The Tektronix TM-504 Power Module is a mainframe for the TM 500 Series of Modular Instrumentation. It accepts up to four independently functional Plug-In modules to form a compact versatile and low cost instrumentation system. The TM 504 is a basic power source and provides an interconnecting scheme for the other members of the TM 500 Series family. Factory options allow customer modification using signal access at the Plug-In module/power module Interface to provide rear panel inputs and outputs, and interconnections between compartments. Power Source This instrument is designed to operate from a power source with its neutral at or near earth (ground) potential with a separate safety-earth conductor. It is not intended for operation from two phases of a multi-phase system, or across the legs of a single phase, three-wire system.