Fisher IM Supplement: FIELDVUE Digital Valve Controller Split Ranging (HART) Manuals & Guides

Instruction Manual Supplement

D103262X012

FIELDVUE Digital Valve Controllers

January 2020

FIELDVUE™ Digital Valve Controller Split Ranging

Supplement to HART® Communicating Fisher™ FIELDVUE
Digital Valve Controller Instruction Manuals

Processes requiring extended rangeability may use two or more control valves. The first valve operates over the first
portion of the range. When the first valve is nearly 100% open, the control action begins to open the second valve. This
is called split ranging. When split ranging is performed a single signal from the control system is supplied to the valve
actuators. This single signal is split among the positioning devices on each valve. The positioning devices are used to
achieve full valve travel over the reduced input range. Most applications need a predictable overlap region in the
middle of the signal range to avoid a zone of no control. Positioning devices provide accuracy to ensure the correct
overlap exists.

When split ranging FIELDVUE digital valve controllers, each device requires a minimum voltage available at the
instrument terminals of at least 12 volts (See the appropriate instruction manual for details). Many control systems
provide a compliance voltage that is less than the minimum requirement of 24 volts for two instruments wired in
series. This supplement describes two methods to address the potential discrepancy between the compliance voltage
and the requirements of FIELDVUE instruments.

Limitations of Split Ranging FIELDVUE Digital Valve Controllers

Split ranging FIELDVUE digital valve controllers provides the following limitations:

D Most control systems cannot provide the 24 volt compliance voltage. A device to boost the compliance voltage may

be required, such as the Eurotherm Action Pak

D To communicate with multiple HART instruments on one pair of wires may require the polling addresses for the

instruments be nonzero. This means the instrument polling addresses must be reset and the HART devices (either
the Trex
instruments at polling addresses other than zero.

D Use of the HART Tri‐Loop™ is limited to communication with only one instrument.

D The instrument polling addresses may be nonzero, thus limiting communication with a HART multiplexer.

™

/475/375 communicator or ValveLink™ software with a HART modem) must be programmed to poll for

®

AP4380-2000.

Guidelines for Split Ranging FIELDVUE Instruments

1. Check the voltage available at the instrument terminals using the method in the instrument instruction manual.

2. Verify that the control system compliance voltage is at least 24 volts DC at 20 mA output. If the compliance voltage
is less than 24 volts DC, add a device to boost the compliance voltage (such as the AP4380‐2000). (Note: the
AP4380‐2000 does not have any hazardous area approvals. Appropriate non‐hazardous installations should be
selected.)

3. Verify that cable length will not limit HART communication. See the instruction manual for details.

www.Fisher.com

FIELDVUE Digital Valve Controllers

January 2020

4. For intrinsically safe installations, install approved intrinsic safety barriers between the non‐hazardous and
hazardous areas.

Instruction Manual Supplement

D103262X012

Methods of Split Ranging

Two methods are available for split ranging FIELDVUE instruments: the traditional method and an alternate method.

Traditional Method

The traditional method uses the traditional split‐range wiring where the two instruments are connected in series to a
single pair of wires from the control system, as shown in figure 1. With the traditional method:

D The device used to boost the compliance voltage may be required to operate beyond its specifications.

D To communicate with multiple HART instruments on one pair of wires requires the polling addresses for the

instruments be nonzero. This means the instrument polling addresses must be reset and the HART devices (either
the 375 Field Communicator or ValveLink Software with a HART modem) must be programmed to poll for
instruments at polling addresses other than zero.

D Because the instrument polling address for one of the instruments must be nonzero, communication with a HART

multiplexer is not possible.

D Use of the HART Tri‐Loop is limited to communication with only one instrument. In addition, because the default

polling address for the Tri‐Loop is one (1), the instrument polling addresses must be set in the range of 2 to 15.

Alternate Method

The alternate method requires more field wiring, but this method:

D Allows the device used to boost the compliance voltage to operate within its specifications because it supplies a

single instrument.

D The polling addresses can remain at zero (0) because the two instruments are not on the same HART loop.

D Allows communicating over a HART multiplexer network.

D Multiple Tri‐Loops may be used allowing multiple instruments to operate in burst mode.

D Split ranging with more than two FIELDVUE digital valve controllers is possible.

Installation—Traditional Method

When connecting two instruments in series for a split range application, the compliance voltage must equal at least 24
volts DC. This is generally impractical with most DCS controllers, but quite possible with older analog controllers. For
example, a Fisher TL108 controller with 45 volt DC power supply provides a compliance voltage of 27 volts DC at 20
mA output and a C
other similar device may be used to increase the compliance voltage.

of 10,000 pF. For other controllers or DCS systems, a Eurotherm AP4380-2000 isolator or

master

The published compliance voltage of the AP4380‐2000 is less than 24 volts DC. However, typical compliance voltage
for this isolator is much higher. To ensure sufficient compliance voltage, the AP4380-2000 distributor can check
compliance voltage of the AP4380-2000 prior to shipment. This should be done using the technique described in the

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