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Why 4 to 20 mA ?

Following are the benefits of 4 to 20mA.

· Benefits of the 4-20 mA convention are that is is widely used by many manufacturers, relatively low-cost to implement, and it can reject many forms of electrical noise with twisted pair.

· The "live zero" at 4 mA allows the receiving instrumentation to distinguish between a zero signal and a broken wire or a dead instrument

Digital

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 teleprinters. More than two teletypes could be connected on a single circuit allowing a simple form of networking. Older teletypes used a 60 mA current loop. Later machines, such as the ASR33 teleprinter, operated on a lower 20 mA current level and most early minicomputers featured a 20 mA current loop interface instead of an RS-232 port. 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.

Analog

Analog current loops are used for any purpose where a device needs to be either monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

4-20 mA ("four to twenty milliamp current loop") is an analog electrical transmission standard for industrial instrumentation and communication. The signal is a current loop where 4 mA represents zero percent signal and 20 mA represents the one hundred percent signal. A "mA" is a milliampere, or 1/1000 of an ampere.

The "live zero" at 4 mA allows the receiving instrumentation to distinguish between a zero signal and a broken wire or a dead instrument. This standard was developed in the 1950s and is still widely used in industry today. Benefits of the 4-20 mA convention are that it is widely used by many manufacturers, relatively low-cost to implement, and it can reject many forms of electrical noise. The live zero also allows low-power instruments to be directly powered from the loop, saving the cost of extra wires. Current loop is also much easier to understand and debug than more complicated digital fieldbuses. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop solutions.

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.

Process-control use

For industrial process control 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 devices in series.

Taking the point of view of 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 transmitter instrument such as 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. 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. Occasionally the mapping between engineering units and current was inverted, so that 4 mA represented the maximum and 20 mA the minimum.

Long circuits

Analog current loops were occasionally carried between buildings 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 theory reminds us that current is the same at all points in a circuit. 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 125V DC 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 radio 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.

Hope this information is enough to understand why 4-20 mA is used.