The S80/S100 and S8/S10 have rear seat repeaters (-D) and are interchangeable. i.e. you can mix and match S80/S8/S100/S10 with S80D/S8D/S100D/S10D. The second unit is more than just a repeater display; it has all of the same functionality as the main unit. Front and back units are connected together with a CAN Bus cable carrying both data and power. One of the great features of the S80 and S8 is connectivity for a PDA or Oudie including power and data from the main unit. You can also drive a second rear seat PDA or Oudie using a CAN2PDA box. This small box connects into the CAN Bus between the front and rear S80 or S8 and provides data and power to the second PDA or Oudie. Parameter changes from PDA to and from the vario are possible such as MacCready, QNH etc.
The only difference between the S100D and S80D is both the S100D and S10D include the CAN2PDA box built in.