Instruction Manual
D102796X012
HF340 Filter
July 2011
5
Corrective Maintenance
The most likely malfunction of a loop containing an HF340 filter is reverse polarity installation of one of the wire pairs.
The 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 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.
The 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.
