Process control loop bridge

Abstract

A bridge connected between a first process control loop and a second process control loop, wherein the bridge allows AC digital signals to be transmitted between the first process control loop and the second process control loop, while preventing DC analog signals from being transmitted between the first process control loop and the second process control loop.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based upon and claims the benefit of U.S. Provisional Patent Application Serial No. 62 / 900,025, filed on September 13, 2019, the entire contents of which are incorporated herein by reference. Background Art

[0003] In a process control system, a transmitter measures a process variable and transmits the value of the process variable to a host. In many systems, the transmitter is connected to the host via a two-wire loop. The transmitter can transmit the value of the process variable as an analog signal by setting the DC current on the loop to a value that represents the value of the process variable. The DC current that can be applied to the loop is usually limited by standards to a specific range, for example, 0mA-20mA or 4mA-20mA, etc.

[0004] Additional information can be transmitted over the loop using an AC signal superimposed on the DC current. This additional information is sent using a signal format supported by a protocol (e.g., the HART protocol). One example of such a signal format is frequency shift keying, where the digital signal consists of two frequencies, 1200 Hz and 2200 Hz, representing bit 1 and bit 0, respectively. Thus, by changing the frequency of the AC signal, digital values ​​can be transmitted over the basic 4mA-20mA analog signal. In another example, phase shift keying is used, where the phase of the AC signal is changed to one of eight values, each representing a corresponding 3-bit digital value.

[0005] There are generally two configurations for connecting transmitters to a host: point-to-point and multi-drop. In point-to-point, there is only a single transmitter on each loop. In these configurations, each transmitter is able to send values ​​to the host using both analog 4mA-20mA signals and AC digital signals, but the transmitter cannot receive communications directly from other transmitters. In multi-drop, multiple transmitters are placed in parallel on the loop, which allows the transmitters to receive digital communications directly from each other. However, in such a configuration, any transmitter on the loop cannot send data using a DC current signal because the DC current of each transmitter is required to remain at 4mA, and the host does not measure the DC current on the loop. Therefore, in multi-drop mode, the transmitter can only use AC digital signals to transmit information. Summary of the invention

[0006] A bridge is connected between a first process control loop and a second process control loop, wherein the bridge allows an AC digital signal to be transmitted between the first process control loop and the second process control loop, while preventing a DC analog signal from being transmitted between the first process control loop and the second process control loop.

[0007] According to another embodiment, a process control transmitter includes: a sensor for sensing a property of a fluid in a process control system; a communication interface configured to communicate with a host using a DC analog signal on a process control loop and receive an AC digital signal; and a microprocessor. The microprocessor is configured to control the DC analog signal set by the communication interface to provide a process variable to the host, and the microprocessor is configured to send and receive, through the communication interface, digital values ​​correspondingly received and sent by a second process control transmitter on a second process control loop and transferred through a bridge for use in digital signals between the process control loop and the second process control loop.

[0008] In yet another embodiment, a terminal block includes: a first terminal for receiving a transmitter segment of a first process control loop connected to a first process control transmitter; and a second terminal for receiving a transmitter segment of a second process control loop connected to a second process control transmitter. A bridge circuit in the terminal connects the transmitter segment of the first process control loop to the transmitter segment of the second process control loop, such that a DC analog signal is blocked from being transmitted through the bridge circuit while an AC digital signal is transmitted through the bridge circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 A block diagram of a process control system according to one embodiment is provided.

[0010] Figure 2 supply Figure 1 A more detailed block diagram of a portion of a process control system.

[0011] Figure 3 A block diagram of a second embodiment is provided.

[0012] Figure 4 A perspective view of a terminal module according to one embodiment is provided.

[0013] Figure 5 A circuit diagram of the interior of a terminal module according to one embodiment is provided.

[0014] Figure 6 A circuit diagram of the interior of a terminal module according to a second embodiment is provided.

[0015] Figure 7 A block diagram of a process control system according to a second embodiment is provided. DETAILED DESCRIPTION

[0016] Under current standards, there is currently no way to allow direct communication between two transmitters while still allowing both transmitters to communicate with a host system via both analog (DC) and digital (AC) signals.

[0017] Embodiments provide a bridge for coupling wired digital communication signals between devices on isolated analog current loop circuits, allowing the devices to communicate individually with a host system and digitally with each other via analog direct current (DC) signals and digital alternating current (AC) signals. The point-to-point analog DC current loops are independent and separated from each other. The point-to-point digital AC signals are independent, but the bridge allows digital messages to be transmitted from one loop to another.

[0018] Figure 1 A block diagram of a process control system 100 having a host 102 and three process control transmitters 104, 106, and 108 is provided. The process control transmitter 104 is connected to the host 102 via a two-wire process control loop 110 having a branch 112 and a branch 114. A sense resistor 116 is connected in series with the branch 112, either internally or externally to the host 102, and the host 102 applies a voltage 118 between one end of the sense resistor 116 and the branch 114. In embodiments where the voltage 118 causes the branch 114 to have a higher voltage than the branch 112, the branch 114 is referred to as a positive branch, and the branch 112 is referred to as a negative branch. The process control transmitter 106 is connected to the host 102 via a two-wire process control loop 120 having a branch 122 and a branch 124. A sense resistor 126 is connected in series with branch 122 inside or outside the host 102, and the host 102 applies a voltage 128 between one end of the sense resistor 126 and branch 124. In embodiments where the voltage 128 causes branch 124 to have a higher voltage than branch 122, branch 124 is referred to as a positive branch, and branch 122 is referred to as a negative branch. The process control transmitter 108 is connected to the host 102 via a two-wire process control loop 130 having branch 132 and branch 134. A sense resistor 136 is connected in series with branch 132 inside or outside the host 102, and the host 102 applies a voltage 138 between one end of the sense resistor 136 and branch 134. In embodiments where the voltage 138 causes branch 134 to have a higher voltage than branch 132, branch 134 is referred to as a positive branch, and branch 132 is referred to as a negative branch.

[0019] Branch 112 and branch 122 are connected by bridge 140, while branch 122 and branch 132 are connected by bridge 142. Bridge 140 allows AC digital signals to be transmitted between branch 112 and branch 122, but blocks DC analog signals from being transmitted from process control loop 110 to process control loop 120, and vice versa. Bridge 142 allows AC digital signals to be transmitted between branches 122 and 132, but blocks DC analog signals from being transmitted from process control loop 120 to process control loop 130, and vice versa. The combination of bridge 140 and bridge 142 allows AC digital signals to be transmitted between branch 112 and branch 132, but blocks DC analog signals from being transmitted between process control loop 110, process control loop 120, and process control loop 130.

[0020] Figure 2 A more detailed block diagram of the process control transmitter 104 and the process control transmitter 106 and the host 102 is provided. The process control transmitter 104 includes a sensor 200, an analog-to-digital converter 204, a microprocessor 208, and a communication interface 212 having a DC analog communication interface 216 and an AC digital communication interface 220. The process control transmitter 106 includes a sensor 202, an analog-to-digital converter 206, a microprocessor 210, and a communication interface 214 having a DC analog communication interface 218 and an AC digital communication interface 222.

[0021] Sensor 200 and sensor 202 sense respective properties of a process fluid in a process fluid conduit or tank. In some embodiments, sensor 200 and sensor 202 sense the same property, while in other embodiments, sensor 200 and sensor 202 sense different properties. In some embodiments, sensor 200 and sensor 202 sense properties of the same process fluid, while in other embodiments, sensor 200 and sensor 202 sense properties of different process fluids.

[0022] The analog signals generated by sensors 200 and 202 are provided to respective analog-to-digital converters 204 and 206, which convert the analog signals to a series of digital values ​​that are provided to respective microprocessors 208 and 210. In some embodiments, additional filtering is applied to one or more of the analog signals and the series of digital values.

[0023] Microprocessor 208 and microprocessor 210 use the corresponding series of digital values ​​to determine one or more values ​​for the corresponding process variables. According to some embodiments, the series of digital values ​​are directly used as the series of values ​​of the process variables, while in other embodiments, the series of digital values ​​are used to calculate the values ​​of the process variables.

[0024] Microprocessor 208 uses DC analog communication interface 216 to communicate the process variable value determined by microprocessor 208. DC analog communication interface 216 communicates the process variable value as a DC analog signal over process control loop 110. According to one embodiment, the process variable value is communicated by controlling the DC current on branch 112 and branch 114 so that the current has a value between 4 mA and 20 mA (or 0 mA and 20 mA), which represents the current value of the process variable.

[0025] Microprocessor 210 uses DC analog communication interface 218 to communicate the process variable value determined by microprocessor 210. DC analog communication interface 218 communicates the process variable value as a DC analog signal over process control loop 120. According to one embodiment, the process variable value is communicated by controlling the DC current on branch 122 and branch 124 so that the current has a value between 4 mA and 20 mA, which represents the current value of the process variable.

[0026] The microprocessor 208 uses the AC digital communication interface 220 to transmit process variable values ​​and / or additional digital information determined by the microprocessor 208 and to receive digital signals on the process control loop 110. The AC digital communication interface 220 sends and receives values ​​and / or information as AC digital signals on the process control loop 110. According to some embodiments, the AC digital communication interface 220 sends and receives AC digital signals using frequency shift keying, wherein the digital signal consists of two frequencies of 1200 Hz and 2200 Hz representing bit 1 and bit 0, respectively. Therefore, by changing the frequency of the AC signal on the control loop 110, the AC digital communication interface 220 is able to transmit digital values ​​on top of the basic 4mA-20mA analog signal generated by the DC analog communication interface 216. In other embodiments, the AC digital communication interface 220 sends and receives AC digital signals using phase shift keying, wherein the phase of the AC signal is changed to one of eight values, each value representing a corresponding 3-bit digital value. Thus, by changing the phase of the AC signal on the control loop 110 , the AC digital communication interface 220 is able to transmit digital values ​​over the basic 4 mA-20 mA analog signal produced by the DC analog communication interface 216 .

[0027] The microprocessor 210 uses the AC digital communication interface 222 to transmit process variable values ​​and / or additional digital information determined by the microprocessor 210 and to receive AC digital signals on the process control loop 120. The AC digital communication interface 222 sends and receives values ​​and / or information as AC digital signals on the process control loop 120. According to some embodiments, the AC digital communication interface 222 sends and receives AC digital signals using frequency shift keying, wherein the digital signal consists of two frequencies of 1200 Hz and 2200 Hz representing bit 1 and bit 0, respectively. Therefore, by changing the frequency of the AC signal on the control loop 120, the AC digital communication interface 222 is able to transmit digital values ​​on top of the basic 4mA-20mA analog signal generated by the DC analog communication interface 218. In other embodiments, the AC digital communication interface 220 sends and receives AC digital signals using phase shift keying, wherein the phase of the AC signal is changed to one of eight values, each value representing a corresponding 3-bit digital value. Thus, by changing the phase of the AC signal on the control loop 120 , the AC digital communication interface 222 is able to communicate digital values ​​over the underlying 4 mA-20 mA analog signal produced by the DC analog communication interface 218 .

[0028] The host 102 includes one or more microprocessors 250 that communicate with a corresponding DC analog communication interface and a corresponding AC digital communication interface for each control loop connected to the host 102. For example, for the control loop 110, the host 102 has a DC analog communication interface 252 and an AC digital communication interface 254, and for the control loop 120, the host 102 has a DC analog communication interface 256 and an AC digital communication interface 258.

[0029] The DC analog communication interface 252 is connected across the sense resistor 116 and uses the DC voltage across the sense resistor 116 as a measure of the current on the process control loop 110, and thus as the value of the process variable. The DC analog communication interface 256 is connected across the sense resistor 126 and uses the DC voltage across the sense resistor 126 as a measure of the current on the process control loop 120, and thus as the value of the process variable. The DC analog communication interface 252 converts the measure of the current on the process control loop 110 into a digital value and transmits the digital value to the microprocessor 250, while the DC analog communication interface 256 converts the measure of the current on the process control loop 120 into a digital value and transmits the digital value to the microprocessor 250. Although the DC analog communication interface 252 and the DC analog communication interface 256 are shown as communicating with the same microprocessor 250, in other embodiments, the DC analog communication interface 252 and the DC analog communication interface 256 communicate with separate microprocessors.

[0030] Also shown is an AC digital communication interface 254 that is connected across the sense resistor 116 and uses the AC voltage across the sense resistor 116 to determine the digital value sent on the control loop 110. For example, in some embodiments, the AC digital communication interface 254 uses the frequency of the AC voltage to decode 0 and 1 based on frequency shift keying, while in other embodiments, the AC digital communication interface 254 uses the phase of the AC voltage to decode 0 and 1 based on phase shift keying. An AC digital communication interface 258 is shown connected across the sense resistor 126 and uses the AC voltage across the sense resistor 126 to determine the digital value sent on the control loop 120. For example, in some embodiments, the AC digital communication interface 258 uses the frequency of the AC voltage to decode 0 and 1 based on frequency shift keying, while in other embodiments, the AC digital communication interface 258 uses the phase of the AC voltage to decode 0 and 1 based on phase shift keying.

[0031] AC digital communication interface 254 can also transmit AC digital signals from microprocessor 250 to process control loop 110, for example, using frequency shift keying or phase shift keying. Similarly, AC digital communication interface 258 can transmit AC digital signals from microprocessor 250 to process control loop 120, for example, using frequency shift keying or phase shift keying.

[0032] The bridge 140 is connected between the branch 112 of the control loop 110 and the branch 122 of the control loop 120. The bridge 140 blocks the DC analog signals generated by the DC analog interface 216 and the DC analog interface 218 from crossing between the process control loop 110 and the process control loop 120, while allowing the AC digital signals on the process control loop 110 and the process control loop 120 to cross between the process control loop 110 and the process control loop 120. Therefore, the AC digital signal generated by the AC digital communication interface 220 or the AC digital communication interface 254 on the loop 110 is transmitted through the bridge 140 so that the AC digital signal can be received by both the AC digital communication interface 222 and the AC digital communication interface 258 on the process control loop 120. Similarly, an AC digital signal generated by AC digital communication interface 222 or AC digital communication interface 258 on control loop 120 is transmitted through bridge 140 so that the AC digital signal can be received by both AC digital communication interface 220 and AC digital communication interface 254 on process control loop 110 .

[0033] According to one embodiment, the AC digital communication interface 220, the AC digital communication interface 222, the AC digital communication interface 254, and the AC digital communication interface 258 use the HART communication protocol when sending or receiving AC digital signals. Under the HART protocol, communication is based on a request and reply structure, in which one device makes a request to another device and the other device replies. The device that makes the request is called the master device, and the device that provides the reply is called the slave device. There can be up to two master devices on a process control loop-a primary master device and a secondary master device. The request is a digital value packet, which includes: an indication of whether the request is from the primary master device or the secondary master device, the address of the slave device to which the request is sent, and an indication of what request is being made. The reply is a digital value packet, which includes: the address of the slave device that provides the reply, an indication of whether the reply is for the primary master device or the secondary master device, and the requested information or error code. (Note that additional information can appear in both the request and the reply). The primary and secondary master devices can request that a slave process control transmitter put itself into burst mode, in which the slave device periodically generates reply packets containing the values ​​of process variables sensed by the process control transmitter. Once in burst mode, the process control transmitter will continue to send the latest process variable values ​​until instructed to stop by the master device that sent the burst mode request.

[0034] according to Figure 2In one implementation of an embodiment of the present invention, the host 102 is a primary master device, the process control transmitter 104 is a secondary master device, and the process control transmitter 106 is a slave device. The AC digital communication interface 220 of the process control transmitter 104 is used to issue a request addressed to the process control transmitter 106. Since the request packet is an AC digital signal, the bridge 140 passes the request packet from the process control loop 110 to the process control loop 120 so that it can be received by the AC digital communication interface 222 and can be provided to the microprocessor 210 of the process control transmitter 106. In the request, the process control transmitter 104 designates itself as a secondary master device and instructs the process control transmitter 106 to send a single reply packet containing a process variable value, or instructs the process control transmitter 106 to enter a burst mode and continuously send multiple reply packets, wherein each reply packet contains the latest process variable value. If the request is for a single reply packet, the microprocessor 210 instructs the AC digital communication interface 222 to generate a single reply packet containing the requested process variable value. If the request is for the transmitter 106 to enter burst mode, the microprocessor 210 instructs the AC digital communication interface 222 to generate the requested reply packets, which are broadcast on the loop 120 so that they are received by the AC digital communication interface 258. Since the reply packets are AC signals, the reply packets are passed across the bridge 140 to the process control loop 110, where they are received by the AC digital communication interface 220 and provided to the microprocessor 208, and received by the AC digital communication interface 254. In this manner, the process control transmitter 104 is able to receive the process variable value generated by the process control transmitter 106 directly through the bridge 140 without going through the host 102.

[0035] In other embodiments, the process control transmitter 104 and the process control transmitter 106 are slave devices and the host 102 is the primary master device. In these embodiments, the process control transmitter 104 and the process control transmitter 106 have different addresses. The host 102 sends a request to place the process control transmitter 106 in burst mode. In response, the microprocessor 210 instructs the AC digital communication interface 222 of the process control transmitter 106 to send AC digital reply packets containing the latest process variable values ​​sensed by the sensor 202. These reply packets are sent on the process control loop 120 and addressed to the host 102. However, these reply packets also pass through the bridge 140 to the process control loop 110, where they are received by the AC digital communication interface 220 of the process control transmitter 104. Even though the reply packets are addressed to the host 102, the AC digital communication interface 220 decodes the AC signal and provides the digitized process variable values ​​in the reply packets to the microprocessor 208.

[0036] According to one embodiment, the microprocessor 208 calculates additional process variable values ​​using the process variable values ​​received from the process control transmitter 106 and the process variable values ​​received from the sensor 200. For example, the microprocessor 208 may calculate the mass flow rate using the pressure value from the process control transmitter 106 and the differential pressure value (or any other volume flow value) from the sensor 200. The microprocessor 208 may then send the mass flow rate to the host 102 in the form of a DC analog signal or an AC digital signal.

[0037] Figure 3 A schematic diagram of an embodiment is provided in which the bridge 140 is located within the terminal module 300 of the panel 304. In some embodiments, the panel 304 is a field wiring panel, while in other embodiments, the panel 304 is a marshaling panel. Figure 3 In an embodiment, each positive branch and each negative branch of each process control loop is divided into a host segment and a transmitter segment. For example, Figure 1 The negative branch 112 is divided into a host section 322 and a transmitter section 332, Figure 1 The positive branch 114 is divided into a host section 324 and a transmitter section 334, Figure 1 The negative branch 122 is divided into a host section 342 and a transmitter section 352, and Figure 1 The positive branch 124 is divided into a host section 344 and a transmitter section 354.

[0038] Within the panel 304, the terminal module 302 connects the host segment and the transmitter segment to form the positive branch 114 and the positive branch 124. Thus, the terminal module 302 connects the host segment 324 to the transmitter segment 334 to form the positive branch 114, and connects the host segment 344 to the transmitter segment 354 to form the positive branch 124. The terminal module 300 connects the host segment and the transmitter segment to form the negative branch 112 and the negative branch 122, while also providing a bridge 140 between the negative branch 112 and the negative branch 122. Thus, the terminal module 300 connects the host segment 322 to the transmitter segment 332 to form the negative branch 112, and connects the host segment 342 to the transmitter segment 352 to form the negative branch 122. In addition, terminal module 300 includes circuit elements that allow AC digital signals to be transmitted between negative branch 112 and negative branch 122 while preventing DC analog signals from being transmitted between negative branch 112 and negative branch 122 .

[0039] Figure 4A perspective view of terminal module 300 according to one embodiment is provided. Terminal module 300 includes housing 400, and terminals 402, 404, 406, and 408 for receiving transmitter section 332, transmitter section 352, host section 322, and host section 342, respectively. Bridge 140 is located within housing 400.

[0040] Figure 5 A circuit diagram of the internal wiring of the terminal module 300 according to one embodiment is provided. Figure 5 4, conductor 500 extends from terminal 402 to terminal 406 to connect terminal section 332 to host section 322. Conductor 502, resistor 504, and conductor 506 extend in series from terminal 404 to terminal 408 to connect transmitter section 352 to host section 342. Capacitor 508 extends between conductor 500 and conductor 502. Capacitor 508 and resistor 504 form bridge 140, which allows AC signals to be transmitted between negative branch 112 and negative branch 122 while preventing DC signals from being transmitted between negative branch 112 and negative branch 122.

[0041] Figure 6 A circuit diagram of the internal wiring of the terminal module 300 according to the second embodiment is provided. Figure 6 6, conductor 600, winding 602 of transformer 612, and conductor 604 extend in series from terminal 402 to terminal 406 to connect transmitter section 332 to host section 322. Conductor 606, second winding 608 of transformer 612, and conductor 610 extend in series from terminal 404 to terminal 408 to connect transmitter section 352 to host section 342. Transformer 612 forms bridge 140 and allows AC digital signals to be transmitted between negative leg 112 and negative leg 122 while blocking DC analog signals from being transmitted between negative leg 112 and negative leg 122. In addition, transformer 612 allows negative leg 112 and negative leg 122 to be at different ground planes without affecting communication on control loop 110 and control loop 120.

[0042] In other embodiments, active circuit elements are used in bridge 140 to allow AC digital signals to be transmitted between loops while blocking DC analog signals from being transmitted between loops. In this simple case, additional components can be added to help balance the signal levels between the various HART loops.

[0043] Figure 7A block diagram of another embodiment of a process control system is provided in which there is no process control loop between the process control transmitter 106 and the host 102. Instead, a power supply 700 is provided that is separate from the host 102 and that generates a voltage similar to the voltage 128 for the process control transmitter 106 and includes current protection in the event of a short circuit. In addition, a load resistor 702 having the same resistance as the sense resistor 126 is connected in series between the negative lead of the power supply 700 and the negative lead of the process control transmitter 106 through a negative branch 722. A positive branch 724 connects the positive lead of the power supply 700 to the positive lead of the process control transmitter 106. The negative branch 722 is connected to a bridge 140 that allows an AC digital signal to be transmitted between the negative branch 722 and the negative branch 112 of the process control loop 110. In this manner, the process control transmitter 106 is able to send and receive AC digital signals to and from the process control transmitter 104 and the host 102 through the bridge 140. According to one embodiment, the load resistor 702 is located within a terminal block that includes the bridge 140.

[0044] Any type of process control transmitter may be used with the various embodiments.

[0045] Although various embodiments are discussed separately above, those skilled in the art will recognize that these embodiments can be used together.Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the present invention.

Claims

1. A process control system comprising: a host connected to a first process control loop and a second process control loop, wherein the first process control loop and the second process control loop are used for wired communication; a first process control transmitter connected to the first process control loop and communicating with the host using DC analog signals and AC digital signals on the first process control loop; a second process control transmitter connected to the second process control loop and communicating with the host using DC analog signals and AC digital signals on the second process control loop; as well as a bridge connected between the first process control loop and the second process control loop for coupling a wired digital communication signal, wherein the bridge allows AC digital signals to be transmitted between the first process control loop and the second process control loop, while preventing DC analog signals from being transmitted between the first process control loop and the second process control loop, And wherein the first process control transmitter communicates directly with the second process control transmitter across the bridge using AC digital signals.

2. The process control system according to claim 1, wherein: The bridge is connected between a branch of the first process control loop and a branch of the second process control loop.

3. The process control system of claim 2, wherein: The bridge includes a capacitor connected to the one branch of the first process control loop and the one branch of the second process control loop.

4. The process control system of claim 2, wherein: The bridge includes a transformer connected to the one branch of the first process control loop and the one branch of the second process control loop.

5. The process control system of claim 1, wherein: The bridge is located within the terminal module.

6. The process control system of claim 1, wherein: The host computer functions as a primary master device and the first process control transmitter functions as a secondary master device.

7. The process control system of claim 1, wherein: The host computer acts as a primary master device and the first process control transmitter and the second process control transmitter act as slave devices.

8. The process control system of claim 1, wherein: The bridge includes at least one active element.

9. A process control transmitter comprising: A sensor configured to: sense a property of a fluid in a process control system; a communication interface configured to: communicate with a host computer using a DC analog signal and receive an AC digital signal on a process control loop; as well as a microprocessor, wherein the microprocessor is configured to: control the DC analog signal set by the communication interface to provide a process variable to the host, and wherein the microprocessor is configured to: receive through the communication interface a digital value sent by a second process control transmitter on a second process control loop and transferred to the process control loop through a bridge for use in a digital signal between the process control loop and the second process control loop, wherein the process control loop and the second process control loop are used for wired communication, The bridge is connected between the process control loop and the second process control loop for coupling a wired digital communication signal. wherein the bridge allows AC digital signals to be transmitted between the process control loop and the second process control loop, while preventing DC analog signals from being transmitted between the process control loop and the second process control loop, And wherein the process control transmitter communicates directly with the second process control transmitter across the bridge using alternating current digital signals.

10. The process control transmitter of claim 9, wherein: The microprocessor is configured to receive the digital value by decoding a digital signal sent by the second process control transmitter to the host on the second process control loop.

11. The process control transmitter of claim 9, wherein: The microprocessor is also configured to send a digital packet across the bridge to the second process control transmitter via the communication interface.

12. The process control transmitter of claim 11, wherein: The microprocessor is also configured to designate the process control transmitter as a secondary master device in the digital packet.

13. The process control transmitter of claim 12, wherein: At least one of the digital packets instructs the second process control transmitter to enter a burst mode.

14. The process control transmitter of claim 9, wherein: The microprocessor is further configured to receive, through the communication interface, a digital value sent by a third process control transmitter on a third process control loop and transferred to the process control loop through a second bridge for use in a digital signal between the process control loop and the third process control loop.

15. A terminal module, comprising: a first terminal for receiving a transmitter segment of a first process control loop connected to a first process control transmitter; a second terminal for receiving a transmitter segment of a second process control loop connected to a second process control transmitter; a third terminal, the third terminal being used to receive a host segment of the first process control loop connected to a host; as well as a fourth terminal, the fourth terminal being configured to receive a host segment of the second process control loop connected to the host; and a bridge circuit connecting the transmitter segment of the first process control loop to the transmitter segment of the second process control loop for coupling wired digital communication signals and allowing AC digital signals to be transmitted between the first process control loop and the second process control loop while preventing DC analog signals from being transmitted between the first process control loop and the second process control loop, wherein the first process control loop and the second process control loop are used for wired communication, wherein the first process control transmitter communicates with the host using a DC analog signal and an AC digital signal on the first process control loop; The second process control transmitter communicates with the host using a DC analog signal and an AC digital signal over the second process control loop; and The first process control transmitter communicates directly with the second process control transmitter across the bridge circuit using alternating current digital signals.

16. The terminal module according to claim 15, wherein: The bridge circuit includes a capacitor.

17. The terminal module according to claim 16, wherein: The capacitor is located between the first terminal and the second terminal, and the bridge circuit further includes a resistor located between the first terminal and the third terminal.

18. The terminal module according to claim 15, wherein: The bridge circuit includes a transformer.

Images