This is the X-10 Powerhouse 2-way transceiver module model RR501. It's often referred to as the "big" transceiver. Use this as the heart of any X10 wireless remote control system or to add an additional housecode to an existing system. It receives the RF signals from an X10 remote via the antenna and retransmits them thru your house wiring to be received by other X10 receiver modules. The RR501 has a two-way appliance module built-in to it that reacts to X10 line signals as well as wireless (RF) signals, whereas TM751's appliance module only reacts to RF signals, not X10 line signals. The appliance module can be "polled" for status (Is it on or off?). The appliance module can be set to respond to unit 1 or 9 in whatever housecode that you choose. Can operate up to 16 X10 addresses on one housecode. You can add up to 15 extra transceiver modules for as many as 256 different codes. The specs for the appliance module are the same as the TM751 mini-transceiver (120 Volts AC 15A, 1/3 HP motor load, 500 Watt incandescent lamp, and 400 Watt for a TV) Simply plug the transceiver module into any AC outlet, extend the antenna, select unit codes 1 or 9, and set the code wheel to correspond to the housecode (A thru P) you want it to control. Even though it's about twice the size of the TM751 mini-transceiver, it's still small enough so that you can plug it in the bottom outlet leaving the top outlet free so as not to tie up an entire outlet.