Current loop
Encyclopedia
A current loop describes two different electrical signalling schemes.
For digital
serial communications
, a current loop is a communication interface that uses current instead of voltage
for signaling. Current loops can be used over moderately long distances (tens of kilometres), and can be interfaced with optically isolated
links.
Long before the RS-232
standard, current loops were used to send digital data in serial
form for teleprinter
s. More than two teleprinters could be connected on a single circuit allowing a simple form of networking. Older teleprinters used a 60 mA
current loop. Later machines, such as the Teletype Model 33 ASR
, operated on a lower 20 mA current level and most early minicomputer
s featured a 20 mA current loop interface, with an RS-232 port generally available as a more expensive option. The original IBM PC
serial port
card had provisions for a 20 mA current loop. A digital current loop uses the absence of current for high (space or break), and the presence of current in the loop for low (mark).
The maximum resistance
for a current loop is limited by the available voltage. Current loop interfaces usually use voltages much higher than those found on an RS-232
interface, and cannot be interconnected with voltage-type inputs without some form of level translator circuit.
MIDI (Musical Instrument Digital Interface
) is a digital current loop interface.
Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbus
es, requiring only a handheld digital multimeter
in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.
Additional digital communication to the device can be added to current loop using HART Protocol
. Digital process buses such as FOUNDATION Fieldbus
and Profibus
may replace analog current loops.
instruments, analog
4–20 mA and 10–50 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance
in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure
, temperature
, flow
, pH
or other process variables. A current loop can also be used to control a valve positioner or other output actuator
. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several instruments in series.
Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder
may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. (A 4-wire instrument has a power supply input separate from the current loop.) Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device).
Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).
The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum.
Basic DC circuit theory shows that the current is the same all along the line. It was common to see 4–20 mA circuits that had loop lengths in miles or circuits working over telephone cable pairs that were longer than ten thousand feet end-to-end. There are still legacy systems in place using this technology. In Bell System
circuits, voltages up to 125 VDC were employed.
remote control consoles, can reverse the polarity of currents and can multiplex audio onto a DC current.
These devices can be employed for any remote control need a designer might imagine. For example, a current loop could actuate an evacuation siren
or command synchronized traffic signals.
Current loop circuits are one possible way used to control radio base station
s at distant sites. The two-way radio industry calls this type of remote control DC remote. This name comes from the need for DC circuit continuity between the control point and the radio base station
. The purpose current loop remote control is to save the cost of extra pairs of wires between the operating point and the radio transceiver. Some equipment, such as the Motorola MSF-5000 base station, uses currents below 4 mA for some functions. An alternative type, the Tone remote
, is more complex but requires only an audio path between control point and base station. To confirm the use of the phrase "DC remote" as accurate in describing this type of device, please see . The patent does not describe this tone remote but confirms the use of the phrase to describe this system of signaling.
For example, a taxi
dispatch
base station might be physically located on the rooftop of an eight-story building. The taxi company office might be in the basement of a different building nearby. The office would have a remote control unit that would operate the taxi company base station over a current loop circuit. The circuit would normally be over a telephone line or similar wiring. Control function currents come from the remote control console at the dispatch office end of a circuit. In two-way radio use, an idle circuit would normally have no current present.
In two-way radio
use, radio manufacturers use different currents for specific functions. Polarities are changed to get more possible functions over a single circuit. For example, imagine one possible scheme where the presence of these currents cause the base station to change state:
Note that this circuit is polarity-sensitive. If a telephone company cable splicer accidentally reversed the conductors, selecting channel 2 would lock the transmitter on.
Each current level could close a set of contacts, or operate solid-state logic, at the other end of the circuit. That contact closure caused a change of state on the controlled device. Some remote control equipment could have options set to allow compatibility between manufacturers. That is, a base station that was configured to transmit with a +18 mA current could have options changed to (instead) make it transmit when +6 mA was present.
In two-way radio use, AC signals were also present on the circuit pair. If the base station were idle, receive audio would be sent over the line from the base station to the dispatch office. In the presence of a transmit command current, the remote control console would send audio to be transmitted. The voice of the user in the dispatch office would be superimposed over the DC current that caused the transmitter to operate.
Digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
serial communications
Serial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
, a current loop is a communication interface that uses current instead of voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
for signaling. Current loops can be used over moderately long distances (tens of kilometres), and can be interfaced with optically isolated
Opto-isolator
In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is "an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output"...
links.
Long before the RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
standard, current loops were used to send digital data in serial
Asynchronous start-stop
Asynchronous serial communication describes an asynchronous, serial transmission protocol in which a start signal is sent prior to each byte, character or code word and a stop signal is sent after each code word...
form for teleprinter
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
s. More than two teleprinters could be connected on a single circuit allowing a simple form of networking. Older teleprinters used a 60 mA
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
current loop. Later machines, such as the Teletype Model 33 ASR
ASR33
The Teletype Model ASR-33 was a very popular model of teleprinter. Introduced about 1963 by Teletype Corporation and designed for light-duty office use, it was less rugged and less expensive than earlier Teletype machines or its heavy-duty cousin, the Model 35-ASR.The Model 33's printing mechanism...
, operated on a lower 20 mA current level and most early minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
s featured a 20 mA current loop interface, with an RS-232 port generally available as a more expensive option. The original IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
card had provisions for a 20 mA current loop. A digital current loop uses the absence of current for high (space or break), and the presence of current in the loop for low (mark).
The maximum resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
for a current loop is limited by the available voltage. Current loop interfaces usually use voltages much higher than those found on an RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
interface, and cannot be interconnected with voltage-type inputs without some form of level translator circuit.
MIDI (Musical Instrument Digital Interface
Musical Instrument Digital Interface
MIDI is an industry-standard protocol, first defined in 1982 by Gordon Hall, that enables electronic musical instruments , computers and other electronic equipment to communicate and synchronize with each other...
) is a digital current loop interface.
Analog
Analog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbus
Fieldbus
Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158....
es, requiring only a handheld digital multimeter
Multimeter
A multimeter or a multitester, also known as a VOM , is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance...
in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.
Additional digital communication to the device can be added to current loop using HART Protocol
HART Protocol
The HART Communications Protocol is an early implementation of Fieldbus, a digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4-20 mA analog instrumentation wiring, sharing the pair of wires used by the older system...
. Digital process buses such as FOUNDATION Fieldbus
FOUNDATION fieldbus
Foundation Fieldbus is an all-digital, serial, two-way communications system that serves as the base-level network in a plant or factory automation environment. It is an open architecture, developed and administered by the Fieldbus Foundation....
and Profibus
Profibus
PROFIBUS is a standard for field bus communication in automation technology and was first promoted in 1989 by BMBF...
may replace analog current loops.
Process-control use
For industrial process controlProcess control
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range...
instruments, analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
4–20 mA and 10–50 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, flow
Volumetric flow rate
The volumetric flow rate in fluid dynamics and hydrometry, is the volume of fluid which passes through a given surface per unit time...
, pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
or other process variables. A current loop can also be used to control a valve positioner or other output actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several instruments in series.
Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder
Chart recorder
A chart recorder is an electromechanical device that records an electrical or mechanical input trend onto a piece of paper . Chart recorders may record several inputs using different color pens and may record onto strip charts or circular charts...
may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. (A 4-wire instrument has a power supply input separate from the current loop.) Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device).
Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).
The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum.
Long circuits
Analog current loops were occasionally carried between buildings by dry pairs in telephone cables leased from the local telephone company. 4–20 mA loops were more common in the days of analog telephony. These circuits require end-to-end direct current (DC) continuity. DC continuity is not available over a microwave radio, optical fiber, or a multiplexed telephone circuit connection.Basic DC circuit theory shows that the current is the same all along the line. It was common to see 4–20 mA circuits that had loop lengths in miles or circuits working over telephone cable pairs that were longer than ten thousand feet end-to-end. There are still legacy systems in place using this technology. In Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...
circuits, voltages up to 125 VDC were employed.
Discrete control
Discrete control functions can be represented by discrete levels of current sent over a loop. This would allow multiple control functions to be operated over a single pair of wires. Currents required for a specific function vary from one application or manufacturer to another. There is no specific current that is tied to a single meaning. It is almost universal that 0 mA indicates the circuit has failed. In the case of a fire alarm, 6 mA could be normal, 15 mA could mean a fire has been detected, and 0 mA would produce a trouble indication, telling the monitoring site the alarm circuit had failed. Some devices, such as two-way radioTwo-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
remote control consoles, can reverse the polarity of currents and can multiplex audio onto a DC current.
These devices can be employed for any remote control need a designer might imagine. For example, a current loop could actuate an evacuation siren
Siren (noisemaker)
A siren is a loud noise making device. Most modern ones are civil defense or air raid sirens, tornado sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types: pneumatic and electronic....
or command synchronized traffic signals.
Two-way radio use
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
s at distant sites. The two-way radio industry calls this type of remote control DC remote. This name comes from the need for DC circuit continuity between the control point and the radio base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
. The purpose current loop remote control is to save the cost of extra pairs of wires between the operating point and the radio transceiver. Some equipment, such as the Motorola MSF-5000 base station, uses currents below 4 mA for some functions. An alternative type, the Tone remote
Tone remote
Remote controls are used anytime a two-way radio base station is located away from the desk or office where communication originates. For example, a dispatch center for taxicabs may have an office downtown but have a base station on a distant mountain top...
, is more complex but requires only an audio path between control point and base station. To confirm the use of the phrase "DC remote" as accurate in describing this type of device, please see . The patent does not describe this tone remote but confirms the use of the phrase to describe this system of signaling.
For example, a taxi
Taxicab
A taxicab, also taxi or cab, is a type of vehicle for hire with a driver, used by a single passenger or small group of passengers, often for a non-shared ride. A taxicab conveys passengers between locations of their choice...
dispatch
Dispatcher
Dispatchers are communications personnel responsible for receiving and transmitting pure and reliable messages, tracking vehicles and equipment, and recording other important information...
base station might be physically located on the rooftop of an eight-story building. The taxi company office might be in the basement of a different building nearby. The office would have a remote control unit that would operate the taxi company base station over a current loop circuit. The circuit would normally be over a telephone line or similar wiring. Control function currents come from the remote control console at the dispatch office end of a circuit. In two-way radio use, an idle circuit would normally have no current present.
In two-way radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
use, radio manufacturers use different currents for specific functions. Polarities are changed to get more possible functions over a single circuit. For example, imagine one possible scheme where the presence of these currents cause the base station to change state:
- no current means receive on channel 1, (the default).
- +6 mA might mean transmit on channel 1
- −6 mA might mean stay in receive mode but switch to channel 2. So long as the −6 mA current were present, the remote base station would continue to receive on channel 2.
- −12 mA might command the base station to transmit on channel 2.
Note that this circuit is polarity-sensitive. If a telephone company cable splicer accidentally reversed the conductors, selecting channel 2 would lock the transmitter on.
Each current level could close a set of contacts, or operate solid-state logic, at the other end of the circuit. That contact closure caused a change of state on the controlled device. Some remote control equipment could have options set to allow compatibility between manufacturers. That is, a base station that was configured to transmit with a +18 mA current could have options changed to (instead) make it transmit when +6 mA was present.
In two-way radio use, AC signals were also present on the circuit pair. If the base station were idle, receive audio would be sent over the line from the base station to the dispatch office. In the presence of a transmit command current, the remote control console would send audio to be transmitted. The voice of the user in the dispatch office would be superimposed over the DC current that caused the transmitter to operate.