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Rolta Academy : Engineering Design Services

CHAPTER 5

CURRENT LOOP APPLCATON

5!" ntrod#ctionThe purpose of this current loop application note is to introduce the reader tothe physical aspects of 20 mA current loop communications.

Until the early 1960s, military teleprinters used 60 ma current loops tocommunicate over long distances. n 1962, the !odel "" teletype #asintroduced and 20 mA current loop interfaces $ecame #idely used.Throughout the 60s, %0s, and early &0s, 20 mA current loop interfaces #ereapplied in many types of e'uipment. (urrent loop interfaces $ecame popularat this time $ecause they offered the most cost effective approach to longdistance, noise immune data transmission. The 20 mA current loop is suita$le

for distances to 2000 feet at data rates up to 19.2) $aud #ith careful attentionto interface design. t can $e used at longer distances #hen data rates are aslo# as "00 $aud.*hen the +A 22 -tandard /ecem$er 19%& and the +A & -tandardApril 19&" $rought forth the application of $alanced differential digital datatransmission, the popularity of 20 mA current loop rapidly diminished.

5!$ T%e &asics o' C#rrent Loo(

5!$!$ )#ll*d#(le+ ," mA Circ#itigure 1 is a full3duple4 20 mA current loop circuit. -imultaneous t#o3#aycommunications is possi$le #ith this circuit. T#o 20 mA current generators arenecessary #ith this circuit. t is possi$le to have one of the t#o currentgenerators in one current loop interface and the other current generator in theother interface. or e4ample, the original 5! ( serial adapter card had acurrent loop interface that contained only one current generator. *hen youmade a correct connection to this current loop interface, the second currentloop device #ould need to provide one current loop generator.

)ig 5!$ )#ll D#(le+ ,"mA circ#it

nstrumentation +ngineering 1

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5!$!, T%e Sim(le+ ," mA Circ#itigure .2 is a diagram of a simple4 20 mA current loop circuit. Thefundamental elements of a 20 mA current loop are a current source, a current

s#itch, and a current detector. The transmitter is the current s#itch and thereceiver is the current detector. The interface that contains the current sourceis called the active unit and all other units are referred to as passive units.igure ." is a diagram of the levels in an 7-32"2 interface and ho# theyrelate to the presence and a$sence of current in a 20 mA current loop circuit.n a 20 mA loop the current flo#s #hen the loop is idle no data $eingtransmitted. n a simple4 type circuit a num$er of transmitters and receiversare put in series in a current loop. As long as only one transmitter sends data,all receivers receive the data.

)ig 5!, Sim(le+ ,"mA circ#it

)ig 5!- Com(arison o' signal levels in a RS*,-, circ#it and a ,"mAc#rrent loo( circ#it

5!$!- Pro.lems /it% ," mA C#rrent Loo(The main pro$lem #ith 20 mA current loop is that there is no mechanical orelectrical standard defined for this interface. This ma)es every interfacesome#hat uni'ue and the user must )no# some of the technical details a$out

the circuits used in the interface.


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)ig 5!5 Constant C#rrent generator 'or a ,"mA c#rrent loo(

n a current loop, the sum of all the voltage drops across all the devices must$e less than the voltage source, ;s driving the loop. +ach device in thecurrent loop #hether it is a transmitter current s#itch or receiver currentdetector has some voltage drop across it. or instance, a typical transistors#itch can have typically 0.2 volts drop across it. or most of 5?5 +lectronicsconverters, the voltage drop across the transmitters can $e as much 2." volts#hen the s#itch is turned 8=. The reason for this is that the transmitter s#itchmust provide for the reverse $ias of the internal photo detector diode insidethe opto coupler. An opto coupler used as a current detector #ill have from 1.2to 2.0 volts drop across it.

)ig 5!1 C#rrent Limiter ild into transmitterTransmitter C#rrent Limiter-ome current loop interfaces incorporate current limiting into the transmittercurrent s#itch itself. igure 6 is an e4ample of a circuit that has $uilt3incurrent limiting so that the loop current cannot e4ceed 20 mA. n this circuit7g provides a source of $ias current for B2 so that if the loop current tries to

e4ceed 20 mA B2 #ill shunt B1 $ase $ias current so that B1 #ill not conductmore than 20 mA.


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Rolta Academy : Engineering Design Services(onnection to a passive current loop port is a little harder. Dou must use a 12;/( po#er supply #ith the %0 ohm resistors inside of our converter toEcreateE a 20 ma current source. -ee the follo#ing dra#ing.

)ig 5!6 Connection to a (assive c#rrent Loo( (ort

5!,!- nterconnection o' T/o C#rrent Loo( Convertersnterconnection of t#o 5?5 current loop converters also re'uires the use of a12 ;/( po#er supply since they are $oth passive port. -ee the follo#ingdra#ing.

)ig 5!$" interconnection o' t/o c#rrent loo( converters

5!- 7%at sn8t Digital C#rrent Loo(

5!-!$ 0 to ," mA Analog C#rrent Loo(The diagram sho#n in igure .11 is an analog to 20 mA current loop. Thiscircuit is mentioned here $ecause it is sometimes confused #ith 20 mA digitalcurrent loop. The purpose of to 20 mA analog current loop is to transmit thesignal from an analog sensor over some distance in the form of current signal.8nly t#o #ires are re'uired to send the analog signal and also supply po#erto the sensor. A loop supply voltage 2 volts in igure .11 is used to po#erthe remote sensor. The remote sensor regulates the loop current such that theloop current represents the value of the parameter $eing measured $y thesensor. A series resistor 7> at loop po#er supply converts this current to a

voltage that can $e used $y the electronics to record or distri$ute theparameter $eing measured.


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)ig 5!$$ 0 to ,"mA c#rrent Loo(

5!-!, HART9 0 to ," mA C#rrent Loo(sigure .12 is another e4ample of a type of to 20 mA com$ined analog ?digital current loop. This current loop uses FA7TG (ommunications protocol.

The FA7TG Figh#ay Addressa$le 7emote Transducer protocol is used for-!A7T remote transducers that are compati$le #ith to 20 mA analogcurrent loops $ut also have digital communications on the same t#o #ires.This is accomplished $y superimposing a t#o3tone re'uency -hift Heyed-H digital current signal on the to 20 mA analog signal.

)ig 5!$, 0 to ,"mA analog c#rrent loo( /it% digital comm#nications#sing HART (rotocol


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