On-line serial communication interface from a computer to a current loop

Abstract

An on-line serial communication interface is established from a digital circuit such as a computer or hand held terminal, to a current loop which has lines for connecting a transmitter to a power supply. The interface includes a diode which is connected in series with one of the lines of the current loop. The diode is connected in parallel to the source and drain of a FET. The control gate of the FET is connected to the output of a differential amplifier who receives as one of its inputs, a digital voltage pulse signal from the digital circuit. The other input of the amplifier is held at a selected voltage so that voltage pulses are output from the amplifier when the digital circuit generates its voltage pulses. This turns the FET on and off in synchronism with the voltage pulses, thus cutting the diode in and out of series with the line of the current loop. With the FET in its non-conducting off condition, the diode has a small voltage drop thereacross. This does not effect the current on the current loop. With the FET conducting and in its on state, the diode is cut out and its voltage drop is applied to the transmitter which is connected to the current loop. Communication can thus be established with the transmitter while again avoiding any interruption in the current on the current loop.