# US 2018/0026809 A1 · Portable field maintenance tool with resistor network for intrinsically safe operation > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2018/0026809 A1 - Document ID: US20180026809A1 - Status: Published application - Kind: A1 - Application number: 15/454,725 - Filed: 2017-03-09 - Published: 2018-01-25 - Earliest claimed: 2016-07-25 - Assignees: Fisher-Rosemount Systems, Inc. - Applicants: Fisher-Rosemount Systems, Inc. - Inventors: Roger Benson; Mehul Rajeshbhai Dalal; Anthony Ferguson; Todd M. Toepke - Web page: https://patentgrep.com/patent/US20180026809A1 ## CPC - H04L 12/40039: {Details regarding the setting of the power status of a node according to activity on the bus} - H04B 3/548: {the power on the line being DC (arrangements for feeding power H04L12/10; extracting feeding power from signals H04L25/02)} - H04B 1/3827: Portable transceivers - H04L 2012/4026: {Bus for use in automation systems} - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) - H04B: TRANSMISSION ## Abstract A portable field maintenance tool may perform one or more tasks, such as communicating with a field device, powering a field device, diagnosing a field device, or diagnosing a communication link in a plant environment to which a field device is connected. The portable field maintenance tool may interact with field devices configured according to a number of different communication protocols, such as the HART protocol and the Fieldbus protocol. The portable field maintenance tool may be energy limited and fault tolerant, and may operate in compliance with Intrinsic Safety standards, enabling use of the portable field maintenance tool in hazardous areas. ## Claims ### Claim 1 A portable field maintenance tool comprising: (A) a housing; (B) a communication interface that is disposed through the housing, the communication interface including an internal portion accessible within the housing and a set of terminals accessible outside the housing, the set of terminals 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; (C) a communication circuit disposed within the housing and electrically connected to the internal portion of the communication interface, the communication circuit configured to encode or decode the communication signal; and (D) a power supply disposed within the housing and electrically connected to the internal portion of the communication interface and to the communication circuit, the power supply configured to transmit the power signal; wherein the communication circuit includes a resistor network having a resistance within a range to cause a voltage drop, at the set of terminals, associated with the composite signal, the voltage drop being: (i) above a minimum voltage threshold associated with reading the composite signal, and (ii) below a maximum voltage threshold. ### Claim 2 (depends on 1) The portable field maintenance tool of claim 1, wherein the communication circuit and the power supply are each configured for intrinsically safe operation. ### Claim 3 (depends on 1) The portable field maintenance tool of claim 1, wherein the communication circuit includes a DC current controller and a digital frequency modulation (FM) modem, and wherein the composite signal includes: (i) an analog DC signal that includes the power signal and that varies in amplitude to convey information; and (ii) a digital FM communication signal superimposed on the analog DC signal. ### Claim 4 (depends on 1) The portable field maintenance tool of claim 1, wherein the minimum voltage threshold is a minimum peak-to-peak voltage associated with reading the communication signal. ### Claim 5 (depends on 1) The portable field maintenance tool of claim 1, wherein the maximum voltage threshold is below a voltage sufficient to generate a spark at the set of terminals. ### Claim 6 (depends on 1) The portable field maintenance tool of claim 1, wherein the power supply is configured to transmit the power signal at a voltage that is at or below the maximum voltage threshold. ### Claim 7 (depends on 6) The portable field maintenance tool of claim 6, wherein the maximum voltage threshold is a value between 10 V and 30 V. ### Claim 8 (depends on 7) The portable field maintenance tool of claim 7, wherein the maximum voltage threshold is a value between 21 V and 24 V. ### Claim 9 (depends on 1) The portable field maintenance tool of claim 1, wherein the minimum voltage threshold is a value between 50 mV peak-to-peak and 500 mV peak-to-peak. ### Claim 10 (depends on 1) The portable field maintenance tool of claim 1, wherein the minimum voltage threshold is a value of 120 mV peak-to-peak. ### Claim 11 (depends on 1) The portable field maintenance tool of claim 1, wherein the resistor network includes one or more resistors exceeding 2 mm in length and exceeding 2 mm in width so that the one or more resistors have sufficient surface area to protect against a temperature spike above a temperature threshold. ### Claim 12 (depends on 1) The portable field maintenance tool of claim 1, wherein the range is 75 Ohms to 750 ohms. ### Claim 13 (depends on 1) The portable field maintenance tool of claim 1, wherein the resistor network includes a plurality of resistors. ### Claim 14 (depends on 13) The portable field maintenance tool of claim 13, wherein the plurality of resistors are arranged such that the resistor network maintains the resistance within the range when any one of the plurality of resistors fails. ### Claim 15 (depends on 13) The portable field maintenance tool of claim 13, wherein the resistor network includes a plurality of switches, each of the plurality of switches in series with a one of the plurality of resistors and actuatable to: (i) remove the one of the plurality of resistors from the resistor network, or (ii) add the one of the plurality of resistors to the resistor network. ### Claim 16 (depends on 15) The portable field maintenance tool of claim 15, wherein each of the plurality of switches is a solid state relay. ### Claim 17 (depends on 15) The portable field maintenance tool of claim 15, wherein a resistance of each of the plurality of resistors is selected so that each of the plurality of switches is actuatable to adjust the resistance of the resistor network to a value within a range of 75 ohms and 750 ohms. ### Claim 18 (depends on 1) The portable field maintenance tool of claim 1, wherein the resistor network includes a first resistor sub-network arranged in parallel with a second resistor sub-network. ### Claim 19 (depends on 18) The portable field maintenance tool of claim 18, wherein the resistor network further includes a third resistor sub-network arranged in parallel with the first resistor sub-network and the second resistor sub-network. ### Claim 20 (depends on 19) The portable field maintenance tool of claim 19, wherein: the first resistor sub-network has a resistance between 200 ohms and 300 ohms; the second resistor sub-network has a resistance between 400 ohms and 600 ohms; and the third resistor sub-network has a resistance between 700 ohms and 800 ohms. ### Claim 21 (depends on 1) The portable field maintenance tool of claim 1, further comprising a fuse electrically connected to the communication interface, the fuse configured to limit a current at the communication interface to below a current threshold. ### Claim 22 (depends on 21) The portable field maintenance tool of claim 21, wherein the current threshold is a value between 10 mA and 150 mA. ### Claim 23 (depends on 21) The portable field maintenance tool of claim 21, wherein the current threshold is 50 mA. ### Claim 24 (depends on 1) The portable field maintenance tool of claim 1, wherein the set of terminals includes: (i) a positive terminal connectable to a first wire of the wired link, and (ii) a negative terminal connectable to a second wire of the wired link. ### Claim 25 (depends on 1) The portable field maintenance tool of claim 1, wherein the wired link is connected to the field device via a second wired link connected to the field device. ### Claim 26 A method of communicating with a transmitter field device, the method comprising: 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 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. ### Claim 27 (depends on 26) The method of claim 26, wherein the communication signal is an analog DC signal that varies in amplitude to convey information and that is superimposed on the supplied power. ### Claim 28 (depends on 26) The method of claim 26, wherein activating or deactivating the one or more resistors of the resistor network disposed within the portable field maintenance tool so that the voltage drop remains above the minimum voltage threshold comprises: activating or deactivating the one or more resistors of the resistor network so that the voltage drop exceeds a minimum peak-to-peak voltage associated with reading the communication signal. ### Claim 29 (depends on 28) The method of claim 28, wherein the minimum peak-to-peak voltage is a value between 100 mV peak-to-peak and 250 mV peak-to-peak. ### Claim 30 (depends on 28) The method of claim 28, wherein activating or deactivating the one or more resistors of the resistor network comprises: activating or deactivating one or more switches, each arranged in series with a one of the one or more resistors. ### Claim 31 A method of communicating with an actuator field device comprising: 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 transmitting by the portable field maintenance tool, via the wired link, to the actuator field device a communication signal superimposed on the supplied power. ### Claim 32 (depends on 31) The method of claim 31, wherein limiting the supplied power so that the set of terminals do not exceed a maximum electrical threshold comprises: limiting the supplied power so that a voltage drop at the set of terminals does not exceed a maximum voltage threshold, wherein the maximum voltage threshold is any value between 21 V and 24 V. ### Claim 33 (depends on 31) The method of claim 31, wherein limiting the supplied power so that the set of terminals do not exceed a maximum electrical threshold comprises: limiting the supplied power so that power available at the set of terminals does not exceed a maximum power threshold, wherein the maximum power threshold is any value between 0.25 W and 1.5 W. ### Claim 34 (depends on 31) The method of claim 31, wherein limiting the supplied power so that the set of terminals do not exceed a maximum electrical threshold comprises: 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. ### Claim 35 (depends on 31) The method of claim 31, wherein limiting the supplied power so that the set of terminals do not exceed a maximum electrical threshold comprises: disabling the portable field maintenance tool when a voltage at the set of terminals exceeds a maximum voltage threshold; or disabling the portable field maintenance tool when a current at the set of terminals exceeds a maximum current threshold. ### Claim 36 A portable field maintenance tool comprising: a set 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 set of terminals, that receives or transmits the signal via the set of terminals; and an energy measurement circuit, electrically connected to the set of terminals, that measures one or more electrical characteristics of the signal at the set of terminals. ### Claim 37 (depends on 36) The portable field maintenance tool of claim 36, further comprising: a resistor network; and a control unit, communicatively coupled to the energy measurement circuit, that activates or deactivates one or more resistors of the resistor network based on the measured one or more electrical characteristics. ### Claim 38 (depends on 36) The portable field maintenance tool of claim 36, further comprising a power supply that supplies power via the wired link. ### Claim 39 (depends on 38) The portable field maintenance tool of claim 38, further comprising a control unit, communicatively coupled to the energy measurement circuit and to the power supply, that controls the power supply based on the measured one or more electrical characteristics. ### Claim 40 (depends on 39) The portable field maintenance tool of claim 39, wherein the control unit controls the power supply to prevent the set of terminals from exceeding a maximum power threshold by reducing a supplied voltage in order to prevent the set of terminals from exceeding the maximum power threshold. ### Claim 41 (depends on 36) The portable field maintenance tool of claim 36, wherein the signal is a composite signal including a communication signal and a power signal. ### Claim 42 (depends on 38) The portable field maintenance tool of claim 38, wherein the communication signal is a digital FM communication signal; and wherein the communication circuit includes an FM modem that transmits or receives the digital FM communication signals. ### Claim 43 A method of communicating with a field device and monitoring signals sent or received by the field device, the method comprising: electrically connecting, via a wired link, a field device to a set of terminals of a portable field maintenance tool; transmitting or receiving, at the set of terminals of the portable field maintenance tool, a signal to or from the field device; and measuring, at the set of terminals, one or more electrical characteristics of the transmitted or received signal. ### Claim 44 (depends on 43) The method of claim 43, wherein the measuring occurs simultaneously with the transmitting or receiving. ### Claim 45 (depends on 43) The method of claim 43, further comprising: maintaining a voltage drop at the set 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. ### Claim 46 (depends on 43) The method of claim 43, further comprising 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. ### Claim 47 (depends on 46) The method of claim 46, wherein the maximum electrical threshold is a maximum power threshold or a maximum current threshold, and wherein the minimum electrical threshold is a minimum voltage threshold or a minimum current threshold. ### Claim 48 (depends on 43) The method of claim 43, further comprising: supplying power from the portable field maintenance tool, via the wired link, to the field device. ### Claim 49 (depends on 48) The method of claim 48, further comprising: adjusting the supplied power to prevent the set of terminals from exceeding a maximum electrical threshold, wherein the maximum electrical threshold is a maximum power threshold between 0.25 W and 1.5 W. ### Claim 50 (depends on 48) The method of claim 48, further comprising: 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. ### Claim 51 (depends on 43) The method of claim 43, further comprising: 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 activating the internal loop resistor when the analysis reveals that the field device is not connected to an external loop resistor. ### Claim 52 (depends on 43) The method of claim 43, further comprising: detecting voltage decay at the set of terminals; and enabling activation of a power supply based on the detected voltage decay.