uint8_t subs_head; /* if head == tail, no subs */
uint8_t subs_tail; /* first invalid */
uint8_t subs_tx; /* current emitting device */
uint8_t subs_head; /* if head == tail, no subs */
uint8_t subs_tail; /* first invalid */
uint8_t subs_tx; /* current emitting device */