The ES-4000A is an expansion station for the US-2002 user station.
It interfaces four additional intercom channels to the US-2002, and it provides talk, listen and call buttons for the four additional channels. There are also four additional program inputs on the back of the ES-4000A, one for each added channel.
Up to four ES-4000A expansion stations may be connected to the US-2002 to add up to 16 channels (18 channels total). The ES-4000A typically utilizes the US-2002 microphone and speaker (or microphone headset) for communication with the added channels. The ES-4000A also has individual speaker outputs for independent monitoring of one or more of the four channels if desired.