114 | Baker Hughes.
© 2021 Baker Hughes Company. All rights reserved.
7.6.3 Capacitance vs
.
Length of Cable for HART
The FieldComm
TM
Group specifies cable capacitance requirements to preserve signal
strength. Refer to the standards for detailed calculation methods.
7.6.4 HART Filter Requirements
The control system output interface must allow the HART
®
frequencies to coexist with the
precision 4
-
20 mA DC signal. Circuits that are not designed for HART
®
may need a
HART
®
filter
.
Consult the controller or DCS manufacturer for interfacing to a particular
system. The HART
®
communications can cause a non-HART
®
compliant output circuit to
malfunction, in some cases. In other cases the HART
®
communications tones are disabled
by the control circuit.
The SVI3 can be used with non-HART
®
compliant output circuits but remote
communications functionality are not enabled.
If remote maintenance is desired always isolate the control valve from the process and
disconnect the non-compliant controller before connecting a current source for power and
a HART
®
master device.
If a HART
®
filter is required, then its voltage drop must be considered in calculating the
compliance voltage.
7.7 Split Range Applications
The SVI3 is designed to operate in split range configurations supporting up to three control
valves connected to a single controller output. Minimum input current span for each SVI3
is 5 mA. For each positioner the upper range value is between 8 and 20 mA and the lower
range value is between 4 and 14 mA. For example, three devices might be configured with
input current ranges of 4
-
9 mA; 9
-
14 mA, and 14
-
20 mA. Split range operation with
SVI3 requires special consideration of the compliance voltage
.
The SVI3 requires at least
9
.
0 V
.
Two SVI3 in series requires at least 18
.
0 V in addition to the voltage drops in wiring
and other series devices. Typical controller output current sources rarely deliver 24 V
,
so
the system can become voltage starved. It is possible to boost the compliance voltage of
the DCS using a series wired voltage source power supply
,
as shown in Figure 40 on
The total loop voltage must not exceed the rating for the controller output
current source. Contact the DCS vendor to validate this approach.
CAUTION
Do not connect a HART
®
modem and a PC to a control circuit unless
the controller is HART
®
compatible or has a HART
®
filter. Loss of
control or a process upset can occur if the controller output circuit is
not compatible with HART
®
signals.
NOTE : The internal electronic components are isolated from ground.
Grounding the case is unnecessary for functional purposes. Grounding the
case may be necessary to conform to local codes.
Refer SVI3 DTM Manual to Configure the SVI3 for Split range Applications.