Honeywell XNX - Page 54

XNX Universal Transmitter
Section 2 - Installation and Operation
52
Multidrop Mode
Themultidropmodeofoperationrequiresonlyasinglepairof
wiresand,ifapplicable,safetybarriersandanauxiliarypower
supplyforupto8elddevices(Figure58)�Allprocessvaluesare
transmitteddigitally�Inmultidropmode,allelddevicepolling
addressesare>0,andthecurrentthrougheachdeviceisxedat
aminimumvalue(typically4mA)�
Input/Output (I/O) System
Handheld Terminal
Field Devices
Control System or
Other Host Application
Note: Instrument power is provided by an interface
or external power source that is not shown.
Figure 58. Multidrop Mode of Operation
Note: Usemultidropconnectionforsupervisorycontrolinstallations
thatarewidelyspaced,suchaspipelines,custodytransferstations,
andtankfarms.
Ingeneral,theinstallationpracticeforHARTdevicesisthesame
asconventional4-20mAinstrumentation�Individuallyshielded
twistedpaircable,eitherinsingle-pairormulti-pairvarieties,is
therecommendedwiringpractice�Unshieldedcablesmaybe
usedforshortdistancesifambientnoiseandcross-talkwillnot
affectcommunication�
Theminimumconductorsizeis0�51mmdiameter(#24AWG)for
cablerunsoflessthan5,000ft(1,524m)and0�81mmdiameter
(#20AWG)forlongerdistances�
Cable Length
Mostinstallationsarewellwithinthe10,000ft(3,000m)
theoreticallimitforHARTcommunication�However,theelectrical
characteristicsofthecable(mostlycapacitance)andthe
combinationofconnecteddevicescanaffectthemaximum
allowablecablelengthofaHARTnetwork�Thetablebelow
showstheeffectofcablecapacitanceandthenumberof
networkdevicesoncablelength�Thetableisbasedontypical
installationsofHARTdevicesinnon-ISenvironments,i�e�no
miscellaneousseriesimpedance�
Detailedinformationfordeterminingthemaximumcablelength
foranyHARTnetworkcongurationcanbefoundintheHART
PhysicalLayerSpecications�
Cable Capacitance – pf/ft (pf/m)
Allowable Cable Lengths for 1 mm (18 AWG) Shielded Twisted Pair – feet (meters)
Number of Network
Devices
20 pf/ft
(65 pf/m)
30 pf/ft
(95 pf/m)
50 pf/ft
(160 pf/m)
70 pf/ft
(225 pf/m)
1
9,000ft
(2,769m)
6,500ft
(2,000m)
4,200ft
(1,292m)
3,200ft
(985m)
5
8,000ft
(2,462m)
5,900ft
(1,815m)
3,700ft
(1,138m)
2,900ft
(892m)
8
7,000ft
(2,154m)
5,200ft
(1,600m)
3,300ft
(1,015m)
2,500ft
769m)
