Current loop

Current Loop - is a two-wire data transmission interface, where the data are placed in the current value.


Many thanks to Michael Hooke for interesting books. Once the author begins the study of modern electronics with the encyclopedia and publications of this remarkable man. Without the Internet textbooks had to patiently turning the pages of his hands, and the mouse ran mainly in the subfield.

muRata company continually provides readers with fresh information, so now will be informed about news and readers. The products concerned have already mentioned in the section on reed sensors. We are talking about the latest development - RedRock.

The need for loop

4-20 mA current loop is considered common protocol transmitting sensor information. The industry is often necessary to measure physical parameters, for example:

  • Pressure;
  • Temperature;
  • Fluid flow.

The need arises constantly, when the information to be transmitted over distances of hundreds of meters or more. Current loop is considered to be slow digital interface, and this is due to the charge capacity of the cable source (which manifests itself with increasing frequency), for analog or digital devices quite opportunities missing. The transmitters are supplied with batteries by 12 (rarely) or 24 (most). The latter allow further transmit the information in a meaningful parameter becomes the current rather than voltage. The longer the line, the greater the voltage drop.

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In This misleading technical solutions have a couple of drawbacks. Firstly, it is necessary to use a shielded wire, and secondly, an increase in distance results in a sharp decrease in efficiency. Typical current loop consists of four components:

  1. Source of power. Arbitrary location.
  2. Receiver or monitor.
  3. A transmitter (sensor).
  4. voltage converter current.

The sensors give information proportional to the measured parameter, represented by voltage. Therefore, we need to do in the current transformation. Then the information is encoded either by the level of current, or in binary form: 4 mA - zero to 20 mA - edinichka. On the receiver side of the data is decrypted.

Digital fans say a low speed loop. Indeed, when heat input capacity of 75 pF / m kilometer the segment conductors forms a capacitor with a nominal value of 75 nF. With increasing frequency, the resistance drops, the effect of smoothing and filtering does not give the right to work with the information. For 19 ms condenser filled fully from 5V, causing limitation noticed in the 9.6 kbit / s.

Actually the current loop is considered obsolete protocol in its place ready to Other massively used, for example, MIDI, and little known to the general public industrial HART interface.

general information

The first surprise becomes a lack of uniform standards. They became the dominant protocol 4-20 mA, 0-20 mA and 0-60 mA, no hard and fast rules. The current loop can be transmitted any information. If it is a binary code, a unit corresponds to the presence of a current of 20 mA, depending on system configuration, and zero - no signal or the presence of 4 mA. If the packet transmission line break occurs, it necessarily is identified through a stop byte.

The interface used with 50s originally coded unit like 60 mA DC. Therefore, system efficiency is much lower. Loop 20 mA appeared in 1962 as a signal for the teletype - for remote printing messages (connected the two electric typewriters). Since the beginning of the 80's tried to reduce the current, not always successfully. We decided to make a compromise:

  1. 4 mA means "live" zero. For the system to know exactly, not whether there was a break in the network.
  2. The unit remains 20 mA.

The main limitation is the data transmission distance. On setting affects bitrate: kilometer distances on permissible data transfer rate is 9600 bits / sec. Above the 19.2 kbit / s line is not used. As a result, the range affected by the electrical parameters of the line and the noise level. Current loop would be replaced on the idea of ​​Fieldbus, actually came into use a standard RS-485 today (1983) - COM-port option. Even today, the terminals on the RS-232 protocol are joined by means of the current loop, and on the receiving side is made necessary transformation. Sometimes elected printers work on the protocol. Suppose there is a theoretical limit kbis 115 / s, in practice, applicable to 9600.

The peculiarity of the current loop - the transmitter does not pay attention to stress. Power is different. The main thing - to withstand the current value of 20 mA. Therefore, the longer the line, the lower the efficiency. This is strictly enforced rule. It meets periodically current loop with galvanic isolation. For this purpose, an optocoupler and similar semiconductor design.

Usually shielded cable is used, to avoid parallel capacitive interference, which is not possible to compensate for or track. To create a network well suited shielded twisted pair. Due to the close intertwining of wires, it eliminates the external interference in the form of inductive and common mode noise. To create a duplex channel using two twisted pair interface programmatically controlled via XON / XOFF methods. Worthy specialized applications bypass the difficulty of creating a pre-transmission and answers queries.

At the receiver the current is converted into voltage using a resistive divider. Depending on the voltage applied resistance 125 - 500 ohms. Sometimes on the side of the transmitter or the receiver is put adapter (transmitter signal) to the serial COM-port interface. The voltage drop across the resistor is calculated by Ohm's law, for example, for the nominal value is 250 ohms is 250 x 0.02 = 5 V. Accordingly, the receiver may need to be calibrated at the desired level.

Where applicable current loop

  1. Control of technological processes. In the manufacture of a 4-20 mA current loop is considered the main analog interface. It used "live" zero when the total absence of a signal is a line break. Current of 4 mA is sometimes used as power for the transmitter or sensor input signal is modulated and returned as information. There are chains, where the battery is separate, if it is modulated signal. Neither the receiver nor the transmitter does not spend its own energy.
  2. In the days of analogue current loop telephony remained the favorite interface for connection. And today more are beating the current conductor in the apartments. Here, the phone is powered from the station and modulates a signal to make a call. As with the sensor described above. These lines remained as a legacy of times past. For example, the company Bell System uses DC power supply up to 125 V.
  3. The current loop is sometimes used to transmit the information signal level. For example, 15 mA means "burn!", 6 mA - "all right", 0 mA - line break. Any small-town manufacturer sets its own rules and uses the protocol.
  4. In telephony over a current loop can be controlled by the base station. This is called "remote control DC." For example, Motorola MSF-5000 uses a constant current 4 mA for the transmission of service signals. An example of such a protocol:
  • No current - conduct reception on channel 1.
  • 6 mA - 1 transmit channel.
  • -6 mA - receive information on channel 2.
  • -12 mA - to pass on the 2 channel.

MIDI interface

MIDI format is popular among musicians, a specialized digital audio protocol. On the physical level it is organized according to the scheme 5 mA current loop. Of course, due to the difference unit level directly transfer standard two are not compatible. According to Michael Hooke, MIDI was developed in 1983 and has become the de facto rule of the connection of synthesizers.

Wikipedia reports that in June 1981 Roland Corporation has filed a major manufacturer of synthesizers - Obergeym Electronics - a standard interface idea. Already in October, Smith Obergeym Kakihashi and discussed it with the board Yamaha, Korg and Kawai, and demonstrated the first workable option in November to AES exhibition of the society.

Two years of the interface is being revised, and in January 1983, Smith joined via MIDI two analog synthesizer. It is possible to directly pump arrangement and to create new songs. Later MIDI files introduced in support of the Windows operating system, allowing the authors directly engaged in processing tones, saturating them with new special effects, which are absent in the original synthesizers. The introduction of samples of different instruments allows the player to play the music of any complexity.

The use of MIDI

The MIDI used physical lines on 5 mA. Rare 10. Isolation through optocoupler. A characteristic feature of the recognized inversion signal:

  1. There are current.
  2. No current.

Therefore, MIDI is not directly compatible with the conventional current loop. The physical interface have seen many, but did not know the name. Outlet visually represents a dielectric disk with a side cutaway openings 5 ​​are arranged around the perimeter (DIN). The design screen is covered by a circle. Musicians account for three types of interfaces:

  1. MIDI-In.
  2. MIDI-Out.
  3. MIDI-Thru.

MIDI port is sometimes necessary on the motherboard of a personal computer. Physically, it is utilized in the normal mode is not used contacts 12 and 15 ports DB-15S game adapter. TTL logic used herein requires an adapter for docking with the standard synthesizer protocol loop. converter circuit is not too complicated, comprises an optocoupler, a diode, a series of logical elements.

MIDI port is programmed via UART as a serial COM-port. On sale are sound cards with MIDI or separate expansion card slots are on the loose.

The HART protocol

This development protocol Fieldbus, massively used in the industry. Subbase becomes 4-20 mA current loop, and therefore can use twisted pair, left over from obsolete protocols. Initially I considered ukzospetsializirovannym standard communication interface, but went on public display in 1986. Transmission of HART comes complete package, having the composition:

  1. Preamble - 5-20 bytes. It serves to synchronize and identify the carrier.
  2. Start-byte - 1 byte. Specifies the bus master number.
  3. Address - from 1 to 5 bytes. Assigned to the master, and the servant is a special feature of batch mode.
  4. Expansion - from 0 to 3 bytes. Its length is specified in the start-byte.
  5. Team - 1 byte. The fact that the servant must fulfill.
  6. The number of bytes of data - 1 byte. data field size in bytes.
  7. Data - from 0 to 255 bytes. The data for reading the procedure.
  8. Checksum - 1 byte. It contains the result of the logical operation XOR of all the bytes, except the starting and final data block.

Of course, the packet structure is typical for digital devices, requires for proper interpretation of the command execution.

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