Abstract: An embodiment of the present invention is a site controller that comprises a microcomputer with a RAM for main memory and an EPROM for program memory, an interface for accepting RS-485 data and voice communication on respective differential wire pairs from a daisy chain of student response keypad terminals, a public telephone data access arrangement with hybrid for sending and receiving voice over a dial-up line, a universal asynchronous receiver-transmitter for data communication by modem with a remote host site via an X.25 PAD and an audio processor for half-duplex communication between an instructor and a student that has voltage controlled amplifiers and side chain devices that reduce audio amplifier gain during periods of silence in order to eliminate feedback howl and adjustable delay and threshold devices for gating the respective channels through in the half-duplex mode.

Abstract: A daisy chain voice-data terminal for communicating data and voice signals between remote locations, comprising, according to an embodiment of the present invention, a microcomputer with a RAM for main memory and an EPROM for program memory, a membrane keypad connected to an input port of the microcomputer, a liquid crystal display adjacent to the keypad and connected to an output port of the microcomputer, a microphone and preamplifier, a differential output audio amplifier, a relay controlled by the microcomputer to switch the differential output audio amplifier on and off a daisy chain cable connected to other response keypads and a site controller, an RS-485 serial input/output to interface the microcomputer to the daisy chain cable, a switching regulator to provide five volts to the logic circuits, and a separate three-terminal linear regulator to supply five volts to the audio circuits.