XNX UNIVERSAL XNX Universal Transmitter Technical Manual

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XNX Universal Transmitter

Section 2 - Installation and Operation

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)

9,000 ft

(2,769 m)

6,500 ft

(2,000 m)

4,200 ft

(1,292 m)

3,200 ft

(985 m)

8,000 ft

(2,462 m)

5,900 ft

(1,815 m)

3,700 ft

(1,138 m)

2,900 ft

(892 m)

7,000 ft

(2,154 m)

5,200 ft

(1,600 m)

3,300 ft

(1,015 m)

2,500 ft

769 m)

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XNX Universal Transmitter Technical Manual - Page 54