The serial cable does not carry ANY packets, it only carries voltage between the 2 units.
Without any failsafe mechanism configured (say, out of the box in default set up) the unit will failover under following conditions:
1. Memory utilization goes above (95 or 97%)
2. Processe(s) listed under "system -> High Availability" dies or conditions listed there is met
3. Power supply (provided it is not dual power supply unit and one still functions) failed and the unit shuts off.
4. Disk or some other component failed and stops TMM from normal function
I may be forgetting some, but those are what I can think of.