Filter
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HF300 Series
Instruction Manual
Form 5715
June 2007
4
Corrective Maintenance
The most likely malfunction of a loop containing an
HF300 Series HART filter is reverse polarity
installation of one of the wire pairs. The Type HF340
filter will work with either polarity, but misconnection
could result in the wrong polarity reaching the field
instrument. If the loop is not operating properly,
check the polarity of the voltage at the inputs and
outputs of the filter and at the inputs of the
instrument.
If the instrument appears to operate properly in the
loop, but communication with a non-isolated
multiplexer or PC modem is not possible, it may help
to reverse the wires to the COMM terminals. The
Type HF340 filter inserts a high impedance in the +
side of the loop only. A reversal of the connections
to the SYS and FLD terminals of the filter will result
in the control loop operating properly, but inoperative
HART communication.
Inadequate control system compliance voltage will
not support filter operation. This is an installation
problem that may appear initially to be a filter
malfunction. For the filter to operate properly, the
control system must have a compliance voltage that
is at least 2 volts higher than the voltage required to
drive the loop to maximum current. Refer to the
appropriate instrument instruction manual for
information on determining the control system
compliance voltage.
Excessive load voltage will interfere with filter
operation. A transient clamping device limits the
output voltage to a nominal 15 volts. If the voltage at
the output (due to the total impedances connected)
is greater than 14 volts DC at maximum loop current,
some of the current may be shunted by the clamping
device.
The Type HF340 filter is protected against accidental
over-current from sources up to 30 volts DC. If an
over-current condition occurs, the filter may be
inoperative for several seconds thereafter.
Emerson Process Management
Marshalltown, Iowa 50158 USA
Cernay 68700 France
Sao Paulo 05424 Brazil
Singapore 128461
www.Fisher.com
The contents of this publication are presented for informational purposes only, and while every effort has been made to ensure their accuracy, they are
not to be construed as warranties or guarantees, express or implied, regarding the products or services described herein or their use or applicability.
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EFisher Controls International LLC 2001, 2007; All Rights Reserved Printed in USA
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HART is a mark owned by the HART Communication Foundation. All other marks are the property of their respective owners.