How are sensors in the wheels connected to power and data buses?
For small planes simple systems can be installed that fit on the inflation tubes and replace your standard cap. They use small batteries in the sender units and a little read out.

It looks like Boeing gets their tire pressure/break temp systems from Crane Aerospace. The latest and greatest systems are all wireless 


  The Crane 777 Tire and Brake Monitoring System (TBMS) provides
  continuous real-time monitoring of aircraft tire pressure and brake
  temperature. The design incorporates our SmartStem® wireless tire
  pressure sensor with integrated fill valve. The SmartStem uses sensing
  technology with triple redundant sensing channels to provide high
  reliability and accuracy. The Crane TBMS replaces TPMU and BTMU units
  (from the legacy system) with a single Tire and Brake Monitoring Unit
  (TBMU) that interfaces with the existing brake temperature probes. The
  existing wheel interface unit is replaced with an in-axle transformer
  and relay transformer hubcap assembly that allow the tire pressure
  signal to be transmitted magnetically with no physical contact between
  the components. Retrofit of the Crane TBMS requires no change to the
  AMC, EICAS, and CMC system interfaces.


That does not shed to much light on how it all works though. Their handheld system is RFID based and presumably their built in system is similar.