| 摘要 |
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. Thus, this method of communication provides for selective alternate digital and analog communications with the digital communications being achieved by variations of the current in the communication loop between the 4-20 ma limits, while the analog data is represented by a corresponding direct current magnitude between the 4-20 ma limits. Consequently, the variation of the loop current in a first format produces a digital communication capability while the variation of the loop current in a second format produces an analog data communication capability.
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