Current loop ( current loop ) - a method of transmitting information using the measured values of the electric current . Currently, this method is more common in engineering practice than using voltage for this purpose. As a rule, a controlled current source is used to set the measured current values. The type of information transmitted differs analog current loop and digital current loop.
Content
Digital current loop
Used in telecommunications equipment and computers for serial data transmission.
History
The current loop was used long before the appearance of the RS-232 and V.24 standards . In the 1960s, teletypes began using the 60 milliamp current loop standard. Subsequent models (one of the first - Teletype Model ASR-33) used the standard 20 mA. This standard has found widespread use in mini-computers that originally used teletypes for dialogue with the operator. Gradually, teletypes gave way to text-based video terminals , retaining the current loop interface. In the 1980s, the RS-232 standard finally replaced the current loop. .
Working Principles
The digital current loop standard uses the absence of current as the SPACE value (low level, logical zero) and the presence of a signal as the value of MARK (high level, logical unit). The absence of a signal for a long time is interpreted as a BREAK state (line break). Data is transmitted using the start-stop method, the sending format is the same as RS-232, for example, 8-N-1: 8 bits, no parity, 1 stop bit.
The current loop can be used at considerable distances (up to several kilometers). To protect the equipment, galvanic isolation is used on optoelectronic devices, such as optocouplers .
Due to the imperfection of the current source, the maximum permissible line length (and maximum line resistance) depends on the voltage from which the current source is powered. For example, with a typical supply voltage of 12 volts, the resistance should not exceed 600 ohms.
The current source can be located at the receiving or transmitting end of the current loop. The node with the current source is called active. Depending on the design, both the transmitter and the receiver can be either active (to power the current loop) or passive (to be powered by the current loop).
For computers of the DVK family, by default it is assumed that the transmitter is active, the receiver is passive.
Standardization
The IRPS / IFSS standard (OST 11 305.916-84) uses a 20 mA current loop for data transmission. This standard was widely used in computers manufactured in the USSR and the CMEA countries until the 1990s. For example, DVK , Electronics-60 , Electronics D3-28 , SM computers , etc. The physical performance of the IRPS connectors is not fixed in the standard, which gave rise to a lot of options. Often used connector CHO53-8-2.
Abroad current loop (Current Loop) is specified in IEC 62056-21 / DIN 66258 standards.
MIDI (Musical Instrument Digital Interface) uses the current loop standard on the DIN 41524 5-pin connector at 31.25 kbit / s.
For the IBM PC and IBM PC XT, there was an IBM Asynchronous Communications Adapter that supports RS-232 serial transmission or a current loop. To transmit the current loop signals, there are unused contacts on the DB25 connector . In later developments, only RS-232 remained.
Analog current loop
An analog current loop is used to transmit an analog signal over a pair of wires in laboratory equipment , production control systems , etc.
An offset range of 4–20 mA is used, that is, the smallest signal value (for example, 0) corresponds to a current of 4 mA, and the largest value is 20 mA. Thus, the entire range of acceptable values is 16 mA. The zero value of the current in the circuit means a broken line and makes it easy to diagnose this situation.
The analog current loop interface allows the use of a variety of sensors (pressure, flow, acidity, etc.) with a single electrical interface. Also, this interface can be used to control the recording and actuating devices: recorders , gates, etc.
The ranges of currents and voltages are described in GOST 26.011-80 "Instrumentation and automation. Current and voltage electrical signals continuous input and output."
The main advantage of the current loop (compared to cheaper parametric voltage transmission) is that the accuracy does not depend on the length and resistance of the transmission line, since the controlled current source will automatically maintain the required current in the line. This scheme allows you to power the sensor directly from the transmission line. Several receivers can be connected in series , the current source will support the required current in all at the same time (according to Kirchhoff's law ). But if leaks appear in the circuit, the operation of the current loop will be broken, and this will not be detected by means of the implementation of the current loop itself, which must be taken into account when designing responsible production sites.
Digital information can be transmitted over an analog current loop. This data transfer method is described in the HART protocol . The competing protocols capable of supplanting HART in the future are various digital field buses, such as FOUNDATION fieldbus or PROFIBUS .
Links
- Current Signal Systems. Chapter 9 - Electrical Instrumentation Signals (English)
- http://www.ni.com/white-paper/6940/en/ (eng.)
- https://www.predig.com/indicatorpage/back-basics-fundamentals-4-20-ma-current-loops (eng.)
- https://www.electronics-notes.com/articles/connectivity/serial-data-communications/20ma-current-loop-introduction-what-is.php (eng.)
- https://www.acromag.com/sites/default/files/Acromag_Intro_TwoWire_Transmitters_4_20mA_Current_Loop_904A.pdf (eng.)
- http://www.bookasutp.ru/Chapter2_4.aspx
- http://easyelectronics.ru/tokovaya-petlya.html