ON-LINE SERIAL COMMUNICATION INTERFACES

Abstract: An on-line serial communication interface (22) effects interfacing between a transmitter (10) of a two-line current loop to a digital circuit, such as a computer or hand-held termi­ nal which receives and processes digital voltage pulses. The interface (22) includes a comparator (32) having a non-invert­ ing input connected by a capacitor (C1) to one (14) of the cur­ rent lines, and a inverting input connected to a source of small positive voltage. An output of the comparator (32) is connect­ ed to an input port of the digital device. The transmitter (10) modulates the current on the current loop by a small amount with respect to the analog current communicated by the trans­ mitter (10). This generates voltage pulses in the capacitor (C1) which are compared to the selected voltage and are amplified to form large voltage pulses used for digital communication with the digital circuit.

Description: This invention relates to on-line serial communication interfaces and to methods of establishing on-line serial communication. Two-wire analog transmission systems are known (see e.g. US-A-4520488). Such systems include a transmitter which is connected to a power supply by two wires which form a current loop. The transmitter includes, as at least one of its features, a transducer which senses a condition such as pressure or temperature. This condition is known as a process variable (PV). The power supply is connected to the two wires to close the current loop. It is also known to provide a resistor in the current loop. The transmitter amplifies the signal from its transducer and this amplified signal is used to draw a certain current from the power supply which is proportional or otherwise related to the process variable. It is also known to draw from a minimum of 4 mA to a maximum of 20 mA. The current between 4 and 20 mA passes through the resistor to produce a voltage drop across the resistor. This voltage drop can be measured to give a value for the process variable. It is noted that the 4 mA minimum current is required to energise the circuitry of the transmitter. Any excess current above this 4 mA level is taken as a value which can be used to determine the process variable. It is known that such 4-20 mA two-wire systems have an accuracy which is limited to around 0.1% at best. These systems are also essentially unidirectional with the transmitter being essentially uncontrolled and transmitting continuously. The transmitters in such circuits are generally limited in accuracy to about 0.1% and their functionality is limited to only continuous reading and sensing of the process variable. According to one aspect of the invention there is provided an on-line serial communication interface for communication between a transmitter of a current loop and a digital circuit for receiving voltage pulses, the transmitter being connected by lines of the current loop to a power supply to draw current from the power supply according to a process variable sensed by the transmitter and the transmitter being capable of modulating the current on the current loop by an amount which is small with respect to the current drain, the interface comprising: a resistor connected in one of the lines of the current loop for establishing a voltage drop on the one line which varies with the modulated current from the transmitter; a capacitor connected to the one line for receiving the varying voltage drop; and a comparator having an output and two inputs, one of the inputs being connected to the capacitor and the other of the inputs being connected to a selected small voltage, the comparator being operative to generate voltage pulses on an output thereof which are synchronised with the variations of the voltage drop on the one line, and the output of the comparator being connected to the digital circuit for applying the voltage pulses to the digital circuit. According to a second aspect of the present invention there is provided an on-line serial communication interface for a digital circuit such as a computer or hand-held terminal, which digital circuit receives voltage pulses, from a current loop having lines for connecting a transmitter to a power supply for drawing current from the power supply according to a process variable sensed by the transmitter and for modulating the current on the current loop, the interface comprising a resistor connected in series in one of the lines of the current loop for establishing a voltage drop on the one line which depends on current drawn by the transmitter, a capacitor connected to the one line for receiving the voltage drop on the one line, the voltage drop being modulated into voltage pulses when the transmitter modulates the current on the one line, and a comparator having an output connected to the digital circuit, and having two inputs, one of the inputs being connected to the capacitor for receiving the voltage pulses and the other input being connected to a selected constant voltage whereby the comparator outputs voltage pulses which are applied to the digital circuit. According to a third aspect of the invention there is provided a method of establishing on-line serial communication from a terminal of a current loop to a digital circuit for receiving voltage pulses, the terminal being of the type for drawing a current within a selected range corresponding to a process variable sensed by the terminal, the current being drawn on the current loop, the method comprising: modulating the current drawn by the terminal by an amount which is small with respect to the selected current range; passing the modulated current through a resistor for establishing a modulated voltage on one line of the current loop; comparing the modulated voltage to a fixed small voltage compared to the modulated voltage for generating voltage pulses; amplifying the voltage pulses; and applying the voltage pulses to the digital circuit. A preferred embodiment of the present invention described hereinbelow utilises microprocessor technology to improve the overall accuracy and expand the functionality of transmitter devices. The preferred embodiment provides an apparatus for interfacing a computer or hand-held terminal with a current loop for digital communication from a two-wire current loop transmitter while the transmitter is still on-line (sending analog information) to a controller or some other monitoring device. The preferred embodiment provides a serial communication interface between a current loop and a digital circuit which is simple in design, rugged in construction and economical to manufacture. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawing, in which: Figure 1 is a block diagram showing a known current loop having a communication device such as a computer or hand-held terminal connected to the current loop; and Figure 2 is a schematic diagram of an on-line serial communication interface embodying the invention connected between the communication device and the current loop. The drawing shows an on-line serial communication interface 22 between a digital circuit, shown in Figure 1 as a communication device 24 (such as an IBM PC, a Radio Shack Model 100 or a Termiflex hand held Terminal) and a current loop formed by a Bailey Controls Company Smart transmitter 10, lines 12,14 and any 12 to 48 Vdc power supply 16. As is known, one of the lines, for example the line 14, may include a resistor R0, which has a voltage drop thereacross proportional to a current flowing in the lines 12,14. The transmitter 10 may include a transducer such as a pressure or temperature transducer (not shown) which receives process variable PV. The transducer may be connected to a microprocessor in the transmitter 10 which controls the amount of current to be drawn from the power supply 16 on the lines 12 and 14. The voltage drop across the resistor R0 is measured by an analog-to-digital converter 18. This voltage drop can be displayed on a display unit 20 as a measurement of the process variable PV. The on-line serial communication interface 22 is connected to the current loop line 14 by connecting lines 26 and 28. The communicating or communication device 24 is connected to the serial interface 22 by a line 30. The communicating device 24 is a digital circuit such as a computer, microprocessor, or hand-held terminal. The communicating device 24 receives digital information in the form of voltage pulses on the line 30 for establishing digital communication with the current loop. The communicating device 24 may be an RS-232C device which receives and/or sends a digital signal one bit at a time. A logic high is between +3 and +12 volts, a logic low is between -3 and -12 volts. Figure 2 illustrates the on-line serial communication interface 22 in more detail. When the microprocessor or other circuitry of the transmitter 10 is to communicate with the communicating device 24 shown in Figure 1, the transmitter modulates the current on the line 14 by about 1 mA. Depending on the value of a resistor R1, or any other resistor on the line 14, this modulation of current produces a voltage modulation of about 80 mV. This is in the form of voltage pulses which appear at a line 28 connected to the line 14. A capacitor C1 is connected to the line 28 for receiving the voltage pulses. The voltage pulses are applied to a non-inverting or positive terminal of a comparator or differential amplifier 32. An output of the amplifier 32 is connected to the communicating device 24, for example over the line 30. As noted above, the communicating device 24 may be in the form of a computer or hand-held terminal and is structured to receive and process large voltage pulses for RS-232C communication. An inverting or negative terminal of the comparator 32 is held slightly positive by a resistive divider circuit made up of resistors R2 and R3. This divider circuit R2,R3 is connected to a source of constant voltage V+. This prevents any noise across the resistor R1 from tripping the comparator 32. It also holds the output of the comparator 32, at the line 30, at earth potential, which is necessary for idle operation of the RS-232C device 24. The comparator 32 converts the 80 mV pulses into large voltage pulses needed for communication with the communication device (digital circuit) 24. A point 34 is also connected to earth via a resistor R4 for proper functioning of the comparator 32. A major advantage of the embodiment of the present invention described above is that communication can be achieved while the transmitter is still on-line with a controller. This is possible because there is little effect on the current in the loop. Communication is effected by modulating the current in the loop by a small amount. That is, the current modulation is small with respect to the 4-20 mA current needed for analog communication from the transmitter 10. Another advantage is that the interface is powered by using a request-to-send line from an RS-232C port of the operator's communicating device 24. No extra power supply is necessary.

Claims: An on-line serial communication interface for communication between a transmitter (10) of a current loop and a digital circuit (24) for receiving voltage pulses, the transmitter (10) being connected by lines (12,14) of the current loop to a power supply (16) to draw current from the power supply (16) according to a process variable (PV) sensed by the transmitter (10) and the transmitter (10) being capable of modulating the current on the current loop by an amount which is small with respect to the current drain, the interface (22) comprising: a resistor (R1) connected in one of the lines (14) of the current loop for establishing a voltage drop on the one line (14) which varies with the modulated current from the transmitter (10); a capacitor (C1) connected to the one line (14) for receiving the varying voltage drop; and a comparator (32) having an output and two inputs, one of the inputs being connected to the capacitor (C1) and the other of the inputs being connected to a selected small voltage, the comparator (32) being operative to generate voltage pulses on an output (30) thereof which are synchronised with the variations of the voltage drop on the one line (14), and the output of the comparator (32) being connected to the digital circuit (24) for applying the voltage pulses to the digital circuit. An interface according to claim 1, for a said digital circuit (24) which comprises an RS-232C computer. An interface according to claim 1 or claim 2, wherein the comparator (32) comprises a differential amplifier for amplifying the variations in voltage drop to produce amplified voltage pulses. An interface according to claim 1, claim 2 or claim 3, for a said current loop which is a 4-20 mA current loop and the transmitter (10) of which is operative to modulate the current by 1 mA, the resistor (R1) being selected so that the voltage drop for a current modulation of 1 mA equals about 80 mV. An interface according to claim 4, including a voltage divider (R2,R3) connected between a voltage source and the other input of the comparator (32). A method of establishing on-line serial communication from a terminal (10) of a current loop to a digital circuit (24) for receiving voltage pulses, the terminal (10) being of the type for drawing a current within a selected range corresponding to a process variable (PV) sensed by the terminal (10), the current being drawn on the current loop, the method comprising: modulating the current drawn by the terminal (10) by an amount which is small with respect to the selected current range; passing the modulated current through a resistor (R1) for establishing a modulated voltage on one line (14) of the current loop; comparing the modulated voltage to a fixed small voltage compared to the modulated voltage for generating voltage pulses; amplifying the voltage pulses; and applying the voltage pulses to the digital circuit (24). A method according to claim 6, wherein the range for current drawn by the terminal is from 4 to 20 mA, the transmitter modulates the current by 1 mA, and the resistor (R1) is selected to establish a maximum voltage drop of about 80 mV. A method according to claim 7, including amplifying the voltage pulses to produce an RS-232C digital pulse signal.

Inventors: STERLING, EDWARD LEE, JR., THOMPSON, WILLIAM LEE
Applicants: INTERNATIONAL CONTROL AUTOMATION FINANCE S.A.
Application year: 1986
Main classification: G08C19
IPC codes: H04B3, G08C19, G06F13