Instruction Manual
Instruction Manual
D103557X012
Wiring Practices
January 2015
11
2. Set the system to provide maximum output current.
3. Increase the resistance of the 1 kW potentiometer, shown in figure 2‐2, until the current observed on the
milliammeter begins to drop quickly.
4. Record the voltage shown on the voltmeter. This is the compliance voltage.
For specific parameter information relating to your control system, contact your Emerson Process Management sales
office.
Maximum Cable Capacitance
The maximum cable length for HART communication is limited by the characteristic capacitance of the cable.
Maximum length due to capacitance can be calculated using the following formulas:
Length(ft) = [160,000 - C
master
(pF)] [C
cable
(pF/ft)]
Length(m) = [160,000 - C
master
(pF)] [C
cable
(pF/m)]
where:
160,000 = a constant derived for FIELDVUE instruments to ensure that the HART network RC time constant will be no
greater than 65 μs (per the HART specification).
C
master
= the capacitance of the control system or HART filter
C
cable
= the capacitance of the cable used (see table 2‐1)
The following example shows how to calculate the cable length for a Foxboro
t
I/A control system (1988) with a C
master
of 50, 000 pF and a Belden 9501 cable with characteristic capacitance of 50pF/ft.
Length(ft) = [160,000 - 50,000pF] [50pF/ft]
Length = 2200 ft.
The HART communication cable length is limited by the cable characteristic capacitance. To increase cable length,
select a wire with lower capacitance per foot. Contact your Emerson Process Management sales office for specific
information relating to your control system.
Table 2‐1. Cable Characteristics
Cable Type
Capacitance
(1)
pF/Ft
Capacitance
(1)
pF/m
Resistance
(2)
Ohms/ft
Resistance
(2)
Ohms/m
BS5308/1, 0.5 sq mm 61.0 200 0.022 0.074
BS5308/1, 1.0 sq mm 61.0 200 0.012 0.037
BS5308/1, 1.5 sq mm 61.0 200 0.008 0.025
BS5308/2, 0.5 sq mm 121.9 400 0.022 0.074
BS5308/2, 0.75 sq mm 121.9 400 0.016 0.053
BS5308/2, 1.5 sq mm 121.9 400 0.008 0.025
BELDEN 8303, 22 awg 63.0 206.7 0.030 0.098
BELDEN 8441, 22 awg 83.2 273 0.030 0.098
BELDEN 8767, 22 awg 76.8 252 0.030 0.098
BELDEN 8777, 22 awg 54.9 180 0.030 0.098
BELDEN 9501, 24 awg 50.0 164 0.048 0.157
BELDEN 9680, 24 awg 27.5 90.2 0.048 0.157
BELDEN 9729, 24 awg 22.1 72.5 0.048 0.157
BELDEN 9773, 18 awg 54.9 180 0.012 0.042
BELDEN 9829, 24 awg 27.1 88.9 0.048 0.157
BELDEN 9873, 20 awg 54.9 180 0.020 0.069
1. The capacitance values represent capacitance from one conductor to all other conductors and shield. This is the appropriate value to use in the cable length calculations.
2. The resistance values include both wires of the twisted pair.