Instruction Manual

Installation
September 2013
2-29
The voltage available at the instrument is not the voltage measured at the instrument terminals. Once the instrument is
connected, the instrument limits the measured voltage to approximately 9.0 to 10.5 volts.
Obtain filter voltage drop. The measured drop will be different than this value. The measured filter voltage drop
depends upon control system output voltage, the intrinsic safety barrier (if used), and the instrument. See note 3.
HART
FILTER
(if used)
CONTROL
SYSTEM
+
−
COMPLIANCE VOLTAGE
VOLTAGE
AVAILABLE AT
THE
INSTRUMENT
+
−
R
INTRINSIC SAFETY
BARRIER
(if used)
Control system compliance voltage
= Voltage available at the instrument
– Filter voltage drop (if used)
Example Calculation
18.5 volts (at 21.05 mA)
– 2.3 volts (for HF300 filter)
– Intrinsic safety barrier resistance (if used) x maximum loop current
– 2.55 volts (121 ohms x 0.02105 amps)
TOTAL LOOP
CABLE RESISTANCE
– Total loop cable resistance x maximum loop current
– 1.01 volts (48 ohms x 0.02105 amps for
1000 feet of Belden 9501 cable)
= 15.19 volts available—if safety barrier (2.55 volts)
is not used
1
NOTES:
1
Calculate Voltage Available at the Instrument as
follows:
2
2
Figure 2-24. Determining Voltage Available at the Instrument (Point-to-Point Mode Only)
Table 2-1 lists the resistance of some typical cables.
The following example shows how to calculate the
voltage available for a Honeywell TDC2000 control
system with a HF340 HART filter, and 1000 feet of
Belden 9501 cable:
Voltage available = [18.5 volts (at 21.05 mA)] − [2.3
volts] − [48 ohms  0.02105 amps]
Voltage available = [18.5] − [2.3] − [1.01]
Voltage available = 15.19 volts
Compliance Voltage
If the compliance voltage of the control system is not
known, perform the following compliance voltage test.
1. Disconnect the field wiring from the control system
and connect equipment as shown in figure 2-25 to the
control system terminals.
2. Set the control system to provide maximum output
current.
3. Increase the resistance of the 1 KΩ potentiometer,
shown in figure 2-25, until the current observed on the
milliammeter begins to drop quickly.
4. Record the voltage shown on the voltmeter. This is
the control system 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 insure 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)
2