ON LINE SERIAL COMMUNICATION INTERFACE DEVICE TO A TRANSMITTER FROM A CURRENT LOOP

摘要

A serial communication interface establishes communication to the microprocessor of a transmitter, from a current loop which is powered by a power supply. The microprocessor outputs an analog signal corresponding to a process variable and also can receive a serial communication voltage pulse signal. The analog signal is applied to a current regulating circuit which draws current from the power supply onto the current loop in an amount which is proportional to the process variable. A comparator in the form of an operational amplifier has an output connected to the microprocessor for applying the serial communication voltage pulse signal to the microprocessor. The comparator has one input which is held at a slight negative voltage and another input which is connected to a line of the current loop connected to the negative terminal of the power supply. To establish communication from the current loop to the microprocessor, voltage on the current loop is modulated by a small amount to generate small voltage pulses which are applied to a positive input of the comparator. The comparator amplifies the small voltage pulses into larger voltage pulses which are then applied to the input of the microprocessor to establish digital communication with the microprocessor. In this way the analog signal on the current loop can be maintained simultaneously with digital communication between the loop and the microprocessor.