Downlink Core:

States:

IDLE

Nothing is happening
FIFO isn't being written to or read from. It should be empty.
Move into ARM when the arm command is received.

ARM

Pop things into the FIFO. If the FIFO's more than halfway full, pop them out as well. This ensures the buffer remains half full.
Move into FILL when trigger condition is met.

FILL

Waiting for the FIFO to fill
Move to DOWNLINK once full.

DOWNLINK

Pop things off the FIFO until it's empty.
Move to IDLE once FIFO is empty.

Uplink Core:

Just two states, IDLE and RUN. Unless we want it to be reconfigurable, in which case we'll need states for being uninitialized or uploading. But this can be specified in the YAML.