RING GENERATOR PSU The Ring Generator PSU is intended for use with the External Ring Generator assembly (90-5945-01), and 3600 system power trays (90-3669-xx). The Ring Generator PSU includes one ringing power supply unit (PSU) and grounding wire. There are some switches on the product to select; DC bias, and operation frequency (16Hz, 20Hz, or 25Hz). The External Ring Generator configuration requirements: dual output, redundant operation = 4 Ring Generator PSU; single output, redundant operation = 2 Ring Generator PSU; dual output, non-redundant operation = 2 Ring Generator PSU.