Patent Publication Number: US-10764083-B2

Title: Portable field maintenance tool with resistor network for intrinsically safe operation

Description:
RELATED APPLICATIONS 
     This application claims priority to and the benefit of Indian Application No. 201621025383, filed Jul. 25, 2016 and titled “Portable Field Maintenance Tool with Resistor Network for Intrinsically Safe Operation,” the entire disclosure of which is expressly incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure generally relates to a portable field maintenance tool, and in particular, to a portable field maintenance tool capable use in a wide variety of environments and situations. 
     BACKGROUND 
     Process control systems, like those used in chemical and petroleum processes, typically include one or more process controllers communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital, or combined analog/digital communication links. 
     A process controller (sometimes referred to as a “controller”), which is typically located within the plant environment, receives signals (sometimes referred to as “control inputs”) indicative of process measurements and uses the information carried by these signals to implement control routines that cause the controller to generate control signals (sometimes referred to as “control outputs”) based on the control inputs and the internal logic of the control routines. The controllers send the generated control signals over buses or other communication links to control operation of field devices. In some instances, the controllers may coordinate with control routines implemented by smart field devices, such as Highway Addressable Remote Transmitter (HART®), Wireless HART®, and FOUNDATION® Fieldbus (sometimes just called “Fieldbus”) field devices. 
     The field devices, which may be, for example, valves, valve positioners, switches, and transmitters (e.g., including temperature, pressure, level, or flow rate sensors), are located within the plant environment and generally perform physical or process control functions. For example, a valve may open or close in response to a control output received from a controller, or may transmit to a controller a measurement of a process parameter so that the controller can utilize the measurement as a control input. Smart field devices, such as field devices conforming to the Fieldbus protocol, may also perform control calculations, alarming functions, and other control functions commonly implemented within a process controller. Field devices may be configured to communicate with controllers and/or other field devices according to various communication protocols. For example, a plant may include traditional analog 4-20 mA field devices, HART® field devices, or Fieldbus field devices. 
     Traditional analog 4-20 mA field devices communicate with a controller via a two-wire communication link (sometimes called a “loop” or “current loop”) configured to carry an analog 4-20 mA DC signal indicative of a measurement or control command. For example, a level transmitter may sense a tank level and transmit via the loop a current signal corresponding to that measurement (e.g., a 4 mA signal for 0% full, a 12 mA signal for 50% full, and a 20 mA signal for 100% full). The controller receives the current signal, determines the tank level measurement based on the current signal, and takes some action based on the tank level measurement (e.g., opening or closing an inlet valve). Analog 4-20 mA field devices typically come in two varieties: four-wire field devices and two-wire field devices. A four-wire field device typically relies on a first set of wires (i.e., the loop) for communication, and a second set of wires for power. A two-wire field device relies on the loop for both communication and power. These two-wire field devices may be called “loop powered” field devices. 
     Process plants often implement traditional 4-20 mA systems due to the simplicity and effectiveness of the design. Unfortunately, traditional 4-20 mA current loops only transmit one process signal at a time. Thus, a set-up including a control valve and a flow transmitter on a pipe carrying material may require three separate current loops: one for carrying a 4-20 mA signal indicative of a control command for the valve (e.g., to move the valve to 60% open); a second for carrying, to the controller, a 4-20 mA signal indicative of the valve&#39;s actual position (e.g., so the controller knows the degree to which the valve has responded to control commands); and a third for carrying, to the controller, a 4-20 mA signal indicative of a measured flow (e.g., so the controller knows how a change in valve position has affected the flow). As a result, a traditional 4-20 mA set-up in a plant having a large number of field devices may require extensive wiring, which can be costly and can lead to complexity when setting up and maintaining the communication system. 
     More recently, the process control industry has moved to implement digital communications within the process control environment. For example, the HART® protocol uses the loop DC magnitude to send and receive analog signals, but also superimposes an AC digital carrier signal on the DC signal to enable two-way field communication with smart field instruments. As another example, the Fieldbus protocol provides all-digital communications on a two-wire bus (sometimes called a “trunk,” “segment,” or “Fieldbus segment”). This two-wire Fieldbus segment can be coupled to multiple field devices to provide power to the multiple field devices (via a DC voltage available on the segment) and to enable communication by the field devices (via an AC digital communication signal superimposed on the DC power supply voltage). 
     These digital communication protocols generally enable more field devices to be connected to a particular communication link, support more and faster communication between the field devices and the controller, and/or allow field devices to send more and different types of information (such as information pertaining to the status and configuration of the field device itself) to the process controller. Furthermore, these standard digital protocols enable field devices made by different manufacturers to be used together within the same process control network. 
     Regardless of the communication protocol utilized, field devices may require on-site setup, configuration, testing, and maintenance. For example, before a field device can be installed at a particular location at a process control plant, the field device may need to be programmed and may then need to be tested before and after the field device is installed. Field devices that are already installed may also need to be regularly checked for maintenance reasons or, for example, when a fault is detected and the field device needs to be diagnosed for service or repair. Generally speaking, configuration and testing of field devices are performed on location using a handheld maintenance tool, such as a portable testing device (“PTD”). Because many field devices are installed in remote, hard-to-reach locations, it is more convenient for a user to test the installed devices in such remote locations using a PTD rather than using a full configuration and testing device, which can be heavy, bulky, and non-portable, generally requiring the installed field device to be transported to the site of the diagnostic device. 
     When a user, such as a service technician, performs maintenance testing and/or communications with a field device, the PTD is typically communicatively connected to a communication link (e.g., a current loop or Fieldbus segment) or directly to a field device (e.g., via communication terminals of the field device). The PTD initially attempts to communicate with the field device, such as by sending and/or receiving digital communication signals along the loop or segment. If the current loop or segment is in proper operating condition, the communications signals may be sent and/or received without problem. However, if the loop, segment, or field device contains an electrical fault, such as a short or a break, communications may be impeded, and it may be necessary to diagnose the loop, segment, and/or field device to identify the fault. 
     When such a fault is identified, a technician might need to use a variety of other tools to test the field device and/or communication link. As an example, the technician may need to carry a multimeter to diagnose the actual signals transmitted or received by the field device. The multimeter is necessary because traditional PTDs are incapable of accurately analyzing the electrical characteristics of signals sent or received by a field device. As another example, the technician may need to use a portable power supply to power an isolated field device. The technician may need to power an isolated field device, for example, when the field device loses power due to a plant-wide power outage or due to an issue with a local power supply. As another example, the technician may simply need to take a field device offline for troubleshooting in order to avoid negatively effecting other field devices and the rest of the process control system. The technician may also need to carry a multimeter to measure the current available on a segment or loop, etc. Each of these tools can take up a fair amount of space, and may be inconvenient for a technician to carry in the field. To address this problem with carrying multiple tools, manufacturers have developed PTDs that include a power supply for providing power to a HART loop. Unfortunately, these powered PTDs are typically incapable of providing power to Fieldbus field devices. Further, typical portable power supplies and powered PTDs often fail to comply with Intrinsic Safety (IS) standards, and thus cannot be safely used in hazardous areas (e.g., an environments or atmospheres that are potentially explosive due to explosive gas or dust). 
     If a field device is located in a hazardous area, the technician may need to verify that each of his or her tools operates in an intrinsically safe manner. When in a hazardous area, a technician&#39;s tools may need to comply with IS standards to ensure safe operation. Generally speaking, IS standards require that plant personnel analyze all equipment attached to a loop or segment (including any PTDs or other tools that will be attached to the loop or segment) to verify that all attached equipment will operate in a safe manner in a hazardous environment. More particularly, IS standards impose restrictions on electrical equipment and wiring in hazardous environments to ensure that the electrical equipment and wiring does not ignite an explosion. To comply with IS standards, electrical equipment generally needs to be designed with two core concepts in mind: energy limitation and fault tolerance. 
     The first IS concept dictates that an IS device be designed such that the total amount of energy available in the device be below a threshold sufficient to ignite an explosive atmosphere. The energy can be electrical (e.g., in the form of a spark) or thermal (e.g., in the form of a hot surface). While IS standards can be complex, they generally require that any voltage within a circuit be less than 29 V; that any current within a circuit be under 300 mA; and that the power associated with any circuit or circuit component be under 1.3 W. A circuit having electrical characteristics exceeding these thresholds may pose an explosion risk due to arcing or heat. 
     The second IS concept dictates that that an IS device be designed in a fault tolerant manner, such that it maintains safe energy levels even after experiencing multiple failures. In short, IS standards reflect a philosophy that circuit faults are inevitable and that energy levels of the circuit must be limited to safe levels when these circuit faults occur. 
     Generally speaking, portable power supplies and powered PTDs are not IS compliant and thus cannot be used in hazardous areas because: (i) portable power supplies and powered PTDs are typically designed such that one or more components may exceed energy levels sufficient to risk igniting an explosive atmosphere, and/or (ii) the portable power supplies and powered PTDs are vulnerable to component failures that would result in the portable power supplies or powered PTDs exceeding energy levels sufficient to risk igniting the explosive atmosphere. 
     For example, a typical portable power supply may generate a voltage across its terminals sufficient to risk an explosion in a hazardous environment (e.g., above 29 V). Even when designed to supply a voltage of under 29 V, a typical portable power supply does not include fail-safe mechanisms guaranteed to prevent the supplied voltage or current from spiking. Consequently, when in a hazardous environment, technicians needing to provide power to a field device generally must uninstall the field device and transport the field device to a safe area where it can be powered and tested. 
     SUMMARY 
     This disclosure describes a portable field maintenance tool configured for use in industrial process control systems, environments, and/or plants, which are interchangeably referred to herein as “automation,” “industrial control,” “process control,” or “process” systems, environments, and/or plants. Typically, such systems and plants provide control, in a distributed manner, of one or more processes that operate to manufacture, refine, transform, generate, or produce physical materials or products. 
     The described portable field maintenance tool may power, communicate with, and/or diagnose field devices and/or communication links connected to field devices. The portable field maintenance tool may be configured for use with field devices configured according to multiple communication protocols, such as the Fieldbus protocol and the HART protocol. Accordingly, rather than being forced to carry multiple tools for servicing different types of field devices, a user need only carry the portable field maintenance tool. In some instances, the portable field maintenance tool may be energy limited and fault tolerant sufficient to comply with IS standards. Accordingly, unlike many prior art portable power supplies and PTDs, the portable field maintenance tool can safely be used in hazardous areas. 
     In an embodiment, the portable field maintenance tool comprises any one or more of: a housing; a communication interface; a communication circuit; and/or a power supply. The communication interface may be disposed through the housing, and may include an internal portion accessible within the housing as well as a set of terminals accessible outside the housing. The set of terminals may be electrically connectable to a field device by way of a wired link configured to carry a composite signal including: (i) a communication signal transmitted to or from the field device, and (ii) a power signal transmitted to the field device. The communication circuit may: be disposed within the housing and electrically connected to the internal portion of the communication interface; be configured to encode or decode the communication signal; and/or include a resistor network having a resistance within a range (e.g., any value between 75 ohms and 750 ohms) to cause a voltage drop, at the set of terminals, associated with the composite signal that is: (i) above a minimum voltage threshold associated with reading the composite signal, and (ii) below a maximum voltage threshold. The power supply may: be disposed within the housing; be electrically connected to the internal portion of the communication interface and to the communication circuit; and/or be configured to transmit the power signal. The composite signal may include an analog DC signal that includes the power signal and that varies in amplitude to convey information, as well as a digital FM communication signal superimposed on the analog DC signal. The minimum voltage threshold may be a minimum peak-to-peak voltage associated with reading the communication signal, and may be any value between 50 mV peak-to-peak and 500 mV peak-to-peak (e.g., 120 mV peak-to-peak). The maximum voltage threshold may be a value selected to remain below a voltage sufficient to generate a spark at the set of terminals. The power supply may be configured to transmit the power signal at a voltage that is at or below the maximum voltage threshold. The maximum voltage threshold may be any value between 10 V and 30 V. The resistor network may include a plurality of resistors, which may be arranged in a plurality of sub-networks. The resistor network may include switches (one or more of which may be solid state relays) for switching one or more resistors in or out of the resistor network. 
     In an embodiment, a method of communicating with a transmitter field device comprises any one or more of: communicatively connecting, via a wired link, a set of terminals of a portable field maintenance tool to a transmitter field device; supplying power from the portable field maintenance tool, via the wired link, to the transmitter field device; receiving by the portable field maintenance tool, via the wired link, a communication signal superimposed on the supplied power; and/or limiting a voltage drop at the set of terminals, associated with the communication signal and the supplied power, to between a minimum voltage threshold and a maximum voltage threshold by: (i) limiting the supplied power so that the voltage drop does not exceed the maximum voltage threshold; and (ii) activating or deactivating one or more resistors of a resistor network disposed within the portable field maintenance tool and electrically connected to the set of terminals so that the voltage drop remains above the minimum voltage threshold (e.g., so that the voltage drop exceeds a minimum peak-to-peak voltage associated with reading the communication signal). The communication signal may be an analog DC signal that varies in amplitude to convey information and that is superimposed on the supplied power. Limiting the supplied power so that the voltage drop does not exceed the maximum voltage threshold may comprise limiting the supplied power so that the voltage drop remains below a voltage sufficient to generate a spark at the set of terminals. 
     In an embodiment, a method of communicating with an actuator field device comprises any one or more of: communicatively connecting, via a wired link, a set of terminals of a portable field maintenance tool to an actuator field device; supplying power from the portable field maintenance tool, via the wired link, to the actuator field device; limiting the supplied power so that the set of terminals do not exceed a maximum electrical threshold; and/or transmitting by the portable field maintenance tool, via the wired link, to the actuator field device a communication signal superimposed on the supplied power. Limiting the supplied power so that the set of terminals do not exceed a maximum electrical threshold may comprise: (i) limiting the supplied power so that a voltage drop at the set of terminals does not exceed a maximum voltage threshold (e.g., any value between 21 V and 24 V); (ii) limiting the supplied power so that power available at the set of terminals does not exceed a maximum power threshold (e.g., any value between 0.25 W and 1.5 W); (iii) limiting the supplied power so that a current at the set of terminals does not exceed a maximum current threshold (e.g., any value between 25 mA and 31 mA); (iv) inducing a first voltage drop across an internal resistor to keep a second voltage drop at the set of terminals below a maximum voltage threshold; and/or (v) disabling the portable field maintenance tool when a voltage at the set of terminals exceeds a maximum voltage threshold or when a current at the set of terminals exceeds a maximum current threshold. 
     In an embodiment, a portable field maintenance tool comprises any one or more of: a pair of terminals electrically connectable, via a wired link, to a field device that transmits or receives a signal via the wired link; a communication circuit, electrically connected to the pair of terminals, that receives or transmits the signal; an energy measurement circuit, electrically connected to the pair of terminals, that measures one or more electrical characteristics of the signal; a resistor network; a control unit; and/or a power supply that supplies power via the wired link. The control unit may: activate or deactivate one or more resistors of the resistor network based on the measured one or more electrical characteristics; and/or control the power supply based on the measured one or more electrical characteristics (e.g., to prevent the pair of terminals from exceeding a maximum electrical threshold, such as a maximum power threshold). When a current draw at the pair of terminals increases, the power supply may prevent the pair of terminals from exceeding a maximum power threshold by reducing a supplied voltage. 
     In an embodiment, a method of communicating with a field device and monitoring signals sent or received by the field device comprises any one or more of: (i) electrically connecting, via a wired link, a field device to a pair of terminals of a portable field maintenance tool; (ii) transmitting or receiving, at the pair of terminals of the portable field maintenance tool, a signal to or from the field device; (iii) measuring one or more electrical characteristics of the transmitted or received signal; (iv) maintaining a voltage drop at the pair of terminals to a value above a minimum voltage threshold necessary to read the signal by activating or deactivating one or more resistors of the portable field maintenance tool based on the measured one or more electrical characteristics; (v) disabling the portable field maintenance tool when the signal on the wired link exceeds a maximum electrical threshold or drops below a minimum electrical threshold; (vi) supplying power from the portable field maintenance tool, via the wired link, to the field device; (vii) adjusting the supplied power to prevent the pair of terminals from exceeding a maximum electrical threshold; and/or (viii) stopping the supplying of power and raising a loop resistance to bleed off voltage associated with the supplying of power by activating or deactivating one or more resistors of the portable field maintenance tool. In an embodiment, the method includes performing an analysis of the one or more electrical characteristics to determine whether or not the field device is connected to an external loop resistor; preventing an internal loop resistor from activating when the analysis reveals that the field device is connected to an external loop resistor; and/or activating the internal loop resistor when the analysis reveals that the field device is not connected to an external loop resistor. The method may include detecting voltage decay at the pair of terminals and enabling activation of a power supply based on the detected voltage decay. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Each of the figures described below depicts one or more aspects of the disclosed system(s) and/or method(s), according to an embodiment. Wherever possible, the Detailed Description refers to the reference numerals included in the following figures. 
         FIG. 1A  depicts an example portable field maintenance tool connected to a field device. 
         FIG. 1B  is a block diagram of an example process control system where the portable field maintenance tool shown in  FIG. 1A  may be utilized to communicate with, diagnose, or power one or more field devices. 
         FIG. 2  is a schematic of a prior art passive PTD communicatively connected to a HART field device. 
         FIG. 3  is a schematic of a prior art passive PTD communicatively connected to a Fieldbus field device. 
         FIG. 4  is a block diagram of the portable field maintenance tool shown in  FIG. 1A , depicting an example in which the portable field maintenance tool includes an active communicator for powering and communicating with field devices. 
         FIG. 5A  is a block diagram of an active communicator, configured for digital frequency modulation communication, that may be found in the portable field maintenance tool shown in  FIG. 1A . 
         FIG. 5B  is a block diagram of an active communicator, configured for digital amplitude modulation communication, that may be found in the portable field maintenance tool shown in  FIG. 1A . 
         FIG. 5C  is a block diagram of an active communicator, configured for analog communication, that may be found in the portable field maintenance tool shown in  FIG. 1A . 
         FIG. 6  is a schematic of an active communicator that may be found in an example portable field maintenance tool and that may enable communication via a digital frequency modulation communication protocol, such as the HART protocol. 
         FIG. 7  is a schematic of a resistor network shown in  FIG. 6 . 
         FIG. 8A  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to a transmitter, depicting an example in which the transmitter is powered by the active communicator of the portable field maintenance tool. 
         FIG. 8B  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to an actuator, depicting an example in which the actuator is powered by the active communicator of the portable field maintenance tool. 
         FIG. 9A  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to a transmitter, depicting an example in which the transmitter is not powered by the active communicator of the portable field maintenance tool. 
         FIG. 9B  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to an actuator, depicting an example in which the actuator is not powered by the active communicator of the portable field maintenance tool. 
         FIG. 10  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to a field device, depicting an example in which a power monitor of the portable field maintenance tool may be connected to the field device in parallel to measure electrical characteristics of signals sent or received by the field device. 
         FIG. 11  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to a field device, depicting an example in which the power monitor of the portable field maintenance tool may be connected to the field device in series to measure electrical characteristics of signals sent or received by the field device. 
         FIG. 12  is a schematic of the portable field maintenance tool shown in  FIG. 6  connected to I/O devices, depicting an example in which the portable field maintenance tool may test the I/O devices. 
         FIG. 13  is a schematic of an active communicator that may be found in an example portable field maintenance tool and that may enable communication via a digital amplitude modulation communication protocol, such as the Fieldbus protocol. 
         FIG. 14A  is a schematic of the portable field maintenance tool shown in  FIG. 13 , demonstrating an example in which the portable field maintenance tool may be connected to a field device connected to an operational bus. 
         FIG. 14B  is a schematic of the portable field maintenance tool shown in  FIG. 13 , demonstrating an example in which the portable field maintenance tool may power and communicate with a field device via an internal bus of the portable field maintenance tool. 
         FIG. 15  is a view of a communication interface of the portable field maintenance tool shown in  FIG. 1A  from a perspective external to the portable field maintenance tool. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a portable field maintenance tool and various techniques for implementing the portable field maintenance tool.  FIG. 1A  depicts an example portable field maintenance tool  100  (“tool  100 ”) that may be connected to a field device  160  via a communication link  150 . Advantageously, the tool  100  is capable of not only communicating with the field device  160 , but of also powering the field device  160 . The tool  100  may utilize a single composite signal, transmitted via the link  150 , for both powering and communicating with the field device  160 . In some cases, the tool  100  can diagnose problems with the field device  160  or with a communication link in the plant environment to which the field device  160  is connected (e.g., a HART loop or Fieldbus segment; not shown). In some instances, the tool  100  may communicate with or diagnose field devices configured according to different protocols. For example, the tool  100  may be capable of communicating with, powering, and diagnosing traditional 4-20 field devices, HART field devices, and Fieldbus field devices. Unlike many prior art PTDs that force a user to utilize multiple devices and/or to connect multiple cables and wires to various different terminal sets if he or she wants to communicate with a field device, power the field device, and perform diagnostics on signals sent or received by the field device, the tool  100  may utilize a single terminal set for communications, power, and diagnostics, simplifying configuration and use of the tool  100  for users. 
     Moreover, the tool  100  may be energy limited and fault tolerant sufficient to comply with IS standards. For example, the tool  100  may be designed so that all components of the tool  100  and so that all signals (e.g., including power and/or communication signals) transmitted and/or received by the tool  100  are energy limited to ranges compliant with IS standards. Further, the tool  100  may “self-monitor” components of the tool  100  and/or signals transmitted or received by the tool  100  to ensure that the components and/or signals remain IS compliant. To illustrate, the tool  100  may disable one or more components (or disable the tool  100  entirely) when a component or signal approaches or exceeds a threshold associated with IS standards. Accordingly, when the tool  100  is IS compliant, a user can connect the tool  100  to the field device  160  or to a link (e.g., a HART loop or Fieldbus segment) to which the field device  160  is connected with confidence that he or she will not violate IS standards and with confidence that he or she will not ignite an explosive atmosphere. In short, unlike many traditional portable power supplies and PTDs, the tool  100  may safely be used in hazardous areas. 
     The communication link  150  may be a two-wire communication link capable of carrying a communication signal and/or a power signal, each of which may be part of a composite signal. As used herein, the term “signal” may refer to a communication signal, a power signal, or a composite signal conveying both power and information. Generally speaking, the term “communication signal” refers to any signal conveying information (such as a control signal that commands an actuator to actuate), and may be analog or digital and AC or DC. The term “power signal” refers to any electrical energy transmitted for the purpose of supplying power, and may be AC or DC. The tool  100  may have a terminal set for connecting to the link  150  and field device  160 , and in some cases may have multiple terminal sets for connecting to field devices configured to various different protocols (e.g., a terminal set for HART field devices and a terminal set for Fieldbus field devices). 
     To power the field device  160 , the tool  100  may include a power supply configured to supply a voltage across terminals of the tool  100  to which the communication link  150  is connected. 
     The tool  100  may be configured to communicate with the field device  160  via a composite signal (transmitted via the link  150 ) including a communication signal (to facilitate communication between the tool  100  and the field device  160 ) and a power signal (to provide power to the field device  160 ). The communication signal may be a digital signal, an analog signal, or a composite analog and digital signal. Said another way, the tool  100  may transmit and/or receive a first composite signal including a power signal and a second composite signal that includes an analog and digital signal. 
     For example, the tool  100  may include a first terminal set for transmitting and/or receiving a first composite signal (e.g., a HART signal) including: (i) a DC power signal (e.g., 4 mA), and (ii) a second composite signal for communications (e.g., an AC digital communication signal superimposed on a 0-16 mA DC communication signal) superimposed on the 4 mA power signal. In such an example, the power signal generally remains constant at 4 mA and represents a live zero, resulting in the first composite signal having a current magnitude range of 4-20 mA. The tool  100  may additionally or alternatively have a second terminal set for transmitting and/or receiving a composite signal according to other protocols, such as the Fieldbus protocol. For example, the tool  100  may transmit and/or receive a composite signal including: (i) a DC power signal (e.g., 10-25 mA), and (ii) an AC digital communication signal (e.g., modulated at 15-20 mA peak-to-peak) superimposed on the DC power signal. In some cases, the tool  100  includes one or more terminal sets for transmitting analog and/or digital communication signals without providing power (e.g., for situations where the field device  160  is already powered). 
     As noted, the tool  100  may operate in compliance with IS standards. That is, the tool  100  may safely be used in hazardous areas because the components of the tool  100  may be energy limited and fault tolerant in accordance with IS standards. For example, the components of the tool  100  may be (i) current limited to a current limit (e.g., 250 mA, 300 mA, 350 mA, etc.) (ii) voltage limited to voltage limit (e.g., 25 V, 29 V, 35V, etc.) and (iii) power limited to a power limit (e.g., 1 W, 1.3 W, 1.5 W, etc.). The tool  100  may have one or more built-in redundancies (e.g., automatic shutdown, redundant components, etc.) to ensure that component failure does not result in these energy limitations being exceeded. 
     The tool  100  may include any one or more of: a display  122 , a housing  128 , input keys  132 , and a folding stand  152 . The housing  128  may be shaped and sized as a handheld unit. The housing  128  may have a generally rectangular cubic shape, or any other desirable shape or size (e.g., 5 inches, 7 inches, or 11 inches measured diagonally). 
     The display  122  and input keys  132  may be disposed on a front face of the housing. The display  122  may be a touchscreen, such as a capacitive touchscreen that detects touch input via capacitive sensing, or a resistive touchscreen that detects touch input via applied pressure. The input keys  32  may be physical keys, such as push buttons or multi-directional buttons. In some cases, the tool  100  does not include the input keys  32 . 
     The folding stand  152  may pivot between a flat position against the back of the housing  128  and an outwardly pivoted position from the back of the housing  128 . In the flat position, a user can carry the tool  100  and use the tool  100  in a similar manner that one would use a tablet. In the outwardly pivoted position, the folding stand  152  can be used to prop the maintenance tool  100  in an upright position. In some instances, the tool  100  does not include the folding stand  152 . 
       FIG. 1B  is a block diagram of an example process control system  10  where the tool  100  may be utilized to communicate with, diagnose, or power one or more field devices. The process control system  10  includes a process controller  11  connected to a data historian  12  and to one or more host workstations or computers  13  (which may be any type of personal computers, workstations, etc.), each having a display screen  14 . The process control system  10  may include a plurality of field devices  160 , including field devices  15 - 22 . 
     The controller  11  may be connected to field devices  15 - 22  via input/output (“I/O”) cards  26  and  28 . The data historian  12  may be any desired type of data collection unit having any desired type of memory and any desired or known software, hardware, or firmware for storing data. The controller  11  is, in  FIG. 1B , communicatively connected to the field devices  15 - 22 . 
     Generally, the field devices  15 - 22  may be any types of devices, such as sensors, valves, transmitters, positioners, etc., while the I/O cards  26  and  28  may be any types of I/O devices conforming to any desired communication or controller protocol. For example, the field devices  15 - 22  and/or I/O cards  26  and  28  may be configured according to the HART protocol or to the Fieldbus protocol. The controller  11  includes a processor  23  that implements or oversees one or more process control routines  30  (or any module, block, or sub-routine thereof) stored in a memory  24 . Generally speaking, the controller  11  communicates with the devices  15 - 22 , the host computers  13 , and the data historian  12  to control a process in any desired manner. Moreover, the controller  11  implements a control strategy or scheme using one or more function blocks  32 - 38 , wherein each function block is an object or other part (e.g., a subroutine) of an overall control routine  30 . The function blocks  32 - 38  may be stored in and executed by the controller  11  or other devices, such as smart field devices. 
     The tool  100  may be communicatively connected via the link  150  to a communication link (e.g., a HART loop or Fieldbus Segment) connecting one of the field devices  15 - 22  to the I/O cards  26  and  28 . Alternatively, the tool  100  may be communicatively connected directly to one of the field devices  15 - 22  (e.g., via communication terminals present on the field devices  15 - 22 ). If desired, the tool  100  may provide power to the field devices  15 - 22  to which the tool  100  is connected, or to a bus (e.g., a Fieldbus segment) to which the field devices  15 - 22  are connected. The tool  100  may enable a user to communicate with and/or diagnose any one of the field devices  15 - 22 . In some instances, the tool  100  only powers a single one of the field devices  15 - 22  at any given time. 
       FIG. 2  is a schematic of a prior art PTD  205  that is connected, via a HART loop  200 A, to a HART field device  215  and that requires the use of a portable power supply  220 . Unlike the tool  100 , the PTD  205  cannot supply power to the field device  215 , and is thus inconvenient for technicians. Further, the portable power supply  220  may not comply with IS standards, making it unsuitable for use in hazardous areas. Finally, unlike the tool  100 , the PTD  205  requires a loop resistor  210 , connected in parallel with the PTD  205  to the loop  200 A, to communicate with the field device  215 . 
     As noted, the PTD  205  does not supply power to the field device  215 . The field device  215  is instead powered by a portable power supply  220 .  FIG. 2  represents a scenario in which the field device  215  is being bench tested or in which the field device  215  is isolated from its normal power source in the field. Because the PTD  205  does not supply power to the field device  215 , a technician may need to carry the portable power supply  220 , in addition to the PTD  205 , to the field device  215  when servicing it in the field. 
     As further noted, the power supply  220  may not comply with IS standards. Thus, if the field device  215  is in a hazardous area, the technician may not be able to supply power to the field device  215 , and consequently may not be able to utilize the PTD  205  to service the field device  215 . Typical portable power supplies often cannot safely be used in hazardous areas because they are usually not compliant with IS standards. In particular, typical portable power supplies are often vulnerable to component failures that may result in voltage, current, and/or temperature spikes sufficient to ignite an explosive atmosphere. It should be noted that if the PTD  205  were to be made “active” by adding a power supply, it would suffer many of the same problems suffered by portable power supplies regarding IS standards. 
     Finally, the PTD  205 , like many prior art PTDs, requires the external 250 ohm loop resistor  210  to communicate with the HART field device  215 . By comparison, the tool  100  may include an internal resistor network that provides sufficient resistance to read a signal on a link such as the loop  200 A, and thus does not require the use of the external resistor  210 . The external resistor  210  provides sufficient loop resistance to enable the PTD  205  to detect a voltage on the loop  200 A, which is necessary for reading the signal carried by the loop  200 A (i.e., the PTD  205  interprets the detected voltage as a signal value). In this example, the PTD  205  might interpret an analog value (e.g., a tank level measurement between 0% full and 100% full) based on the particular value of the detected voltage within a range of 1 V-5 V (e.g., wherein 1 V=0% and 5V=100%). For example, when the loop current is 20 mA, the PTD  205  detects 5 V (i.e., 20 mA*250) and when the loop current is 4 mA, the PTD detects 1 V (i.e., 4 mA*250). Further, the PTD  205  might interpret the digital component of a HART signal based on the detected voltage. The digital component of a HART signal generally varies by about 1 mA peak-to-peak. Thus, the 250 ohm resistor  210  enables the PTD  205  to detect a voltage, corresponding to this digital component, of about 250 mV (1 mA*250). If a smaller resistor were used (or no resistor were used), the voltage associated with the signaling on the loop  200 A might drop to levels undetectable by the passive PTD. By comparison, the tool  100  may utilize an internal resistor network having a resistance below 250 ohms, enabling the tool  100  to read a signal on the HART loop  200   a  while complying with IS energy limitations. 
       FIG. 3  is a schematic of a prior art PTD  305  communicatively connected to a Fieldbus field device  310  that is powered by a Fieldbus power supply  315  via a Fieldbus segment  300 . The PTD  305  is similar to the PTD  205  in that it does not supply power to the field device  310 , and is thus inconvenient for technicians. That is, when a technician is servicing the field device  310 , he or she generally relies on the power supply  315  or a portable power supply (not shown) to power the field device  315 . By comparison, the tool  100  can supply power to a field device such as the field device  310 , even when the field device  310  is located in a hazardous area. 
       FIG. 4  is a block diagram of the tool  100 , depicting an example in which the tool  100  includes an active communicator  404  and a physical communication interface  406  electrically connected via electrical connections  416  and  417  to the active communicator  404  so that the active communicator  404  can power and communicate with the field device  160  via the physical communication interface  406 , as well as measure one or more electrical characteristics of signals sent or received by the active communicator  404 . As shown, the communication interface  406  may be disposed through the housing  128 , such that an external portion of the interface  406  is accessible outside the housing  128 , enabling the communication link  150  and field device  160  to be connected to the interface  406 . 
     The active communicator  404  enables the tool  100  to communicate with the field device  160 , diagnose the field device  160 , power the field device  160 , and/or diagnose a communication link in a plant environment to which the field device  160  is connected (not shown). In some cases, the active communicator  404  may be configured to communicate with and diagnose multiple different types of field devices (e.g., HART field devices and Fieldbus field devices), and/or may be configured to comply with IS standards so that it can be used to communicate with, diagnose, and power field devices located in hazardous areas. The one or more power supplies of the active communicator  404  may include switches for disabling the power supplies. 
     The active communicator  404  may include a power supply for supplying power to the field device  160 , a signal encoder and decoder (e.g., a modem) for communicating with the field device  160 , and/or energy measurement circuitry (e.g., a voltmeter and/or ammeter) for measuring electrical characteristics of signals sent and received by the active communicator  404 . The active communicator  404  may transmit or receive communication signals to or from the field device  160  via the electrical connections  416  and  417 . The active communicator  404  may encode communication signals by modulating a current magnitude or a frequency to represent an analog or digital value, and may superimpose the communication signal on a power signal to create a composite signal. 
     The tool  100  may include a control unit  402 , communicatively coupled to the active communicator  404  via a communication bus  414 , configured to control and monitor the active communicator  404 . At a high level, the control unit  402  may activate and deactivate components of the active communicator  404  to: (i) configure the active communicator  404  so that it remains energy limited in accordance with IS standards; (ii) configure the active communicator  404  to communicate according to a desired communication protocol (e.g., HART or Fieldbus); (iii) configure the active communicator  404  in response to a connection made at the physical communication interface  406  (e.g., based on whether a user connects the communication link  150  to a terminal set for HART or a terminal set for Fieldbus); and/or (iv) configure the active communicator  404  for a particular field device configuration or field device type (e.g., actuator or transmitter). Generally speaking, a transmitter is a field device configured to obtain a measurement (e.g., via a temperature sensor, pressure sensor, flow sensor, level sensor, etc.) and to transmit the measurement. The field device configuration or type may be determined based on user input or based on communication with the connected field device. 
     The control unit  402  may include a processor  422 , a memory  424  storing one or more routines, and an I/O interface  426  communicatively coupled to other components of the tool  100  via the bus  414 . The routines stored at the memory  424  may include a circuit manager routine  462  for activating and deactivating components of the active communicator  404  as described above and a diagnostics manager routine  464  for diagnosing signals sent and received by the active communicator  404 . 
     The tool  100  may also include a user interface (“UI”)  410 , communicatively coupled to the control unit  402  via the bus  414 , for providing a user interface and/or for detecting user input received at the UI  410  (e.g., touch input). The control unit  402  may provide the user interface at the UI  410  and detect the user input at the UI  410  by executing a UI manager  466  stored at the memory  424 . The UI  410  may include the display  122  shown in  FIG. 1A , where the control unit  402  may render visual output; and an audio device  444  for providing audio output. For example, the UI  410  may render a graphical user interface that enables a user to select a communication protocol for communicating with the field device  160 , to select a command to transmit to the field device  160 , to view information transmitted from the field device  160  to the tool  100 , etc. The audio device  444  may generate audio alarms or notifications, for example, in response to alarms transmitted by the field device  160 . 
     Further, the tool  100  may include a power monitor  408  (e.g., an ammeter), communicatively coupled to the control unit  402  via the bus  414 , for measuring a current or voltage associated with the communication link  150  connected to the interface  406 . The diagnostics manager  464  of the control unit  402  may utilize the power monitor  408  to measure a signal transmitted and/or received by the tool  100  to determine whether the signal has electrical characteristics within an expected range for a particular protocol. For example, if a user utilizes the tool  100  to attempt to command a HART valve to open to 50%, the power monitor  408  may be utilized to verify that the transmitted signal has a current at or near a level that will enable the HART valve to properly interpret the signal (e.g., 12 mA). The UI manager  464  may display measurements obtained by the power monitor  408 . In some cases, the tool  100  does not include the power monitor  408 . However, regardless of whether the tool  100  includes the power monitor  408 , the tool  100  may rely on electrical measurements obtained by the active communicator  404 . 
     The tool  100  may also include a wireless communication interface  412 , communicatively coupled to the control unit  402  via the bus  414 , for transmitting and/or receiving wireless signals, enabling the tool  100  to communicate with other components of the plant  10 . The wireless interface  412  may support one or more suitable wireless protocols, such as Wi-Fi (e.g., an 802.11 protocol), Bluetooth (e.g., 2.4 to 2.485 GHz), near-field communications (e.g., 13.56 MHz), high frequency systems (e.g., 900 MHz, 2.4 GHz, and 5.6 GHz communication systems), etc. In some cases, the tool  100  does not include the wireless communication interface  412 . 
       FIGS. 5A-5C  are block diagrams of active communicators  501 ,  511 , and  521 , each of which is an example of the active communicator  404  shown in  FIG. 4 , that are configured to communicate according to different communication schemes. Depending on the embodiment, each of the active communicators  501 ,  511 , and  512  may be communicatively coupled to the control unit  402  via the bus  414  and may be electrically connected to the communication interface  406  via the electrical connections  416  and  417  shown in  FIG. 4 . 
     The active communicator  501  shown in  FIG. 5A  is a digital frequency modulation (“FM”) circuit, and may include a power supply  502  and an FM modem  504 , each of which may be electrically connected, directly or indirectly, to the electrical connection  416  and  417 . The power supply  502  may provide power to the field device  160  connected to the interface  406  via a DC signal. The FM modem  504  may transmit information to and/or receive information from the field device  160  (via the interface  406 ) using a frequency modulation scheme, such as the HART protocol. For example, to transmit information, the FM modem  504  may superimpose an AC communication signal onto a DC signal provided by the power supply  502 . The FM modem  504  may encode digital communication signals by modulating the frequency of an AC communication signal, wherein a first frequency (or frequency range) represents a digital 0 and a second frequency (or frequency range) represents a digital 1. For example, the FM modem  504  may encode a communication signal by modulating the frequency of the communication signal at 1200 Hz (representing a digital 1) and 2200 Hz (representing a digital 0). To receive information, the FM modem  504  may interpret a first frequency or frequency range as a digital 0, and may interpret a second frequency or frequency range as a digital 1. 
     The active communicator  511  shown in  FIG. 5B  is a digital amplitude modulation (“AM”) circuit, and may include a power supply  512  and an AM modem  514 , each of which may be electrically connected, directly or indirectly, to the electrical connections  416  and  417 . The power supply  512  may provide power to the field device  160  connected to the interface  406  via a DC signal. The AM modem  514  may transmit information to and/or receive information from the field device  160  using an amplitude modulation scheme, such as the Fieldbus protocol. For example, to transmit information, the AM modem  511  may superimpose an AC communication signal onto a DC signal provided by the power supply  512 . The AM modem  511  may encode digital communication signals by modulating the amplitude of an AC communication signal, wherein a first amplitude (or amplitude range) represents a digital 0 and a second amplitude (or amplitude range) represents a digital 1. For example, the first range may be 7.5 mA to 10 mA and the second range may be −7.5 mA to −10 mA. In some circumstances, transitions from the first amplitude or amplitude range to the second amplitude or amplitude range may represent a digital 0, and transitions from the second amplitude to the first amplitude may represent a digital 1. Thus, the AM modem  514  may control the current magnitude of the communication signal to cause transitions between the first and second range to encode digital 1s and 0s onto the communication signal. To receive information, the AM modem  514  may interpret a first amplitude, amplitude range, and/or transition between amplitudes (e.g., high-to-low) as a digital 0, and may interpret a second amplitude, amplitude range, and/or transition between amplitudes (e.g., low-to-high) as a digital 1. 
     Turning to  FIG. 5C , the active communicator  521  is an analog circuit, and may include a power supply  522 , a DC current controller or current sink  524 , and/or a DC current monitor  526 , each of which may be electrically connected, directly or indirectly, to the electrical connections  416  and  417 . The active communicator  521  may encode information by causing the current controller  524  to draw current at a particular magnitude within a range (e.g., 4-20 mA). Example information encoded by the active communicator  521  includes a command to open a valve to 100% open (e.g., 20 mA) or to 0% open (e.g., 4 mA). The field device  160  that receives the encoded signal may be configured to receive the signal and interpret the current magnitude as a particular command or value. The active communicator  521  may additionally or alternatively decode a signal from a field device  160  by measuring, via the current monitor  526 , the current magnitude of the received signal. Example information encoded by a field device  160  (to be decoded by the active communicator  521  and/or control unit  402 ) includes a flow measurement (e.g., wherein the field device  160  is calibrated to report measurements within a range of 0-100 gallons per minute by transmitting a corresponding 4-20 mA signal). 
     In some cases, the tool  100  may include only one of the active communicators  501 ,  511 , and  521 ; while in other cases, the tool  100  may include two or more of the active communicators  501 ,  511 , and  521 . When the tool  100  includes multiple ones of the active communicators  501 ,  511 , and  521 , the tool  100  may be capable of communicating with and/or diagnosing multiple field devices that operate according to different protocols (e.g., HART field devices and Fieldbus field devices). Advantageously, a user can carry a single tool in the plant for testing multiple types of field devices, saving the user the trouble of carrying a different tool for each different type of field device. 
     When the tool  100  includes multiple ones of the active communicators  501 ,  511 , and  521 , the interface  406  may include a terminal set for each of the active communicators  501 ,  511 , and  521 . In some instances, the active communicators  501  and  521  may share a power supply and/or a terminal set. In such instances, the active communicators  501  and  521  may utilize a single composite signal that is modulated in both current amplitude and frequency to carry information. For example, the active communicator  521  may transmit and/or receive information by varying or measuring an amplitude of a DC signal between 4-20 mA. The active communicator  501  may then superimpose an AC communication signal (e.g., 1 mA peak-to-peak) onto the modulated DC signal. 
       FIG. 6  is a schematic of an active communicator  600  (which may be an example of the active communicator  404  shown in  FIG. 4 ) for the tool  100  that may be electrically connected to the field device  160  shown in  FIG. 1A  via the communication interface  406  to: (i) supply power to the field device  160  by way of a DC signal (e.g., 4 mA); (ii) communicate with the field device  160  by way of a current modulated signal biased (e.g., at 4-20 mA) on top of the DC power signal; and (iii) communicate with the field device  160  by way of a digital FM signal superimposed on the analog current modulated signal. Advantageously, the active communicator  600  may be utilized to communicate with and/or diagnose HART field devices. In some cases, the active communicator  600  may be energy limited and fault tolerant according to IS standards, and enabling the active communicator  600  and the tool  100  to power, communicate with, and/or diagnose field devices and communication links located in hazardous areas. 
     As noted, the active communicator  600  may communicate with the field device  160  utilizing two simultaneous communication channels: a current modulated analog signal and a frequency modulated digital signal superimposed on the analog signal. Generally speaking, the analog signal communicates a primary measured value (e.g., a flow, pressure, temperature, etc.) when the field device  160  is a transmitter and communicates a command (e.g., to open or close a valve) when the field device  160  is an actuator. The digital signal may contain information from the field device  160  including a device status, diagnostics, additional measured or calculated values, etc. 
     The active communicator  600  may include one or more of the following, each of which may be communicatively coupled to the control unit  402  via a communication bus (not shown): a power supply  602 , a resistor  604 , and a communication circuit  609 . The communication circuit  609  may include energy measurement circuitry (e.g., the voltage monitors  611  and  616 ) that measures electrical characteristics of signals sent and/or received by the tool  100 . The diagnostics manager  662  may analyze the electrical characteristics to verify the signals are within an expected range for a given protocol (e.g., 4-20, HART, Fieldbus, etc.) or for IS standards. The power supply  602  may supply the power signal provided by the active communicator  600 , and the communication circuit  609  may encode and decode communication signals transmitted and/or received by the active communicator  600 . 
     The power supply  602 , which may be designed to supply any desired voltage (e.g., any value between 20 V and 29 V), may be communicatively coupled to the control unit  402 . In some cases, the power supply  602  may be designed to never exceed: (i) a maximum voltage threshold (e.g., any desirable value between 23 V and 30 V), or (ii) a maximum current threshold (e.g., any desirable value between 20 mA an 35 mA). In some cases, the power supply  602  may be designed to be voltage limited and/or current limited even when experiencing one or more faults, and/or may be designed to perform a ramped start-up or soft start during which it ramps to a desired voltage over a period of time, thus mitigating against the chance of current spikes. In some instances, the current and/or voltage of the power signal provided by the power supply  602  may be measured so that the control unit  402  can shut down the power supply  602  when a measured voltage or current exceeds a maximum threshold or fails to exceed a minimum threshold. A measured voltage exceeding a maximum threshold may indicate that someone added an external power source to the field device to which the tool  100  is connected, which may cause the loop to violate IS standards and/or may damage components of the tool  100  or of other devices connected to the loop. A measured voltage failing to exceed a minimum threshold may indicate that a circuit has shorted or that the capacity of the power supply  602  has been exceeded. A measured current exceeding a maximum threshold may indicate that a circuit has shorted or that the power supply  602  is limited at its maximum load. A measured current failing to exceed a minimum threshold may indicate that no field device is connected to the tool  100 . 
     The power supply  602  may be electrically connected to the communication interface  406  via the resistor  604 , which may have any desirable resistance (e.g., 200-300 ohms). The resistor  604  may function to induce a voltage drop sufficient to ensure that a voltage drop at the communication interface  406  remains below a threshold. For example, if the power supply  602  is supplying a current of 25 mA, the resistor  604  may induce a voltage drop of 6.25 V (i.e., 0.025 A*250 ohms). In cases where the power supply  602  is a 23 V power supply, for example, this may result in a maximum potential output voltage of 16.75 V. As shown, the active communicator  600  also includes a voltage monitor  605  that measures a voltage drop across the resistor  604  and transmits the measured voltage drop to the control unit  402 . The voltage monitor  605  may function as a current monitor for the power supply  602 . For example, the control unit  604 , which may be communicatively coupled to the voltage monitor  605 , may rely on the measured voltage drop to calculate a current flowing through the resistor  604 . In some cases, the active communicator  600  may include an ammeter placed between the power supply  602  and the resistor  604 . 
     As noted, the active communicator  600  may include the communication circuit  609 , which may include: (i) a DC current controller  610  (such as the DC current controller  524  in  FIG. 5 ) to transmit a signal by controlling the current magnitude of an analog DC signal; (ii) a resistor  613  and voltage monitor  611  for measuring a voltage drop across the resistor  613 , which is utilized by the control unit  402  and diagnostics manager  662  to calculate a current transmitted by the DC current controller  610 ; (iii) a resistor network  618  for receiving and interpreting an analog DC signal, and (iv) an FM modem  612  (such as the FM modem  504  in  FIG. 5 ) to communicate by encoding or decoding a digital FM signal. One or more components of the communication circuit  609  may be switched out if desired. Generally speaking, the DC current controller  610  is used to transmit DC signals (e.g., 4-20), the resistor network  618  is used to receive and interpret DC signals (e.g., 4-20), and the FM modem  612  is used to transmit and receive digital signals (e.g., the digital component of a HART signal superimposed on a 4-20 signal). Accordingly, when connected to an actuator, the circuit manager  661  of the control unit  402  may activate the DC current controller  610  and may disable the resistor network  618 . When connected to a transmitter, the circuit manager  662  may activate the resistor network  618  and disable the DC current controller  610 . Generally speaking, the FM modem  612  will be enabled when connected to both actuators and transmitters. 
     The DC current controller  610  may be configured to draw a DC current (e.g., 4-20 mA) when the active communicator  600  is connected to an actuator, and may be controlled by the control unit  402  (e.g., in response to a user&#39;s input provided at the UI  410 ). For example, a user may initiate a command to open a valve to 75% open. Based on this detected input, the control unit  402  may cause the DC current controller  610  to draw a current corresponding to a command to open the valve to 75% open (e.g., 16 mA). The DC current controller  610  may abruptly adjust current levels or gradually ramp current levels, depending on a user specified value. When the active communicator  600  is connected to a transmitter, the DC current controller  610  may be switched out of the communication circuit  609  (via a switch not shown) to avoid interfering with the DC current modulation of the transmitter. 
     The resistor network  618  may have any desired resistance (e.g., any desired value between 100 ohms and 1000 ohms) and may be adjustable (e.g., by the control unit  402 ). In some instances, the resistor network  618  has a resistance of 167 ohms. The voltage monitor  616  may measure a voltage drop across the resistor network  618 , which may be utilized to measure current flowing through the resistor network  618 . As an example, in a HART implementation, the voltage monitor  616  may measure a voltage between 3.34 V (20 mA*167 ohms) and 0.668 V (4 mA*167 ohms). The digital component of the HART signal, which typically modulates at 1 mA peak-to-peak, may be detected as a peak-to-peak voltage across the resistor network  618  of 0.167 V (1 mA*167 ohms). Importantly, this is above 0.12 V, a minimum voltage threshold typically needed to read the digital component of a HART signal. The resistor network  618  is described in more detail with reference to  FIG. 7 . 
     The FM modem  612 , like the FM modem  504 , may transmit information and/or receive information using a frequency modulation scheme, such as the HART protocol. The capacitor  614  filters DC current. 
     The communication circuit  609  may further include a capacitor  614  in series with the FM modem  612  to filter DC current so that the FM modem  612  can receive and transmit the AC component of the signal, and a voltage monitor  616 , communicatively coupled to the control unit  402 , configured to measure a voltage drop across the resistor network  618 . The control unit  402 , knowing the resistance of the resistor network  618 , may calculate the current flow through the resistor network  618  based on the measured voltage. 
     As noted, the communication circuit  609  may be electrically connected to the communication interface  406  to send and receive communication signals. Further, the power supply  602  may be electrically connected to the communication interface  406  to supply a power signal to the field device  160  via the communication interface  406 . The communication interface  406  may include terminals  631 - 636  for connecting to one or more field devices, and/or fuses  641  and  642  to limit current flowing through the active communicator  600 . Generally speaking, the field device  160  may be a transmitter configured to report a measurement by modulating a DC current (e.g., 4-20 mA) or an actuator configured to actuate in a particular manner in response to the magnitude of a received DC current (e.g., 4-20 mA). 
     The communication interface  406  may be electrically connected to the power monitor  408  shown in  FIG. 4 , which may include a resistor  651  and a voltage monitor  652 . The resistor  651  may have a resistance sufficiently low to avoid a significant voltage drop. In some cases, the resistor  651  has a resistance between 0 and 10 ohms (e.g., 2.43 ohms), which may be selected to minimize the voltage drop over the resistor  651 . The voltage monitor  652  may measure the voltage drop across the resistor  651  and transmit the measured voltage to the control unit  402 . The control unit  402 , knowing the resistance of the resistor  651 , may calculate the current flow through the resistor  651 . In some cases, a fuse may be placed between the terminal  635  and the power monitor  408 . 
     In operation, the active communicator  600  may be configured to operate in a “tool-power mode” (i.e., where the active communicator  600  provides power to the connected field device  160 ) and in a “loop-power mode” (i.e., where the connected field device  160  relies on, e.g., a portable external power supply instead of the active communicator  600  for power). The communication interface  406  may include a first terminal set (e.g., terminals  631  and  632 ) for tool-power mode and a second terminal set (e.g., terminals  633  and  634 ) for loop-power mode. Further, the active communicator  600  may be configured to operate in a “transmitter connection mode” and an “actuator connection mode.” Thus, the communication interface  406  may be configured to facilitate four different configurations or types of connections: (i) tool-power transmitter connection, wherein the active communicator  600  supplies power to a transmitter; (ii) loop-power transmitter connection, wherein the active communicator  600  connects to a powered transmitter field device (in some scenarios the loop to which the loop-powered transmitter is connected includes a loop resistor, enabling communication; in others the loop does not include a loop resistor. When a loop resistor is not present, the tool  100  may activate or adjust the resistor network  618  to provide sufficient loop resistance for communication); (iii) tool-power actuator connection, wherein the active communicator  600  supplies power to an actuator; and (iv) loop-power actuator connection, wherein the active communicator  600  connects to a powered actuator (in some scenarios the loop-powered actuator includes a DC current controller; in others it does not). 
     For a tool-power transmitter connection, the active communicator  600  may activate a “tool-power” mode and/or a “transmitter connection” mode. The active communicator  600  may activate one or both of these modes in response to user input (e.g., via the screen  122  and/or via the buttons  132  shown in  FIG. 1A ). In some cases, the active communicator  600  may activate “tool-power” mode in response to detecting that the terminals  631  and  632  have been connected to the field device  160 . In other cases, the active communicator  600  instead activates “tool-power” mode based on input from a user. If desired, the active communicator  600  may perform one or more power or communications checks or verifications before activating “tool-power mode.” In some instances, the active communicator  600  may activate the “transmitter connection” mode in response to detecting that a connected field device is a transmitter. In other instances, the active communicator  600  instead activates the “transmitter connection” mode based on input from a user. Further, the active communicator  600  may perform a power verification after power is enabled to verify that the field device  160  is behaving as expected (e.g., behaving as expected for a transmitter or actuator). The active communicator  600  may also verify that the field device  160  is connected, that power supply limits are not exceeded, and/or that no circuits have unexpectedly shorted. 
     For a tool-power transmitter connection, a user may connect a transmitter to the terminals  631  and  632 . When connected to a transmitter, the current controller  610  may be switched out of the network via a switch (not shown) because the active communicator  600  is not modulating DC current to transmit a command. After the transmitter has been connected, the power supply  602  may ramp up power. Power may be ramped slowly to avoid current spikes. Current may flow from the power supply  602 , through the resistor  604  and terminal  631 , to the transmitter. The transmitter may draw a certain level of baseline current for power (e.g., up to 4 mA). The transmitter may then draw additional current based on its configuration and based on a measurement it has performed (e.g., a measured flow, pressure, tank level, etc.). As an example, a current draw by the transmitter of 4 mA may represent a live-zero for the transmitter&#39;s configured measurement range (e.g., 0 gpm), and a current draw by the transmitter of 20 mA may represent a measurement at the top of the configured measurement range (e.g., 100 gpm). A current draw between 4-20 mA may represent a proportional measurement within the configured measurement range (e.g., 12 mA=50 gpm). In some instances, the tool  100  generates a high alarm when a current of 22.5 mA or higher is detected and/or generates a low alarm when a current of 3.75 mA or lower is detected. 
     Current may flow from the terminal  631  to the transmitter and back through the terminal  632 , through a switch  641 . The received current may flow to the circuit  609 . As noted, the capacitor  614  filters DC current and the DC current controller  610  may be switched out when the active communicator  600  is connected to a transmitter. Accordingly, the DC component of the received signal flows through the resistor network  618 , where the voltage monitor  616  may measure the voltage drop across the resistor network  618  so that the control unit  402  can determine the magnitude of the received DC current. The control unit  402  may determine a variable value (e.g., a flow rate) based on the determined magnitude. 
     Because the capacitor  614  allows AC current to pass, the AC component of the signal may flow to the FM modem  612 . The FM modem  612  may then decode a digital signal carried by the received AC component in a manner similar to that described regarding the FM modem  504 . Further, the FM modem  612  may also transmit information to the transmitter by encoding a digital signal (superimposed onto the DC signal) in a manner similar to that described regarding the FM modem  504 . 
     For a loop-power transmitter connection, a user may connect a powered transmitter to the terminals  633  and  634 . The active communicator  600  may activate one or both of “loop-power mode” and/or “transmitter connection mode” in response to user input (e.g., via the screen  122  and/or via the buttons  132  shown in  FIG. 1A ). In some cases, the active communicator  600  activates “loop-power” mode in response to detecting that the terminals  633  and  634  have been connected to a field device (e.g., via a link  150 ). In other cases, the active communicator  600  instead activates “loop-power” mode based on input from a user. The active communicator  600  may activate “loop-power” mode after verifying that no voltage exists at any other terminals of the communication interface  406 . 
     The tool  100  may include a fuse  642 , electrically connected to the terminal  634 , configured to limit current to a particular threshold. In some cases, the fuse  642  is not included. In some cases, the fuse  642  is placed between the terminal  634  and ground. In the same manner as that described with respect to the tool-power transmitter connection, the circuit  609  may decode a DC component of a received signal (e.g., 4-20 mA) and may modulate and demodulate an AC component of the signal (e.g., a superimposed frequency modulated 1 mA peak-to-peak signal) to transmit information to and receive information from the transmitter. 
     For a tool-power actuator connection, a user may connect an actuator to the terminals  631  and  632 . The active communicator  600  activates one or both of “tool-power mode” and/or “actuator connection mode” in response to user input (e.g., via the screen  122  and/or via the buttons  132  shown in  FIG. 1A ). The power supply  602  may supply power to the actuator in a manner similar to that described regarding a tool-power transmitter connection. 
     In this mode of operation, a switch (not shown) may activate the DC current controller  610  if the DC current controller  610  is switched out of the circuit  609 . The DC current controller  610  may draw a DC current (e.g., 4-20 mA), which may be supplied by the power supply  602  and may flow through the terminal  631  to the actuator, and then back through the terminal  632  to the DC current controller  610 . The magnitude of the current acts as a command for the actuator. The current resistor network  618  and voltage monitor  616  may be switched out of the circuit  609  by the control unit  402  via a switch (not shown) when the DC current controller  610  is active because the circuit  609  is not interpreting a modulated DC current in this mode. 
     Because the capacitor  614  allows AC current to pass, the AC component of the received signal may flow to the FM modem  612 . The FM modem  612  may then decode a digital signal carried by the received AC component in a manner similar to that described regarding the FM modem  504 , and the FM modem  612  may transmit information to the actuator by encoding a digital signal (superimposed on the DC signal) in a manner similar to that described regarding the FM modem  504 . 
     For a loop-power actuator connection, a user may connect a powered actuator to the terminals  633  and  634 , and the active communicator  600  may activate one or both of “loop-power mode” and/or “actuator connection mode” in response to user input (e.g., via the screen  122  and/or via the buttons  132  shown in  FIG. 1A ). In the same manner as that described with respect to a tool-power transmitter connection, the circuit  609  may modulate a DC component of a signal (e.g., 4-20 mA) to transmit a command to the actuator, and may modulate and demodulate an AC component of the signal (e.g., a superimposed frequency modulated 1 mA peak-to-peak signal) to transmit information to and receive information from the actuator. 
     The control unit  402  may include a circuit manager routine  661  for managing the active communicator  600 , and/or a diagnostics manager routine  662  for analyzing signals sent and/or received by the active communicator  600 . The diagnostics manager  662  may analyze the signals based on measurements obtained from the voltage monitors  605 ,  611 , and/or  616 . In some cases, the active communicator  600  includes one or more voltage monitors that measure a voltage drop across one or more of the terminals  631 - 634  (e.g., across terminals  631  and  632  or across terminals  633  and  634 ). The diagnostics manager  662  may analyze one or more of these voltage drops prior to the circuit manager  661  activating (i.e., switching in) one or more resistors of the network  618  in order to (i) protect against activation of the resistor network  618  in parallel with an external loop resistor, and/or (ii) manage a multi-step process for activating the resistor network  618 . 
     First, the circuit manager  661  may protect a user from enabling the resistor network  618  in parallel with an external loop resistor on an externally powered loop, which might result in a disturbance in the loop current and/or a loss in communication due to insufficient loop resistance. That is, activating the resistor network  618  while connected in parallel with an external loop resistor may drop the total loop resistance to a value too low for detecting and interpreting digital communications. To illustrate, in some cases the resistor network  618  may have a resistance of 250 ohms. Typical external loop resistors have a resistance of 250 ohms. Thus, if the resistor network  618  is activated in parallel with an external loop resistor, the total loop resistance drops to 125 ohms, which may not be sufficient resistance to induce a readable voltage drop associated with digital communications. For example, HART digital communications, which typically modulate at 1 mA peak-to-peak, generally require a voltage drop of at least 120 mVp-p. Thus, if the total loop resistance is 125 ohms, there is little margin of error before a HART digital signal becomes unreadable. To prevent the tool  100  from activating the resistor network  618  in parallel with an external loop resistor, the control unit  402  may cause the FM modem  612  to attempt digital communication upon connection. If the digital communication does not succeed, the control unit  402  may prompt the user (e.g., via the display  122  shown in  FIGS. 1 and 4 ) to activate the resistor network  618 . In some cases, the tool  100  may prevent the resistor network  618  from activating in parallel with a loop resistor by prompting the user with one or more questions and/or instructions to cause the user to connect the tool  100  with the field device in series. 
     Second, the measured voltages may be used to manage a multi-step process for activating the resistor network  618 , which may help the tool  100  avoid exceeding voltage and/or current thresholds (which might otherwise, for example, blow the fuse  642 ). For example, the diagnostics manager  662  may obtain a voltage measurement from the voltage monitor  605  or from a voltage monitor across the terminals  631  and  632  (not shown). If the diagnostics manager  662  determines the power supply voltage for the power supply  602  or for an external power supply exceeds a voltage threshold (e.g., 24 V), the circuit manager  661  may prevent the resistor network  618  from activating. Activation of the resistor network  618  in such a scenario could result in excessive current (e.g., more than 50 mA) flowing through the resistor network  618 , which may blow the fuse  642 . If the measured power supply voltage is under the voltage threshold, the circuit manager  661  may activate the resistor network  618  with a resistance of 500 ohms. The diagnostics manager  662  may then measure current flow (e.g., based on measurements from the voltage monitor  616 ) and, if the measured current is below a threshold (e.g., 22.5 mA) the circuit manager  661  may adjust the resistor network  618  to a resistance of 250 ohms or 167 ohms, as desired. The fact that the measured current is below a threshold generally indicates that the connected field device is controlling the current. 
     The circuit manager  661  may detect a voltage decay across the terminals  631  and  632  (e.g., 0.01 V to 0.1 V drop every 50 to 100 msec). The circuit manager may take two to ten measurements over a period over 100 msec to 1 second. In response to detecting the voltage decay, the circuit manager  661  may give a user an option to activate the power supply  602  immediately rather than waiting until the voltage decays to zero. 
     The circuit manager  661  may activate and/or switch out one or more resistors in the network  618  when the power supply  602  is turned off to raise the resistance of the network  618  to bleed off voltage from a field device connected to the active communicator  600 . Bleeding off voltage may reduce the wait time needed before the power supply  602  can be reactivated. 
     The circuit manager  661  may rely on a temperature sensor (not shown) disposed near the network  618  to compensate for changes in resistance attributable to temperature changes, enabling the circuit manager  661  to more accurately calculate a current flow based on measurements from the voltage monitor  616 . 
     The circuit manager  661  may cause the DC current controller  610  to gradually change current. For example, the circuit manager  661  may implement a gradual current change in response to user input specifying a change in current over a certain number of seconds (e.g., 1, 2, 3, 4, . . . , 60 seconds, etc.). 
     The diagnostics manager  662  may perform one or more of the following: a loop test, a device simulation, a recorder calibration, a valve stroking, a DCS output check, a DCS input check, a zero trim calibration, or a check for isolating cable damage. 
     For the loop test, a user may connect a field device to: (i) the terminal set  631  and  632  or the terminal set  633  and  634 , and (ii) the power monitor  408 . The diagnostics manager  662  may cause the FM modem  612  to transmit a communication signal to the field device commanding the field device to draw current at various levels (e.g., 4 mA, 12 mA, 20 mA). The power monitor  408  measures the current transmitted by the field device in response. The diagnostics manager  662  may cause the tool  100  to display the requested current level and the transmitted current level, enabling the user to determine if the transmitter is appropriately responding to the commands. 
     For device simulation, a user may wire the tool  100  (e.g., via the terminals  631  and  632  of the active communicator  600 ) to a communication link in place of a field device. The diagnostics manager  662  may cause the DC current controller  610  to transmit DC current at a number of levels (e.g., in response to user input). The user, or a second user, may then verify that the connected process controller received the appropriate values. 
     For recorder calibration, a user may wire the tool  100  to an analog recorder and may cause the tool  100  to transmit via the DC current controller  610  preselected current values to the recorder. The user may then verify that the audio recorder received the preselected values. 
     For valve stroking, a user may wire the tool  100  to a valve. The diagnostics manager  662  may transmit a 4 mA signal to the valve, and the user may set a full closed stop on the valve (e.g., thereby calibrating the valve so that the full closed position corresponds to a 4 mA signal). The diagnostics manager  662  may transmit a 20 mA signal to the valve (e.g., in response to user input), and the user may set a full open stop on the valve (e.g., thereby calibrating the valve so that the full open position corresponds to a 20 mA signal). The diagnostics manager  662  may then perform a step test (e.g., in response to user input) with the DC current controller  610 , transmitting current at a number of levels between 4 mA and 20 mA, to verify that the valve is appropriately calibrated to a 4-20 mA signal. 
     To check a DCS output, a user may wire the tool  100  to an I/O card that is typically connected to a field device (e.g., an actuator) and that is configured to transmit to the field device a 4-20 mA signal for controlling the field device. A user may coordinate (e.g., via radio) with a second user to cause a number of commands to be sent to the disconnected field device (e.g., to open or close a valve). The diagnostics manager  662  may read the received signal, and may display a current measurement, enabling the user to verify that I/O card is transmitting appropriate signals when attempting to control the field device. The tool  100  may utilize either the power monitor  408  (and terminals  635  and  636 ) or the voltage monitor  616  (and terminals  631  and  632 ) for the DCS output check. 
     To check DCS input, a user may wire the tool  100  to an I/O card that is typically connected to a field device (e.g., a transmitter) and that is configured to receive from the field device a 4-20 mA signal representing a measurement. The user may cause the tool  100  to send (e.g., via the DC current controller  610 ) a signal, and may coordinate with a second user to confirm that the controller connected to the I/O card is receiving the proper values. 
     To perform a zero trim calibration, the tool  100  may implement a procedure similar to that implemented for the DCS input check. For example, a user may connect the tool  100  to a field device, and may cause the field device to run a “device method” that causes the field device to output current at a number of levels. The tool  100  measures and displays the current from the field device. The user then enters at the field device the displayed current so that the field device can calibrate itself based on what it attempted to transmit and what was actually transmitted. The tool  100  may activate the power supply  602  to power the field device and the resistor network  618  to measure current from the field device, and may measure the current across the resistor network  618 . 
     To isolate a damaged cable, a user may utilize the tool  100  to perform voltage measurements at various locations for a cable, at field device terminals, at power supply terminals, etc. A large voltage drop indicates damage. 
       FIG. 7  is a schematic of the resistor network  618  shown in  FIG. 6 , which includes multiple resistors and is configured to withstand failure of one or more resistors within the network  618  without significantly affecting the overall resistance of the network  618 . By utilizing multiple resistors, the resistor network  618  may avoid dramatic increases or decreases in resistance due to resistor failure, thus avoiding dramatic increases or decreases in voltage drops in the circuit. For example, if the resistor network  618  were a single resistor that failed and shorted, the result may be an increased voltage drop across terminals  631  and  632 . Such an increase in the voltage drop at the terminals  631  and  632  may exceed IS standards and may risk igniting an explosive atmosphere. Further, by utilizing multiple resistors, each individual resistor receives only a portion of the current that enters the network  618 , which may prevent the resistors from overheating and exceeding IS standards. 
     The resistor network  618  may include a resistor network  702  and a resistor network  704  arranged in parallel, each of which may be switched out of the resistor network  618  via a switch  706  and a switch  708 , respectively. In some instances, the entire resistor network  618  may be switched out of the circuit  609  (e.g., during power up or when the tool  600  is communicating with an actuator). The resistor network  618  may have any desirable resistance, e.g., within a range of 100 and 300 ohms (e.g., 167 ohms). 
     The resistor network  702  may include resistors  712 - 714 , arranged in parallel, and may have a total resistance between 200 and 300 ohms (e.g., 250 ohms). For example, each of the resistors  712 - 714  may have a resistance of 750 ohms, giving the network  702  a total resistance of 250 ohms. Note, the resistor network  702  may have any other combination of a plurality of resistors (including combinations and/or subcombinations of resistors arranged in series and/or in parallel) resulting in a total resistance of the network  702  between 200 and 300 ohms. If desired, the resistor network  702  may include a single resistor having a resistance between 200 and 300 ohms. 
     The resistor network  704  may include a resistor network  721  and a resistor network  723 , arranged in series, and may have a total resistance between 400 and 600 ohms (e.g., 500 ohms). The resistor network  721  may include resistors  722  and  724 , arranged in parallel, and the resistor network  723  may include resistors  726  and  728 , arranged in parallel. Each of the resistors  722 - 728  may have a resistance of 500 ohms, which may give each of the resistor networks  721  and  723  a resistance of 250 ohms (i.e., 1/X=1/500+1/500). Because the resistor networks  721  and  723  may be arranged in series, the resistor network  704  may have a total resistance of 500 ohms. Note, the resistor network  704  may have any other combination of a plurality of resistors that results in a total resistance between 400 and 600 ohms. For example, the resistor network  704  may include two 1000 ohm resistors arranged in parallel, or may include a single resistor having a resistance of 500 ohms. 
     Regarding the switches  706  and  708 , the control unit  402  may: (i) actuate the switch  706  to switch out the resistor network  702  to increase the resistance of the network  618  (i.e., to the resistance of the network  704 ), or (ii) actuate the switch  708  to switch out the resistor network  704  to increase the resistance of the network  618  (i.e., to the resistance of the network  702 ). The control unit  402  may increase the resistance of the network  618  to verify that a field device attached to the terminals  633  and  634  is current-controlled below the rating of the fuse  642 , or to bleed off voltage from the active communicator  600  when the power supply  602  is disabled. 
     In operation, the control unit may switch out one of the networks  702  or  704  depending on which of the terminals  631 - 634  are connected to a field device and/or based on whether the active communicator  600  is providing power to the field device. For example, in some situations, the active communicator  600  may switch out either the network  702  or  704  to increase the resistance of the network  618  when the active communicator  600  is connected to an externally powered field device. 
     When one or both of the resistor networks  702  and  704  are switched out of the network  618 , the control unit  402  may actuate one or both of the switches  706  and  708  to “activate” both the networks  702  and  704 , giving the resistor network  618  a lower resistance than that of either the network  702  or the network  704 . The lower resistance may be desirable when operating the active communicator  600  in “tool-power mode” in a hazardous area. IS standards require that the output voltage at the terminals  631  and  632  be lower than what might otherwise be used in normal operation. However, if the voltage at the terminals  631  and  632  is dropped too low, the communication signal transmitted or received via the terminals  631  and  632  may be unreadable. Accordingly, it may be advantageous to lower the resistance of the network  618  from a value that might be used with a traditional communicator (e.g., 250 ohms) to a value that will result in a lower (but still readable) voltage drop over the resistor network  618  (e.g., 167 ohms). In some instances, the resistor network  618  may include additional resistor networks (e.g., 1000 ohms) and/or switches, which may be enable the tool  100  to activate the resistor network when a power supply voltage exceeds 24 V. 
     One or both of the switches  706  and  708  may be solid state relays, which may offer a number of advantages over typical mechanical relays. For example, solid state relays can be switched by a lower voltage and lower current than most mechanical relays, making it easier to keep the electrical signals generated by the tool  100  at levels compliant with IS standards. Further, unlike typical mechanical relays, solid state relays generally do not generate a spark when operated. Thus, by utilizing solid state relays with the resistor network  618 , the tool  100  can avoid violating IS standards that might otherwise be violated with mechanical relays. 
     One or more resistors in the resistor network  618  may have a large surface area designed to facilitate efficient heat dissipation. For example, one or more resistors in the resistor network  618  may be size  2512  resistors (e.g., 6.3 mm×3.1 mm×0.6 mm). In some cases, one or more resistors in the network  618  may be size  2010  resistors, size  2020  resistors, and/or size  2045  resistors. In some cases, one or more resistors (or sub-networks) within the resistor network  618  may be rated for 2.5 W. 
     The tool  100  may include a temperature sensor (not shown) to be used in conjunction with the active communicator  600 . For example, the temperature sensor may measure a temperature at or near the resistor network  618 , which may be utilized by the control unit  402  when calculating a current through the network  618 . This temperature measurement is beneficial because current calculations obtained based on measurements from the voltage monitor  616  may be inaccurate when a temperature change increases or decreases the resistance of the network  618  to a value different than that assumed by the control unit  402 . In short, the temperature sensor enables the control unit  402  to compensate for changes in resistance of the network  618  attributable to temperature changes. 
       FIGS. 8A-12  are schematics of the tool  100 , when it includes the active communicator  600  shown in  FIG. 6 , connected to various field devices and I/O devices.  FIGS. 8A-12  may not show one or more components of the active communicator  600  or tool  100 . For example, components that are “switched out” or not active may not be shown. Some components may be active, but may not be shown. 
       FIG. 8A  illustrates an example in which the active communicator  600  is connected to a transmitter  805  and in which the active communicator  600  provides power to the transmitter  805  (i.e., a tool-power transmitter connection). In such a configuration, a user may connect the active communicator  600  to the transmitter  805  via the terminals  631  and  632 . The active communicator  600  may power the transmitter  805  by way of the power supply  602 , e.g., transmitting a DC signal of up to 4 mA to the transmitter  805 . Further, the active communicator  600  may engage in one-way analog communication and/or two-way digital communication with the transmitter  805 . To engage in one-way analog communication, the active communicator  600  may activate a “transmitter connection” mode in which the active communicator  600  expects the transmitter  805  to communicate by modulating the current magnitude (e.g., between 4-20 mA) of the DC signal provided by the power supply  602 . The control unit  402  (not shown) may switch the DC current controller  610  (not shown) out of the circuit  609  because the transmitter  805 , not the circuit  609 , may modulate the DC current flowing between the transmitter  805  and tool  100 . Note, the FM modem  612  may remain connected to the circuit  609 , and may facilitate two-way digital communication by: (i) transmitting information to the transmitter  805  by modulating the frequency of an AC signal superimposed on the DC signal, and/or (ii) receiving information from the transmitter  805  by demodulating a frequency modulated AC signal superimposed on the DC signal by the transmitter  805 . The diagnostics manager  602  may analyze a signal received from the transmitter  805  based on measurements obtained from the voltage monitor  616 . 
       FIG. 8B  illustrates an example in which the active communicator  600  is connected to an actuator  855  and in which the active communicator  600  provides power to the actuator  855 . In such a configuration, a user may connect the active communicator  600  to the actuator  855  via the terminals  631  and  632 . The active communicator  600  may power the actuator  855  by way of the power supply  602 , e.g., transmitting a DC signal of up to 4 mA to the actuator  855 . The active communicator  600  may engage in one-way analog communication and/or two-way digital communication with the actuator  855 . To engage in one-way analog communication, the active communicator  600  may activate an “actuator connection” mode in which the active communicator  600  expects the actuator  855  to receive information by interpreting changes in the current magnitude of the DC signal provided by the active communicator  600  (e.g., between 4-20 mA). Because the active communicator  600  may expect the actuator  855  to receive rather than transmit a current modulated DC signal, the control unit  402  may switch the resistor network  618  (not shown) and voltage monitor  616  (not shown) out of the circuit  609  because the network  618  and monitor  616  may not be needed to receive and interpret a current modulated DC signal. Note, the FM modem  612  may remain connected to the circuit  609 , and may facilitate two-way digital communication by: (i) transmitting information to the actuator  855  by modulating the frequency of an AC signal superimposed on the DC signal, and/or (ii) receiving information from the actuator  855  by demodulating a frequency modulated AC signal superimposed on the DC signal by the actuator  855 . 
       FIG. 9A  illustrates an example in which the circuit  609  of the active communicator  600  may be connected to a transmitter  905  and in which the transmitter  905  is not powered by the active communicator  600 . The transmitter  905  may be powered by a power supply  910 , which may be a portable power supply or a rack-mounted power supply. In some instances, the transmitter  905  may rely on both loop power and the power supply  910  for power. A user may connect the circuit  609  to the transmitter  905  via the terminals  633  and  634 . Because the terminals  631  and  632  are not connected, a closed circuit including the power supply  602  is not formed. The circuit  609  may engage in one-way analog communication and/or two-way digital communication with the transmitter  905  in a manner similar to that discussed regarding  FIG. 8A . The configuration shown in  FIG. 9A  may be useful when a user encounters a conventional loop that lacks a loop resistor. In some instances, the tool  100  may be connected to a loop that already has a loop resistor (e.g., connected to the negative terminal of the power supply  910 ). In such cases, the power supply  910  may be wired to the terminals of the transmitter  905  as one would expect in normal operation, and the user may connect the terminals  633  and  634  to the terminals of the transmitter  905 . Alternatively, the user may connect the terminals  633  and  634  across the already-existing external loop resistor. 
       FIG. 9B  illustrates an example in which the circuit  609  of the active communicator  600  is connected to an actuator  955  and in which the actuator  955  is not powered by the active communicator  600 . The actuator  955  may be powered by a power supply  960 , which may be a portable power supply or a rack-mounted power supply. A user may connect the circuit  609  to the transmitter  905  via the terminals  633  and  634 . Because the terminals  631  and  632  are not connected, a closed circuit including the power supply  602  is not formed. The circuit  609  may engage in one-way analog communication and/or two-way digital communication with the actuator  955  in a manner similar to that discussed regarding  FIG. 8B . In some instances, the power supply  960  may include a DC current controller that transmits a DC signal (e.g., 4-20 mA). In such instances, the terminals of the power supply  960  may be connected to the terminals of the actuator  955  to form a loop. Further, in such instances a user may connect the tool  100  to the terminals of the actuator  955 , effectively placing the tool  100  in parallel with the power supply  960 . Advantageously, such a connection enables a user to utilize the tool  100  without breaking an already existing loop between the actuator  955  and power supply  960 . 
       FIG. 10  illustrates an example in which the circuit  609  of the tool  100  may be connected to a field device  1010  powered by a power supply  1020 , and in which the power monitor  408  of the tool  100  may be connected to the field device  1010  in parallel with the power supply  1020  so that it can measure electrical characteristics of signals sent or received by the field device  1010  without disconnecting the field device  1010  from the power supply  1020 . The field device  1010  may be an actuator or a transmitter; may be powered by a power supply  1020 ; and may include a positive terminal  1012 , a negative terminal  1014 , and a test terminal  1015 . The test terminal  1015  may enable detection of current through the field device  1010  and/or detection of voltage across the terminals  1012  and  1014 . 
     The circuit  609  may be connected to the field device  1010  via the terminals  633  and  634 , and via the positive and negative terminals  1012  and  1014  of the field device  1010 . The power monitor  408  can be connected to the field device  1010  without breaking the loop between the circuit  609  and the field device  1010 , enabling a user to simultaneously communicate with the field device  1010  via the circuit  609  and to verify that current, voltage, and/or power measurements associated with communication signals between the field device  1010  and the circuit  609  are within an expected range. Advantageously, there is no need to disconnect the field device  1010  from the power supply  1020  when utilizing the power monitor  408  to measure the electrical characteristics of the signals received or transmitted by the field device  1010 . In some cases, the power supply  1020  may include a DC current controller that send a 4-20 mA signal for controlling the field device  1010  (e.g., when the field device  1010  is an actuator). In such cases, the DC current controller  610  may be switched out of the circuit, and the tool  100  may function primarily as a digital communicator using the FM modem  612 . 
       FIG. 11  illustrates an example in which the circuit  609  of the tool  100  may be connected to a field device  1105  powered by a power supply  1120 , and in which the power monitor  408  of the tool  100  may be connected to the field device  1105  in series with the power supply  1120  so that it can measure electrical characteristics of signals sent or received by the field device  1010 . The power monitor  408  may be connected to the field device  1105  via the terminals  635  and  636 . Because the field device  1105  does not include a test terminal like the field device  1010  shown in  FIG. 10 , the power monitor  408  may be connected to the field device  1105  in series. That is, the positive terminal of the power supply  1120  may be disconnected from the positive terminal of the field device  1105 , and a first terminal from the power monitor  408  (e.g., terminal  635  or  636 ) may be connected to the positive terminal of the field device  1105  and a second terminal from the power monitor  408  (e.g., the terminal  635  or  636 ) may be connected to the positive terminal of the power supply. In some cases, the power monitor  408  may be connected in series between the negative terminal of the field device  1105  and a loop resistor (not shown) connected to the negative terminal of the power supply  1120 . Typically, such a loop resistor can be found when the field device  1105  is a transmitter. In such cases, the resistor network  618  may be switched out of the circuit  609 . 
       FIG. 12  illustrates an example in which the circuit  609  of the active communicator  600  may be used to test I/O devices. In particular, the circuit  609  may be connected to an AI card  1205  (which may be connected to a transmitter) to verify that a signal transmitted by the DC current controller  610  (intended to simulate a transmitter&#39;s signal) is properly received by the AI card  1205 . Further, the power monitor  408  may be connected to an AO card  1210  (which may be connected to a controller) to verify communications sent by the controller via the AO card  1210 . 
       FIG. 13  is a schematic of an active communicator  1300  (which may be an example of the active communicator  404  shown in  FIG. 4 ) for the tool  100  that may be electrically connected to the field device  160  shown in  FIG. 1A  via the communication interface  406  to: (i) supply power to the field device  160  by way of a DC signal (e.g., 10-25 mA); and (ii) communicate with the field device  160  by way of a digital AC signal superimposed on the DC signal. Advantageously, the active communicator  1300  may be utilized to communicate with and/or diagnose Fieldbus field devices. Further, unlike typical PTDs configured for AM communication, the active communicator  1300  can measure current, as well as DC and AC voltages at the same terminal set that is used for communicating with the field device  160 . In some cases, the active communicator  1300  may be energy limited and fault tolerant according to IS standards, and enabling the active communicator  1300  and the tool  100  to power, communicate with, and/or diagnose field devices and communication links located in hazardous areas. 
     The active communicator  1300  may be communicatively coupled to the control unit  402 , and may include: (i) a bus  1302  electrically connected to the communication interface  406 , (ii) a power supply  1304  configured to transmit a power signal via the bus  1302 , and (iii) a communication circuit  1309  configured to communicate with the field device  160  via the bus  1302 . The communication circuit  1309  may be configured to send and receive digital communication signals, which may be amplitude modulated AC signals (e.g., 15-20 mA peak-to-peak). 
     The bus  1302  may be referred to as an “internal bus” or “mini-bus,” and may be disposed at least partially within the housing  128  shown in  FIG. 1A . The bus  1302  may enable the tool  100  to connect to, communicate with, and/or power a field device, even when the field device does not have an active and healthy connection to a bus (such as the fieldbus segment  300  shown in  FIG. 3 ) in the plant environment. By contrast, traditional PTDs that can communicate with a bus-based field device typically require that the field device be connected to a functioning external bus located in the process plant. The bus  1302  may include terminators  1321  and  1322  that provide sufficient resistance to enable communication on the bus  1302 . Each of the terminators  1321  and  1322  may include a resistor (e.g., having a resistance between 90 ohms and 105 ohms) in series with a capacitor (e.g., having a capacitance of 1 μF). The bus  1302  may include switches  1322  and  1324  for switching the terminators  1321  and  1322  out of the bus  1302 , or for switching the bus  1302  completely out of the active communicator  1300  (e.g., when the active communicator  1300  is connected to a field device already connected to a healthy bus). Advantageously, the bus  1302  enables a field device to be tested in isolation, allowing a user to more easily identify the source of communication problems. For example, a user may utilize the active communicator  1300  to measure signals transmitted by an externally powered field device, to remove the field device from the external power source (e.g., from the powered segment), and to take the same measurements on the bus  1302 , which can be compared to the previously obtained measurements. 
     The circuit  1309  may communicate with the field device  160 , which may be a Fieldbus device, via the bus  1302 , and may include an AM modem  1310  connected in series to a capacitor  1314 , the combination of which are electrically connected in parallel to a DC current controller or sink  1312 . The AM modem  1310  may transmit to and/or receive information from the field device  160  using a digital amplitude modulation scheme (such as the Fieldbus protocol), and may be the same as or similar to the AM modem  514  shown in  FIG. 5 . As an example, the AM modem  1310  may modulate and/or demodulate a signal at 15-20 mA (e.g., −10 mA to 10 mA). The capacitor  1314  may filter DC current, allowing only the digital communication signal to pass to the AM modem  514 . The DC current controller  1312  may be configured to draw a DC current sufficient to enable the AM modem  1310  to superimpose the communication signal on the DC current without the current dropping below 0 mA. For example, the DC current controller  1312  may draw a DC current of 11 mA, enabling the AM modem  1310  to superimpose a 20 mA signal on the DC current so that the total current on the wires connected to the communication interface  406  varies from 1 mA to 21 mA. 
     The communication interface  406  may include terminals  1331 - 1333 . The field device  160  may be connected to the terminals  1332  and  1333  to establish a communication link between the field device  160  and the circuit  1309 . If a user wishes to supply power to the field device  160  using the tool  100 , the user may place a shunt between the terminal  1331  and the terminal  1332 . Placing the shunt between the terminals  1331  and  1332  may activate the bus  1302 , which may create sufficient load impedance to ensure communication signals transmitted via the terminals  1332  and  1333  are within an expected range such that the AM modem  1310  and/or the field device  160  can interpret the communication signals (e.g., between 0.5 and 1.5 Vpp). The communication interface  406  may include a fuse  1342 , electrically connected in series with the terminal  1333 , that has a resistance of 11 ohms and that is rated for 50 mA. In some cases, the active communicator  1300  does not include the fuse  1342 . In some cases, the fuse  1342  may be placed between the terminal  1333  and ground. 
     The power supply  1304  may be configured to supply a power signal on the bus  1302  at a voltage between 15 V and 20 V (e.g., 17 V). The power supply  1304  may be a transformer-based power supply, and may “shift” its ground with respect to the terminal  1333 . The power supply  1304  may be configured to limit a voltage drop across the terminals  1332  and  1333  to a threshold consistent with IS standards. For example, in some cases the maximum allowable output voltage is limited to 15 V at no load. The output voltage at full load may be 10.5 V. The power supply  1304  may be current limited (e.g., 38 mA) to avoid exceeding voltage and/or power thresholds at the terminals  1331 - 1333 . The active communicator  1300  may include a power conditioner  1306 , connected in series to the power supply  1304 , configured to prevent the power supply  1304  from filtering out communication signals (e.g., from the AM modem  1310  and/or a connected field device). 
     In operation, the active communicator  1300  may operate in tool-power mode and external-power mode. In tool-power mode, a user may connect the field device  160  to the terminals  1332  and  133 , and may connect a shunt to terminals  1331  and  1332  so that current will flow from the terminal  1331  to the terminal  1332  and to the connected field device  160 . In external-power mode, a user may connect the field device  160  to terminals  1332  and  1333 , leaving an open circuit between terminals  1331  and  1332 . 
     When in external-power mode, the switches  1332  and  1324  may be activated to switch out the bus  1302  and create an open circuit between the terminals  1321 - 1322  and the wire connecting the power condition  1306  to the terminal  1331 . Further, the DC current controller  1312  may be switched out of the circuit  1309  when the active communicator  1300  is in external-power mode. 
     The active communicator  1300  may operate in communications mode or diagnostics mode. When in communications mode, the active communicator  1300  may communicate with the connected field device  160 . When in diagnostics mode, the active communicator  1300  may measure electrical attributes of signals on a communication bus (e.g., a Fieldbus segment), and/or may measure electrical attributes of signals received at the terminals of the field device  160 . When in diagnostics mode, DC current controller  1312  may not draw a significant current. When in communications mode, the DC current controller  1312  may draw a current (e.g., 11 mA). In some cases, the active communicator  1300  may operate in communications mode and diagnostics mode simultaneously. 
     If desired, the control unit  402  may disable one or more of the power supply  1304 , the power condition  1306 , the terminators  1321 / 1322 , and/or the circuit  1309  when a change in voltage or current is detected at the terminals  1331 - 1333 . For example, a detected high voltage across the terminals  1332  and  1333  may indicate that a user has attached a new power source, which may cause the active communicator  1300  to disable one or more components. As another example, a detected high current at the terminals  1332  and  1333  may indicate that a user has shorted wires or attempted to add another device, and may cause the active communicator  1300  to disable one or more components. For example, a low voltage measurement may indicate that an externally powered field device has lost power. A low current measurement may indicate a device has been removed from the terminals  1331 - 1333 . 
     The control unit  402  may include a circuit manager routine  1361  for managing the active communicator  1300  and/or a diagnostics manager routine  1362  for analyzing signals sent and/or received by the active communicator  1300 . The circuit manager  1361  may switch out the circuit  1309  to protect a current limited connection from having its current limit exceeded due to the current load of the circuit  1309 . Further, a user may interact with the UI  410  to turn off the power supply  1304  at any time. 
     Further, the circuit manager  1361  may compensate for signal measurement error caused by the fuse  1342 . That is, the circuit manager  1361  may account for the resistance of the fuse  1342  (which may be roughly 11 ohms in some cases) as well as the resistance of the terminators  1321  and  1322  when calculating current measurements based on voltage drops on the bus  1302  associated with communication signals. 
     The diagnostics manager  1362  may: (i) identify field devices, (ii) detect and analyze electrical characteristics of signals sent and/or received by the active communicator  1300 , (iii) log measurements and/or analysis performed over time, and/or (iv) perform a noise spectrum analysis. 
     First, the diagnostics manager  1362  may identify field devices connected to the bus  1302  (or connected to an external bus to which the active communicator  1300  is connected) by tag or device ID. The diagnostics manager  1362  may enable a user to create a user selectable device list that the user can select during creation of a log file to define file name, bus name, and/or location name (e.g., a user customizable string such as “storage tank  157 ”). 
     Second, the diagnostics manager  1362  may detect and analyze electrical characteristics of signals sent and/or received by the active communicator  1300 . These measurements may be displayed to a user via the display  122  shown in  FIGS. 1 and 4 . Further, the circuit manager  1361  may rely on these measurements to activate or deactivate components of the tool  100  (e.g., to protect the components and/or to ensure compliance with IS standards). 
     Third, the diagnostics manager  1362  may log the measurements and/or the analysis performed over time. For example, the diagnostics manager may create a health report representing “snapshots” taken over time. As an example, a user may utilize the tool  100  to measure signals associated with a given field device on a fairly regular basis (e.g., every day, once a week, etc.) The diagnostics manager  1362  may log these measurements, enabling plant personnel to identify trends over time associated with the field device. The health report may identify a bus by a tag or ID of the lowest address device on the bus. The health report may include information identifying the user of the tool  100  at the time relevant measurements were taken, the date of the relevant measurements, the bus or segment name where the measurements were taken, and/or the name of the location where the measurements were taken. The tool  100  may retrieve some of this information from the field device (e.g., the tag or segment name). 
     As another example of time-based signal analysis and logging, the diagnostics manager  1362  may create a troubleshooting log for continuously monitoring a field device&#39;s signals over a given time period. For example, a plant may be experiencing issues with a particular field device (e.g., communication disruptions), but may be unable to determine the cause of the issues. A user may connect the tool  100  to the field device and leave the tool  100  for an extended period of time (e.g., for a number of hours or days). The tool  100  may then measure and log electrical characteristics of transmitter and/or received signals on a regular interval and/or based on a trigger (e.g., signals dropping above or below a threshold). A user can later analyze the log to identify when the field device is suffering problems, and to determine what might correlate with the field device suffering problems. For example, by comparing the log to historian data collected by the process control system, the user may determine that disruptions experienced by the field device are associated with a start-up of a nearby motor, which is causing vibrations that disrupt the field device&#39;s communications. The troubleshooting log may include the same type of information as that included in the health report (e.g., a tag, time, date, etc.). 
     Fourth, the diagnostics manager  1362  may perform a noise spectrum analysis. In particular, the diagnostics manager  1362  may detect voltages at a small frequency range (e.g., less than 1 kHz) associated with noise and may display the detected voltages so that a user can identify one or more of: (i) a frequency of the noise; (ii) an amplitude of the noise (e.g., average or maximum); and/or (iii) a time at which a noise burst occurs. 
       FIG. 14A  is a schematic of the active communicator  1300  connected to a field device  1405  via an external bus  1420  (i.e., external to the active communicator  1300 ), demonstrating an example in which the active communicator  1300  is connected to a field device connected to an operational bus. 
     An external power supply  1410  (e.g., a rack-mounted power supply) may supply power to the external bus  1420 , and the field device  1405  may draw power from the external bus  1420 . The field device  1405  may draw a current of 10-25 mA for power. For example, the field device  1405  may draw a current of 20 mA, which may be supplied by the power supply  1410 . In some cases, multiple field devices may be connected to the bus  1420  and may draw power. For example, three field devices may draw 20 mA each from the bus  1420 . In such an example, the power supply  1410  may supply 60 mA of current to the bus  1420  for power. A power conditioner (not shown) may be placed between the power supply  1410  and the bus  1420 . 
     The field device  1405  may be connected to the Fieldbus segment  1420  via a Fieldbus spur. The spur may be a two-wire link that connects to the segment  1420  via a junction box, which may connect multiple other spurs to the segment  1420 . Due to the multiple links and connection points associated with the Fieldbus segment  1420 , it can be difficult to isolate a point of failure. Advantageously, the communicator  1300  can not only communicate with the field device  1405 , but can connect to the segment  1420  in place of the field device to “see” what the field device  1405  is “seeing.” In short, the tool  100  is a “known good device.” If a communication problem exists between a field device and a controller, the tool  100  can connect to and test the field device. If the field device functions properly when connected to the tool  100 , a user of the tool  100  knows the problem exists “upstream” of the field device (e.g., at a spur, junction box, I/O cards, or some other cable or device). Accordingly, the user can keep moving upstream and testing communications to isolate the “bad” device or communication link. 
     The power supply  1410  may supply power to the field device  1405  via the segment  1420 , as well as to potentially other field devices connected to the segment  1420 . 
     The segment  1420  may carry a 10-25 mA DC power signal for powering the field device  1405  and an AC digital communication signal utilized by the field device  1405 . Generally speaking, multiple other field devices may connect to the segment  1420 , each drawing a 10-25 mA DC signal for power. Accordingly, the DC current supplied to the segment  1405  by the power supply  1410  may vary depending on the number of field devices that connect to the segment  1420  and draw power. The segment  1420  may include terminators  1422  and  1424 . Each of the terminators  1422  and  1424  may include, e.g., a 100 ohm resistor in series with a 1 μF capacitor. Accordingly, the terminators  1422  and  1422  may block DC current and act as a current shunt for the AC communication signal. 
     In operation, the active communicator  1300  and field device  1405  may communicate by way of a digital signal (e.g., 20 mA peak-to-peak) superimposed on the power signal on the bus  1420 . Generally, only one device connected to the bus  1420  may communicate at any given time. For example, if the active communicator  1300 , field device  1405 , and two other field devices (not shown) share the bus  1420 , only one of the four connected devices may communicate at a given time. Communication on the bus  1420  may be coordinated by a device designated as a Link Active Scheduler (LAS). The LAS may be responsible for passing a token between devices connected to the bus  1420 , where only the device with the token may communicate over the bus  1420 . In some cases, the field device  1405  may be the LAS; in other cases, the tool  100  may be the LAS. 
     The LAS maintains a list of all devices needing to access the bus  1420 . This list may be called a “Live Device List.” In some circumstances, the bus  1420  may have only a single LAS. Devices capable of becoming a LAS may be called link master devices. All other devices may be referred to as “basic devices.” If desired, the tool  100  may be a link master device. When the active communicator  1300  is connected to the bus  1420 , the tool  100  may bid to become the LAS. The link master that wins the bid (e.g., the one with the lowest address) may begin operating as the LAS immediately upon completion of the bidding process. 
       FIG. 14B  is a schematic of the active communicator  1300  connected to a field device  1455  via the bus  1302 , demonstrating an example in which the active communicator  1300  provides power to the field device  1455 . A user may place a shunt  1460  between terminals  1331  and  1332 , enabling the power supply  1304  to supply power to the bus  1302 . The power supply  1302  may be current limited so that current flowing via the shunt  1460  remains under a maximum threshold consistent with IS standards. For example, the threshold may be between 35-45 mA. As noted, the DC current controller  1312  may draw 11 mA. Further, typical field devices may draw 10-25 mA. The power supply  1302  may be configured to supply up to 36 mA to ensure that the DC current controller  1312  and the field device  1455  receive sufficient current (i.e., based on an expected potential total current draw of 11 mA+25 mA) while remaining IS compliant. The power supply  1304  may be configured to supply a maximum current of 38 mA. A user may utilize the active communicator  1300  to supply power to the field device  1455 , to communicate with the field device  1455 , and/or to perform diagnostics on the field device  1455  while complying with IS standards. 
       FIG. 15  is a view of the communication interface  406  of the tool  100  from a perspective external to the tool  100 . As shown, the tool  100  includes the active communicator  600  and the active communicator  1300 , shown in  FIGS. 6 and 13 , respectively. In some cases, the tool  100  includes only one of the active communicator  600  and the active communicator  1300 . 
     The communication interface  406  may include the terminals  1331 - 1333  shown in  FIG. 13  and the terminals  631 - 636  shown in  FIG. 6 , which may be arranged as terminal sets  1501 - 1504 . 
     In operation, a user may connect the terminal set  1501  to a field device configured to communicate according to an AM communication scheme, such as the Fieldbus protocol. The user may utilize the terminal set  1501  when connecting the tool  100  to a field device relying on an external power supply (i.e., external relative to the tool  100 ), such as a rack-mounted power supply typically found in plant environments. If the user wishes to utilize the tool  100  to power the field device, the user may connect terminals  1331  and  1332  using a shunt. 
     As another example, a user may connect the terminal set  1502  to a field device that requires power and that is configured to communicate according to a DC current signaling scheme (e.g., 4-20 mA) and/or according to a digital FM communication scheme (e.g., the HART protocol) and that requires power. Alternatively, if the user wants to connect the tool  100  to a similarly configured field device that is already powered by an external power supply, the user may connect the field device to the terminal set  1503 . A user may also connect the terminal set  1502  to a field device, or to a communication link connected to the field device, to detect the electrical characteristics (e.g., current, voltage) of signals transmitted by the field device or received by the field device. 
     Finally, a user may connect the terminal set  1504  to a field device to detect the electrical characteristics (e.g., current, voltage) of signals transmitted by the field device or received by the field device. The user may also connect the terminal set  1504  to a communication link (e.g., a HART loop or Fieldbus segment) to detect the electrical characteristics of signals transmitted via the communication link. In some cases, the power monitor  408  connected to the terminal set  1504  does nothing but measure current. 
     As shown, the communication interface  406  enables the tool  100  to communicate with or diagnose field devices configured according to different protocols. Thus, a user may carry the tool  100  to service multiple different types of field devices rather than carrying multiple specialized tools.