Improvements in 2-wire analog communication systems

Abstract

Abstract A data communication system uses a 2-wire transmitter 2 which adjusts the current through the pair of wires 17 between 4 and 20 mA to represent a process variable; this can be sensed by an analog-to-digital converter 70, across a resistor RO, feeding a display 74. A communication device 8 can interrupt the PV analog signalling for digital signalling. Device 8 draws a 16 mA current for 1 bit time, and this is sensed by transmitter 2, which ceases PV transmission. Device 8 then sends 1 or more bytes, by drawing 0 and 16 mA current pulses, which are sensed by transmitter 2. Transmitter 2 can then send data back to device 8 by raising its output to 20 mA, and dropping it to 4 mA for 1 bit time each, then transmitting the data as 1 or more bytes with logic levels 4 and 20 mA. A steady level of 4 mA for a predetermined period t is recognized by either device as the end of digital signalling, and the transmitter 2 resumes its analog PV output

Inventors

• 👤 Felix J. Houvig
• 👤 Ronald H. Rowlands

Bibliographic Details