公开(公告)号：DE69222652D1

公开(公告)日：1997-11-13

申请号：DE69222652

申请日：1992-08-20

Applicant: ROSEMOUNT INC

Inventor： ANDERSON STEPHEN D ,  FRICK ROGER ,  MONZO GLEN ,  WESTFIELD BRIAN

IPC: G08C19/02 ,  G08C19/00

Abstract: A three wire transmitter bidirectionally communicates AC signals to and from a first external device and sends DC signals to a second external device. The transmitter includes a communication circuit which is energized from power and common terminals and includes memory storage for transmitter status and a process variable (PV). The communication circuit receives a sensor output indicative of the PV and provides the DC signal and the AC signal to a signal terminal which connects to both external devices, and also receives AC signals from the first external device. The communications circuit has a characteristic AC impedance between the signal and common terminals over an AC frequency range for receiving and transmitting AC signals to and from the first external device so that the receiving signals are not shorted out and so the transmitted signals can be received. The communications circuit has a characteristic DC impedance between the signal and common terminals over a range of frequencies which include DC and typically extend to approximately 20 Hz. The characteristic impedance is substantially lower than the impedance of the second external device which receives DC signals so that the accuracy of the DC signal is not compromised.