Process transmitter communication circuit

• US 6,006,338 A
• Filed: 07/09/1997
• Issued: 12/21/1999
• Est. Priority Date: 10/04/1996
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Edwards, Grant B., Longsdorf, Randy J., Nelson, Richard L., Pederson, David L.
• Primary Examiner(s): Sheikh, Ayaz R.
• Assistant Examiner(s): PANCHOLI, JIGAR K
• Application Number:
  US08/890,169
• Time in Patent Office: 895 Days
• Field of Search: 395/750.01-750.08, 395/280, 395/306
• US Class Current: 713/340
• CPC Class Codes: G05B 19/4185 characterised by the network communication
• CPC Class Codes: Y02P 90/02 Total factory control, e.g. smart factories, flexible manufacturing systems [FMS] or integrated manufacturing systems [IMS]

Abstract

A process transmitter communications circuit has link terminals which are coupled to a process link, which supplies power to the circuit. A power source is coupled to one of the link terminals and is independent from the power supplied from the process link. Communications circuitry is coupled to the power source and link terminals, which transmits a transmitter signal over the process link, through the link terminals. A power sensing component is coupled to at least one of the link terminals and the communications circuitry and senses whether the power supplied by the process link is inadequate to transmit the transmitter signal. A power control component is coupled to the power source, the link terminals and the communications circuitry and selectively applies power from the power source to the communications circuitry for transmitting the transmitter signal when the power sensing component senses there is inadequate power supplied by the process link.

Claims

1. A process transmitter communications circuit for coupling to a process link which supplies power to the circuit, the circuit comprising:

link terminals which are adapted to couple to the process link;

a power source coupled to at least one one of the link terminals and independent from the power supplied from the process link;

communications circuitry coupled to the power source and link terminals, which is adapted to transmit an analog process signal and a digital transmitter signal over the process link, through the link terminals, wherein analog process signal has an analog level which is controlled by the communications circuitry based on a process variable;

a power sensing component coupled to at least one of the link terminals and the communications circuitry, which is adapted to sense whether the power supplied by the process link is inadequate to transmit the transmitter signal; and

a power control component coupled to the power source, the link terminals and the communications circuitry which is adapted to selectively apply power from the power source to the communications circuitry, the communications circuitry being adapted to transmit the digital transmitter signal, but not the analog process signal, with the power supplied by the power source when the power sensing component senses there is inadequate power supplied by the process link.

2. The process transmitter communications circuit of claim 1 wherein the power control component directs a predetermined level of current from the power source to the communications circuitry when the power sensing component senses there is inadequate power supplied by the process link to transmit the digital transmitter signal and wherein the communications circuitry comprises a modulator which modulates the predetermined level of current to produce the digital transmitter signal.

3. The process transmitter communications circuit of claim 1 wherein the digital transmitter signal comprises status information relating to the transmitter and wherein the communications circuitry continues to transmit the status information during periods of inadequate power supplied by the process link.

4. The process transmitter communication circuit of claim 1 and further comprising a process sensor which has a sensor output, a signal processor which is coupled to the sensor output and has a process signal output, and a modem which is coupled between the process signal output and the communications circuitry.

5. The process transmitter communications circuit of claim 4 wherein the process sensor is selected from the group consisting of level, mass, flow, pressure, and temperature process sensors.

6. The process transmitter communications circuit of claim 1 wherein the link terminals form a process connection selected from the group consisting of a two-wire process loop connection, a process bus connection, a point-to-point process connection, a daisy chain process connection, and a tree process connection.

7. The process transmitter communications circuit of claim 1 wherein the communications circuitry has a commissioning mode of operation and wherein the communications circuitry is powered solely from the power source when in the commissioning mode.

8. The process transmitter communications circuit of claim 1 wherein the power sensing component includes a diode that is connected between the power source and one of the link terminals.

9. The process transmitter communications circuit of claim 8 wherein the control component also includes the diode that is connected between the power supply and one of the link terminals.

10. The process transmitter communications circuit of claim 8 and further comprising:

a process sensor which measures the process variable;

a microprocessor which is coupled between the process sensor and the communications circuitry;

the communications circuitry comprises a modulator coupled to the link terminals; and

wherein the microprocessor has a control output coupled to the communications circuitry which controls a level of current flowing through the modulator as a function of the process variable measured by the process sensor to generate the analog process signal and which modulates the current flowing through the modulator to generate the digital transmitter signal.

11. The process transmitter communications circuit of claim 10 wherein the power source supplies power sufficient for the modulator to generate the digital transmitter signal but insufficient for the modulator to generate the analog process signal when the power sensing component senses that the power supplied by the process link is inadequate.

12. The process transmitter communications circuit of claim 8 wherein the diode is reverse biased when the power supplied by the process link is greater than a threshold level and is forward biased when the power supplied by the process link is less than the threshold level.

13. The process transmitter communications circuit of claim 1 and further comprising:

a process sensor which measures the process variable;

a microprocessor which is coupled between the process sensor and the communications circuitry;

the communications circuitry comprises a modulator coupled to the link terminals;

the power sensing component comprises a sensed voltage output indicative of the voltage supplied by the process link;

wherein the microprocessor forms the power control component and switches between a normal power mode when the sensed voltage output is greater than a threshold and a low power mode when the sensed voltage output is less than the threshold;

wherein the microprocessor has a control output coupled to the communications circuitry which, when in the normal power mode, controls a level of current flowing through the modulator as a function of the process variable measured by the process sensor to generate the analog process signal and which modulates the current flowing through the modulator to generate the digital transmitter signal; and

wherein the control output of the microprocessor, when in the low power mode, sets the level of current flowing through the modulator to a predetermined level independent of the process variable measured by the process sensor and modulates the current flowing through the modulator to generate the digital transmitter signal on the process link.

14. The process transmitter communications circuit of claim 13 wherein the modulator comprises a transistor coupled to the link terminals and wherein the control output of the microprocessor operates the transistor to control current flow through the link terminals to permit transmission of the digital transmitter signal.

15. The process transmitter communications circuit of claim 1 wherein the power sensing component comprises a voltage divider coupled to one of the link terminals.

16. The process transmitter communications circuit of claim 15 wherein the power sensing component further comprises a diode that is connected between the power supply and one of the link terminals.

17. The process transmitter communications circuit claim 16 wherein the diode becomes forward biased when the power supplied by the process link is less than a threshold level;

wherein the communications circuitry comprises a current control element that controls the level of current flowing through the link terminals; and

wherein the power control component comprises a microprocessor coupled to the voltage divider and the current control element for setting the level of current flowing through the link terminals to a predetermined level that will permit transmission of the digital transmitter signal when the power supplied by the process link is less than a threshold level.

18. The process transmitter communications circuit of claim 1 and further comprising a modem having a transmitter output line coupled to the communications circuitry;

and a switch which connects the transmitter output line to one of the link terminals and which is operated by the microprocessor to allow transmission of a voltage signal, which is generated by the modem on the transmitter output line, directly to the process link.

19. A method of transmitting an analog process signal and a digital transmitter signal over a process link with a process transmitter which is powered by the process link, the method comprising:

coupling the process transmitter to the process link through link terminals;

sensing whether the power supplied by process link is above or below a threshold level;

transmitting the analog process signal and the digital transmit signal over the process link with a communications circuit in the transmitter when the power supplied by the process link is above the threshold level;

selectively coupling a power source, which is independent from the power supplied by the process link, to one of the link terminals to supply power from the power source to the communications circuit when the power supplied by the process link is below the threshold level; and

transmitting the digital transmit signal, but not the analog process signal over the process link with the communications circuit when the power supplied by the process link is below the threshold level.

20. A transmitter for use in a process control system, comprising:

a sensor for sensing a process variable and generating a sensor signal which is representative of the process variable;

information means for providing transmitter information;

a microprocessor coupled to the sensor and the information means for receiving the sensor signal and the transmitter information and responsively providing a data output;

an independent power source providing a power output;

communication circuitry comprising;

a data input coupled to the data output of the microprocessor;

first and second loop terminals adapted for coupling to a process control loop;

a modulator coupled to the data input and the first and second loop terminals, which is adapted to control an analog signal level on the process control loop through the first and second loop terminals in response to the data input and based on the sensor signal and is adapted to digitally modulate analog signal level in response to the data input and based on the transmitter information; and

a power sense connection coupled to the microprocessor and the loop providing an output related to power on the process control loop;

wherein the microprocessor controls the communication circuitry based upon the power detected through the power sense connection to selectively operably couple the power output to the loop terminals in response to a condition in which the loop is not powered, and operates the communication circuitry so that the modulator sets the analog signal level to a predetermined level independent of the sensor signal and digitally modulates the predetermined level based on the transmitter information during the condition.