Автор: Айдар
Дата: 07.06.05 11:24
Wiring and Installation
In general, the installation practice for HART communicating 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 0.51 mm diameter (#24 AWG) for cable
runs less than 1,524 m (5,000 ft) and 0.81 mm diameter (#20 AWG) for
longer distances.
CABLE LENGTH Most installations are well within the 3,000 meter (10,000 ft)
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. Table
3 shows the affect 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 configuration can be found in the HART Physical Layer
Specifications.
Cable Capacitance - pf/ft (pf/m)
Cable Length - feet (meters)
No. Network
Devices
20 pf/ft 30 pf/ft 50 pf/ft 70 pf/ft
(65 pf/m) (95 pf/m) (160 pf/m) (225 pf/m)
1 9,000 ft 6,500 ft 4,200 ft 3,200 ft
(2,769 m) (2,000 m) (1,292 m) (985 m)
5 8,000 ft 5,900 ft 3,700 ft 2,900 ft
(2,462 m) (1,815 m) (1,138 m) (892 m)
10 7,000 ft 5,200 ft 3,300 ft 2,500 ft
(2,154 m) (1,600 m) (1,015 m) (769 m)
15 6,000 ft 4,600 ft 2,900 ft 2,300 ft
(1,846 m) (1,415 m) (892 m) (708 m)
Table 3: Allowable cable lengths for 1.02 mm (#18 AWG)
shield twisted pair
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