Method for automatic switching of a communication resistor of a hart device in or out

Abstract

Method for automatic switching of a communication resistor of a HART device in or out, wherein the method comprises steps as follows: transmitting a test signal on an electrical current loop; reading from the electrical current loop a test voltage signal, which is based on the transmitted test signal; comparing the test voltage signal with a reference signal; connecting the communication resistor into the electrical current loop when the test voltage signal exceeds the reference signal.

Claims

1. A method for an automatic switching of a communication resistor of a HART device in or out, comprising: providing a HART device, including: a first interface configured to connect with a service unit; a second interface configured to connect with an electrical current loop; a microprocessor; a HART modem having a carrier detect signal, wherein the HART modem is connected to the microprocessor, including the carrier detect signal connected with the microprocessor; a communication resistor; a switch controlled by the microprocessor, wherein the switch is configured in a first state to connect the communication resistor with the second interface, and wherein the switch is configured in a second state to disconnect the communication resistor from the second interface; a filter element connected to the second interface and configured to filter out disturbance signals from a test voltage signal received via the second interface; and a comparator connected to an output of the filter element, wherein the comparator is configured to compare the filtered test voltage signal with a reference signal, and wherein an output of the comparator is connected to the microprocessor, wherein the microprocessor is configured to transmit a test signal on the electrical current loop, and to set the switch to the first state or to the second state based on the comparison of the filtered test voltage signal with the reference signal; connecting the HART device to an electrical current loop; transmitting the test signal on the electrical current loop; reading from the electrical current loop, via the second interface, the test voltage signal that is based on the transmitted test signal; filtering, via the filter element, disturbance signals from the test voltage signal; comparing, via the comparator, the filtered test voltage signal with a reference signal; and connecting the communication resistor into the electrical current loop or disconnecting the communication resistor from the electrical current loop based on the comparison of the filtered test voltage signal with the reference signal.

2. The method as claimed in claim 1, wherein a preamble within a HART communication is used as the test signal.

3. The method as claimed in claim 1, wherein the test voltage signal is filtered through a bandpass filter with a center frequency of about 1200 Hz.

4. The method as claimed in claim 1, wherein the test voltage signal is filtered through a lowpass filter with a limit frequency of about 1200 Hz.

5. The method as claimed in claim 1, wherein the comparing of the filtered test voltage signal with the reference signal occurs over a predefined duration and only when the filtered test voltage signal exceeds the reference signal for the predefined duration is the communication resistor connected.

6. The method as claimed in claim 1, further comprising: disconnecting the communication resistor from the electrical current loop before the transmitting of a test signal.

7. The method as claimed in claim 1, further comprising after the connecting of the communication resistor: verifying a carrier detect signal indicates a valid carrier signal; and if the carrier detect signal indicates no valid carrier signal, disconnecting the communication resistor from the electrical current loop.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows:

(2) FIG. 1 shows a schematic representation of a HART communication box known from the state of the art and connected to an electrical current loop, or a control loop, with a communication resistor,

(3) FIG. 2 shows a schematic representation of the HART communication box, which is connected via a communication resistor to the electrical current loop, or the control loop, and

(4) FIG. 3 shows a schematic representation a HART device of the invention in the form of a HART communication box with a communication electronics, which is designed to execute the method of the invention.

DETAILED DESCRIPTION

(5) FIGS. 1 and 2 show the two connection variants known from the state of the art of a HART communication box 1 connected to an electrical current loop, or to a control loop, 3. In such case, the HART communication box 1 has been selected for purposes of explanation. The invention, is, however, in principle, applicable for any HART device.

(6) FIGS. 1 and 2 show further, schematically, an installation in principle, in the case of which at least one field device 4 is connected via an electrical current loop 3 with at least one superordinated unit, for example, a control system or, such as shown in FIG. 1, a service unit 2.

(7) In FIG. 1, the communication resistor 6 required for conforming HART communication is located within the HART communication box and not separately shown.

(8) In contrast therewith, FIG. 2 shows an installation, in the case of which the required communication resistor 6, for example, one manually introduced by a service technician, is already present in the electrical current loop 3. In this case, the voltage signal is tapped across this resistor by the HART communication box 1.

(9) Both the HART communication box 1 shown in FIG. 1 as well as also that shown in FIG. 2 have a first interface 15, which is embodied, for example, according to the USB-standard, and a second interface 16 in the form of connection terminals.

(10) The first interface 15 connects the HART communication box 1 with the service unit 2, which serves for configuration and/or parametering of a field device 4. The USB connection shown in FIG. 1 is only by way of example, and other forms of communication connection, e.g. RS232, Bluetooth, etc. likewise provide options.

(11) The HART communication box 1 is connected to the electrical current loop 3 via the second interface 16. Typically for this, the second interface 16 is implemented in the form of connection terminals.

(12) Via the two interfaces 15 and 16, the HART communication box 1 connects the portable computer unit, e.g. service unit 2, with the electrical current loop 3 data conductively, so that via the service unit 2, on which a correspondingly provided service- and/or observation software is running, and the electrical current loop 3, it is possible to communicate with a field device 4 connected to the electrical current loop 3.

(13) FIG. 3 shows a schematic representation of a HART device of the invention, i.e. a HART communication box 1 with a communication electronics located within the communication box 1. The communication electronics includes a computing unit, for example, a microprocessor 13 and a HART modem 9, which together implement the transfer of the data, especially configuration data and/or parametering data, between the first interface 15 and the second interface 16. In this way, configuration data and/or parametering data is transferred between the first interface 15, which is embodied according to the USB standard, and the second interface 16, which is embodied according to the HART standard.

(14) In order to be able to practice the method of the invention described below, the communication electronics further includes a communication resistor, which can be automatically connected in or out via a switch operated by the microprocessor, and a comparator. Additionally, the communication electronics includes a filter element, for example, a bandpass filter- or lowpass filter. The filter element serves to filter out disturbance signals, which are produced, for example, by external EMC influences and, thus, to prevent an unintended influencing of the signal of interest. In principle, the invention can, however, also be practiced without the filter element.

(15) The method of the invention assumes that one starts with the HART communication box 1 in the high-impedance state, i.e. the communication resistor 6 is normally switched out. In this way, only an erroneous scenario must be recognized: the defective connection of two HI devices, thus, for example, a HI field device and the HART communication box without a communication resistor switched in, i.e. with communication resistor switched out. The connection of two HI devices has the result that the signal level of the HART signal is too high. This is recognizable by analysis of peak values. In order to perform this analysis, a test signal is transmitted from the HART communication box 1 via the connection terminals and then a test voltage signal, which is based on the transmitted test signal, is read back. Opportunely, the preamble is used, which is composed of three or more transmitted 0xFF hexadecimals, which serve, in this case, both for synchronization as well as also test signal.

(16) In order that an unintended, or erroneous, switching in of the communication resistor 6 is prevented as much as possible, it is advantageous to filter the test voltage signal (typically 1200 Hz signals) coming from the connection terminals of the HART communication box 1. This filtered signal is then fed to the comparator 12.

(17) The comparator 12 serves for comparing the test voltage signal with a reference signal, or with a reference level. In the case, in which the reference signal, e.g. the reference level, is exceeded, the microprocessor connected to the comparator can use this as switching signal for the switching in of the communication resistor via the switch. Typically, the switching in of the communication resistor by the microprocessor is only performed after the reference level has been exceeded for a predefined duration. In this way, disturbing influences (EMC, etc.) can be minimized.

(18) In order to enable a verification of the performed measure, advantageously a carrier detect signal is used, which is output from the HART modem 9. The carrier detect signal is a control signal, which is produced according to standard by the HART modem 9 and displays that it can receive data. Typically, this signal can only assume the logical states high or low, respectively a binary one or a binary zero.

(19) In the case, in which a valid carrier detect signal is present, it can be assumed therefrom that the signal received via the connection terminals is free of error. Thus, when the carrier detect signal assumes a logical binary one, or high, the measure, thus the switching in of the communication resistor 6, is deemed successful. In contrast is the case, in which the carrier detect signal assumes a logical binary zero, whereupon the measure is deemed not successful and the switched in communication resistor 6 is switched back out.

LIST OF REFERENCE CHARACTERS

(20) 1 HART device, or HART communication box 2 service unit 3 electrical current loop, or control loop 4 field device 5 measurement transmitter, power supply 6 communication resistor, or load 7 connection terminals 8 switch 9 HART modem 10 filter 11 reference signal 12 comparator 13 computing unit, microcontroller 14 carrier detect signal 15 first interface 16 second interface