need it's UID and support any PIDs?

[alvangee]

Probably it's a noob question, but couldn't find any mentions on this topic.
There's a list of required parameters and a whole lot of discussions on how responders should operate.

But if it's a controller, is it required that it has it's own UID, responds to Discovery commands and supports any of the PIDs?

Now I have Enttec MK2 controllers and I can see that they respond to Discovery with their own UID, but to any other PID request they send ACK_TIMER. Not sure why ACK_TIMER though, it seems more appropriate to send NACK with the reason NR_UNKNOW_PID to tell that PID is not supported.