Re: Ah... Serial ports
You can't put useful security on the serial port itself, there isn't the CPU (or the bandwidth) in most devices.
Aside from that, even if you did it could not even begin to protect against man-in-the-middle or replay attacks without making the port itself useless for its intended purpose - namely simple interconnect between disparate systems.
Your TV probably has a serial port - it's for remote control like on/off and channel select when used in places like the Heathrow baggage area.
As long as that network stays private, the risk is easily mitigated. The trouble arises when the network is not private!
The security has to be in the serial-to-internet link, that's the only effective location.