Avoiding HART loop interference when using the Smart Wireless THUM™ Adapter

Avoiding HART loop interference when using the Smart Wireless THUM™ Adapter
White Paper
00840-0100-4075, Rev AA
March 2011
Smart Wireless THUM
Avoiding HART loop interference when
using the Smart Wireless THUM™ Adapter
Cabling Considerations
This document provides guidance on avoidance of
interference on a HART loop that can affect the
performance of the Smart Wireless THUM™ Adapter.
To prevent external interference from being induced
on the HART loop, use only high quality shielded,
twisted pair cable. The cable size should be a
minimum of #24 AWG for runs less than 5,000 ft. and
#20 AWG for longer distances.
The Smart Wireless THUM Adapter employs an
innovative energy scavenging design which uses the
existing 4-20mA signal of the wired HART sub-device
as a power source instead of requiring a separate
power supply. Due to the extremely limited power that
may be available, the Smart Wireless THUM Adapter
is slightly more sensitive to loop interference.
Although the Smart Wireless THUM Adapter is
designed to ensure reliable data transmission from
the wired HART sub-device, interference and
excessive noise on the loop may cause the THUM to
periodically miss a wireless burst message
publication. If this noise and interference is removed,
the THUM will automatically recover and continue to
provide >99% data reliability.
The theoretical limit for HART communications is
10,000 ft (3,000 m). However, cable capacitance will
affect the maximum distance for HART
communication. Table 1 below shows the maximum
distance for different cable capacitance. The
information in the table is based on non-intrinsically
safe installations and considers only the capacitive
affect of the cable on the HART signal. For more
information, see the HART physical layer
Table 1. Maximum cable length based on cable
Cable Capacitance - pf/ft (pf/m)
This white paper will discuss the different potential
sources of interference and provides guidance on
avoiding interference related problems.
Figure 1. Basic HART loop with THUM adapter
Wired device with
HART output
Smart Wireless
THUM Adapter
ft. (m)
20 (65)
9,000 (2,769)
30 (95)
6,500 (2,000)
50 (160)
4,200 (1,292)
70 (225)
3,200 (985)
Improper grounding can also cause interference on
the HART loop so it is important to follow good
grounding practices. The signal loop should only be
connected to ground at one point, if at all. Cable
shields should be grounded at only one point.
Generally speaking, the best location for ground
connection is near the control system on the power
White Paper
00840-0100-4075, Rev AA
March 2011
Smart Wireless THUM
Power Supply
Ripple and noise from a power supply can interfere
with HART communications. Ensure that your power
supply meets the specifications shown in Table 2
below to avoid interference.
The Smart Wireless THUM adapter has been
designed to function on loops that conform to HART
requirements. Avoid interference issues by ensuring
that all wiring and equipment also conform to HART
requirements. With proper planning, a well formed
HART loop can be created and maintained, ensuring
proper THUM adapter performance.
Table 2. Power supply noise requirements for HART conformance
Maximum ripple (47 to 125 Hz)
Maximum noise (500 Hz to 10 kHz)
Maximum series impedance (500 Hz to 10 kHz)
0.2 V p-p
1.2 mV rms
10 Ohm
Certain types of solar power systems are known to
interfere with HART communications. Charge
controllers that rely on pulse width modulation may
not meet the above requirements for supply noise
and should be avoided. When choosing a solar
power system, make sure that it will not create noise
on the HART loop in excess of the limits shown in
Table 2.
For additional information about the HART protocol
and proper installation, refer to the HART
Communication Foundation website at
Other Interference Sources
Other equipment such as loop powered indicators
can also cause interference on a HART loop. Care
should be taken to ensure that all equipment
attached to the HART loop conforms to the noise
limits shown in Table 2.
Loop powered remote displays are often used on
loops where the HART sub-device is installed in an
inaccessible location. Many of these displays were
designed for standard 4-20 mA analog loops and
may create noise on the loop that has the potential of
interfering with HART communications. One example
of this is the Rosemount Model 751 LCD indicator
which should not be used on a HART loop with a
THUM. The analog version of the 751 does not
interfere with HART communication and can be used
with the THUM.
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00840-0100-4075 Rev AA, 3/11
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