Bronkhorst FLOW-BUS interface Manuale Utente Scaricare il pdf (Pagina 15)

BRONKHORST®

Page15

FLOW‐BUSinterface9.17.024

2.4 TERMINATION



ForbestqualityofdatatransferFLOW‐BUSshouldbeterminatedcorrectly.



2.4.1 Terminationresistors

A resistor is added in parallel with the receiver’s “A” and “B” lines in order to match the data line characteristic

impedance specified by the cable manufacturer (120Ωis a common value). This value describes the intrinsic

impedanceofthetransmission lineand isnot afunction of  thelinelength.A terminating resistor of less tha

n90Ω

shouldnotbeused.Terminationresistorsshouldbeplacedonlyattheextremeendsofthedataline(seeTermination

schematicsresistorsRT1andRT2),andnomorethan twoterminationsshouldbeplacedinanysystemthatdoesnot

userepeaters.



2.4.2 Biasingresistors

WhenanRS‐485networkisinanidlestate,allnodesareinlisten(receive)mode.Underthisconditionthereareno

activedriversonthenetwork.Alldriversaretri‐stated.Withoutanythingdrivingthenetwork,thestateofthelineis

unknown.Ifthevoltagelevelatthereceiver

’sAandBinputsislessthan±200mVthelogiclevelattheoutputofthe

receiverswillbethevalueofthelastbitreceived.Inordertomaintaintheproperidlevoltagestate,biasresistors

mustbeappliedtoforcethedatalinestotheidlecondition.Biasresistor

sarenothingmorethanapull‐upresistor

(RB1)onthedataRS485‐Alineandapull‐down(toground)onthedataRS485‐Bline.The“Terminationschematic”

illustratestheplacementofbiasresistorsonatransceiver.Thevalueofthebiasresistorsisdependentontermination

andnum

berofnodesinthesystem.ThegoalistogenerateenoughDCbiascurrentinthenetworktomaintaina

minimumof200mVbetweentheBandAdataline.Considerthefollowingexampleofbiasresistorcalculation.



Idealsituation:

Terminationresistors: 120Ohm

Receiverresistance: omitted

Biassupplyvol

tage: 5Vdc

Wantedsituationisaminimumof200mVbetweenAandBlinesandacommonmodevoltageof2.5V.



Minimumcurrenttherefore:  200mV/60Ohm=3.33mA

Totalmaximumbiasresistorvalueis (5V–0.2V)/3.33mA=1440Ohm.

Themaximumvalueofeachbiasingresistor: 72

0Ohm.



Situationwith127nodes:

Terminationresistors: 120Ohm

Receiverresistance: 12KOhm

Numberofinstruments: 127

Biassupplyvoltage: 5Vdc

Wantedsituationisaminimumof200mVbetweenAandBlinesandacommonmodevoltageof2.5V.



Totalterminationresistance:  120//120//12

000*127=120//120//94.5=36.7Ohm

Minimumcurrenttherefore:  200mV/36.7Ohm=5.45mA

Totalmaximumbiasresistorvalueis (5V–0.2V)/5.45mA=880Ohm.

Themaximumvalueofeachbiasingresistors: 440Ohm.

Lowervaluesmaybeused.(Dependingonmaximumpowercons

umptionoftheresistors)





Bronkhorstadvicesthefollowingresistorvaluesforthefollowingvoltages.

Supplyvoltage

termination

Termination

resistors

BiasPull‐up

resistor

BiasPull‐down

resistor

+5V 121Ohm 392Ohm 392Ohm

+10V 121Ohm 1210Ohm 392Ohm

+15V 121Ohm 2210Ohm 392Ohm

+24V 121Ohm 3480Ohm 392Ohm

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