From AT:

The AT-MX351 SmartMixer® is a microprocessor controlled, automatic switching, five-channel mixer. The four microphone inputs are XLRF-type balanced, with 48-volt phantom power available on pins 2 and 3. The Aux input is an RCA jack accepting auxiliary-level input from sources such as tape decks and VCRs. There is no signal processing of the Aux input. The mixer output is XLRM-type balanced, non-inverting. All AT-MX351 and AT-MX341a SmartMixers can be daisy-chained via the included AT8325/1.0 Link Cable and special connectors on the rear panel. The control bus and the audio are carried between mixers by the link cable. The result is that all microphones on a multi-mixer system can be controlled by one microphone connected to any mixer. Since all of the mixers are independently powered, there is no practical limit to the number of SmartMixers that can be daisy-chained.

The AT-MX351 provides an independent gain control on the front panel for each input channel. Substantial gain reserve and adjustment range permit microphones of widely differing sensitivities to be used together successfully. Automatic threshold setting on all mic/line channels ensures proper gating levels for existing ambient sound conditions.