Compliance Voltage; Maximum Cable Capacitance - Fisher FIELDVUE DVC6200 Instruction Manual

Instruction Manual
D103409X012

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 2‐29 to the control
system terminals.
Figure 2‐29. Voltage Test Schematic
CIRCUIT
UNDER
TEST
A6192‐1/IL
2. Set the control system to provide maximum output current.
3. Increase the resistance of the 1 kW potentiometer, shown in figure 2‐29, 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
Length(m) = [160,000 - C
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 = the capacitance of the control system or HART filter
master
C = the capacitance of the cable used (see table 2‐2)
cable
The following example shows how to calculate the cable length for a Foxboro
of 50, 000 pF and a Belden 9501 cable with characteristic capacitance of 50pF/ft.
kW
1 POTENTIOMETER
VOLTMETER
(pF)] [C
(pF/ft)]
master cable
(pF)] [C
(pF/m)]
master cable
MILLIAMMETER
t
I/A control system (1988) with a C
Installation
May 2013
master
37