Flowserve Logix 1200e LGAIM0044 - Manual (Page 36)

44-36 Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 8611
NOTE: This filter is not rated for use in hazardous
areas. It should be located between the current source
and the barrier in intrinsically safe applications.
Frequently Asked Questions
Q: How do I know if I need a VHF?
A: If the current source is interfering with communica-
tion, it will affect the ValTalk or SoftTools software and
the HART 275 handheld communicator. If the posi-
tioner communicates to ValTalk software or the HART
275 handheld communicator when using one current
source (for example, a 4-20 mA current calibrator) and
not the DCS, it indicates a filter is necessary with that
current source. Some 4-20 mA calibrators that work
without a filter are listed below. If one of these is avail-
able, try to connect with ValTalk or SoftTools software
or the HART 275 handheld communicator again. If
communications are established while using one of
these sources, but fails on the original source, a filter
is needed.
The following handheld 4-20 mA calibrators do not
require a filter:
• Altek Model 334
• Rochester Instrument Systems (RIS) CL-4002
• Unomat UPS-I
HART Modem
The HART modem is a device that connects to the
serial communications port of a computer. This
modem converts the RS-232 COM port signals to the
HART signal. A HART modem is required to use
ValTalk software, and is optional in SoftTools since a
MUX can be used in its place. The HART modem
takes power from the RS-232 COM port lines. If using
a laptop computer that is operating on its internal
battery, HART communication may become erratic as
the batteries begin to lose their charge. This is due to
a reduction in HART modem power. At this point,
recharge the batteries or apply AC adapter power to
correct the problem.
Connections
When using ValTalk or SoftTools software or the HART
275 handheld communicator, the leads can be con-
nected anywhere across the 4-20 mA current signal.
The leads are not polarity sensitive. When using a fil-
ter, the connection must be made between the filter
output and the Logix 1200 positioner (Figure 21).
Cabling
Because HART protocol uses frequencies of 1200 Hz
and 2200 Hz to communicate, cable type and length
must be considered. High capacitance/foot cable or
long runs can cause erratic communication. Refer to
instructions for correct cable requirements.
Intrinsically Safe Barriers
When selecting an intrinsically safe barrier, make sure
the barrier is HART-compatible. Although the barrier
will pass the loop current and allow normal positioner
control, if not compatible, it may prevent HART
communication.
Figure 20: HART VHF Filter
Figure 21: HART VFH Filter Schematic