Abstract: A communication system for communicating analog and digital data with a process variable, e.g., pressure, transmitter over the two wires which supply power to the transmitter apparatus. The digital communication operation is half-duplex, bit serial transmission and is represented by the currents and voltages in the transmitter communication loop which are selectively introduced between process variable analog data transmissions of 4-20 ma. A plurality of bit cells are provided for each data word or byte between a start bit and a parity bit cell. Following the completion of transmission of the digital data, a predetermined time period is introduced to enable the resumption of transmission of process variable data. Multiple byte digital data transmission can also be effected between process variable analog transmissions by having a byte spacing less than the predetermined time period preceeding the 4-20 ma process variable transmissions.

Abstract: A bus structure for the interconnection between a data bus capable of transferring a plurality of words and a local bus which can carry only one word. This structure includes latches for storing the plurality of words for simultaneous transfer over the data bus or for sequential transfer over the local bus as required. Control logic operates to perform the necessary control over this interconnection to effect the required operation.

Abstract: A sensor correction circuit uses a programmable read only memory (PROM) for storing characterizing information for an associated sensor or transducer. The information is stored in the 1.times.2048 bit configuration. The programmable read-only memory (PROM) has its parallel outputs connected to corresponding inputs of a 2048 bit shift register. The programmable read-only memory and the shift register are located in an electronic circuit housing, along with a power supply and sensor signal handling electronics, attached to the sensor structure to form an integral package therewith. An isolation interface is mounted on the housing and is connected between the circuit within the housing and an external data processing and interrogating system which may include a microprocessor.