Current loop

I want to introduct something about Two Port VoiceAdditional digital communication to the device can be
Gateway. Two Port Voice Gateway functionadded to current loop using HART Protocol. Digital
description: The popularization of the Internet drivesprocess buses such as FOUNDATION Fieldbus and
the rapid development of a wide variety of IP-basedProfibus may replace analog current loops.
applications. The IP telephone technology has becomeProcess-control use
the major means for operators to develop voiceFor industrial process control instruments, analog 4-20
services now. Especially, IP technology becomes themA and 10-50 mA current loops are commonly used
core of the next generation network (NGN), so thefor analog signaling, with 4 mA representing the
IP-based voice technology will keep soaring speedinglowest end of the range and 20 mA the highest. The
the future and become the No. 1 choice of newkey advantages of the current loop are that the
operators in exploring services. Upon the marketaccuracy of the signal is not affected by voltage
requirement, Zhuhai Estone Co., Ltd has launched thedrop in the interconnecting wiring, and that the loop
IAD 2FXS gateway integrated access device. As ancan supply operating power to the device. Even if
Integrated Access Device, the upstream port of thethere is significant electrical resistance in the line, the
IAD can be directly connected to the IP network andcurrent loop transmitter will maintain the proper
its downstream port can be connected to multiplecurrent, up to its maximum voltage capability. The
ordinary telephone sets, to provide basic accesseslive-zero represented by 4 mA allows the receiving
for POTS users. This user port gateway can supportinstrument to detect some failures of the loop, and
two telephone lines and connect multiple telephonealso allows transmitter devices to be powered by the
sets. At the same time, this device is capable ofsame current loop (called two-wire transmitters).
Internet gateway and can access data stream, suchSuch instruments are used to measure pressure,
as Email or Internet intemperature, flow, pH or other process variables. A
A current loop describes two different electricalcurrent loop can also be used to control a valve
signalling schemes.positioner or other output actuator. An analog current
Digitalloop can be converted to a voltage input with a
For digital serial communications a current loop is aprecision resistor. Since input terminals of instruments
communication interface that uses current instead ofmay have one side of the current loop input tied to
voltage for signaling. Current loops can be used overthe chassis ground (earth), analog isolators may be
moderately long distances (tens of kilometres), andrequired when connecting several instruments in
can be interfaced with optically isolated links.series.
Long before the RS-232 standard, current loopsDepending on the source of current for the loop,
were used to send digital data in serial form fordevices may be classified as active (supplying power)
teleprinters. More than two teletypes could beor passive (relying on loop power). For example, a
connected on a single circuit allowing a simple form ofchart recorder may provide loop power to a pressure
networking. Older teletypes used a 60 mA currenttransmitter. The pressure transmitter modulates the
loop. Later machines, such as the ASR33 teleprinter,current on the loop to send the signal to the strip
operated on a lower 20 mA current level and mostchart recorder, but does not in itself supply power to
early minicomputers featured a 20 mA current loopthe loop and so is passive. ( A 4-wire instrument has
interface, with an RS-232 port generally available as aa power supply input separate from the current
more expensive option. The original IBM PC serial portloop.) Another loop may contain two passive chart
card had provisions for a 20 mA current loop. A digitalrecorders, a passive pressure transmitter, and a 24 V
current loop uses the absence of current for highbattery. (The battery is the active device).
(space or break), and the presence of current in thePanel mount displays and chart recorders are
loop for low (mark).commonly termed 'indicator devices' or 'process
The maximum resistance for a current loop is limitedmonitors'. Several passive indicator devices may be
by the available voltage. Current loop interfacesconnected in series, but a loop must have only one
usually use voltages much higher than those found ontransmitter device and only one power source (active
an RS 232 interface, and cannot be interconnecteddevice).
with voltage-type inputs without some form of levelThe relationship between current value and process
translator circuit.variable measurement is set by calibration, which
MIDI (Musical Instrument Digital Interface) is a digitalassigns different ranges of engineering units to the
current loop interface.span between 4 and 20 mA. The mapping between
Analogengineering units and current can be inverted, so that
Analog current loops are used where a device must4 mA represents the maximum and 20 mA the
be monitored or controlled remotely over a pair ofminimum.
conductors. Only one current level can be present atLong circuits
any time.Analog current loops were occasionally carried
Given its analog nature, current loops are easier tobetween buildings by dry pairs in telephone cables
understand and debug than more complicated digitalleased from the local...(and so on) To get More
fieldbuses, requiring only a handheld digital multimeterinformation , you can visit some products about
in most situations. Using fieldbuses and solving relatedwholesale st adapter, Automatic Voltage Regulator, .
problems usually requires much more education andThe Two Port Voice Gateway products should be
understanding than required by simple current loopshow more here!
systems.