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Instruction Manual Supplement
D103265X012
FIELDVUE Instruments
May 2019
Audio Monitor for HART® Communications, for
HART Communicating FIELDVUE
The frequencies used for HART communications are in the audible range—you can hear them! The 1200 and 2200 Hz
waveforms sound similar to the sounds you hear from a FAX machine's phone modem. This document outlines how to
assemble a device to monitor these audible waveforms and a few suggestions on how to use it for troubleshooting.
The audio monitor is shown in figure 1. The cable can also be used as an input to a cassette recorder if recordings of
the HART messages are desired.
Figure 1. Audio Monitor for HART Communications
MINI AMPLIFIER/SPEAKER
INPUT JACK
0.1 μF DISC
CERAMIC CAPACITOR
470K OHM RESISTOR
WIRE TIE
BANANA PLUGS
PC BOARD
W6943
1/8‐INCH
PHONE PLUG
™
Instruments
HART Protocol—A Little Background
The HART communication protocol has a “question‐answer” format; the HART master asks a question and the HART
slave answers back. The slave will not communicate unless a master first asks a question. With the audio monitor
powered up and attached to a FIELDVUE instrument which is communicating with a 475/375 Field Communicator or a
PC running ValveLink software or AMS Suite: Intelligent Device Manager, a series of “squawks” are heard. Often the
pattern heard is “dot dot dash ...dot dot dash...dot dot dash...” that occur at intervals of about once a second with a
slight pause in between. The first “dot” is the master asking the question and the remaining “dot dash” is the
instrument's response.
It is possible to have two HART masters communicating simultaneously with one HART slave. This is done by having
one master be “primary” and the second master be “secondary”. The HART protocol provides time for each master to
alternate making “question‐answer” exchanges with the slave. The pattern heard would then be “dot dot dash dot dot
dash dot dot dash dot dot dash” with no pauses in between exchanges. Assuming the communications are within
HART waveform specifications, no communication errors should normally occur.
Some analog output circuits used to generate the 4 to 20 milliamp control signal are adversely affected by the HART
communications waveforms. The usual result is that the impedance of the output circuit is substantially reduced
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FIELDVUE Instruments
May 2019
during communications. Good HART communications depend on the master device creating the waveform in a circuit
where the load from the FIELDVUE instrument and the current source are roughly equal such that the signal is uniform
throughout the loop. If the output circuit's impedance drops, the HART waveform is unevenly split (as in a voltage
divider) and the signal received by the FIELDVUE instrument is reduced. The HART filter blocks the HART waveform
from being detected by the analog output circuitry and prevents the source impedance from dropping.
Another thing to understand is the Field Communicator has a broadcasting mode called the “shout” mode. If it doesn't
receive an answer to its initial “question”, it will ask again with a larger signal strength. Demonstrate this by
connecting the audio monitor to the Field Communicator without a FIELDVUE digital valve controller attached. Power
on the Field Communicator and listen to the HART messages. The pattern will be something like this:
“dot...dot...DOT...DOT...DOT” where the capitalization indicates the louder attempt.
Instruction Manual Supplement
D103265X102
Troubleshooting Communications Problems with the Audio
Monitor
Several HART communication problems can be identified with use of the audio monitor, a little knowledge of the
HART protocol, and a little experience by the user in hearing differences between normal HART communications and
faulty ones.
1. No response by slave device—Polling by a master device that is not answered by a slave device would sound like “
dot...dot...dot...” and would continue for only a few times before the master device would display a message
stating that no device was found. Corrective action would be to determine why slave is not communicating.
2. Two masters of the same gender—If two masters set to the same “gender” (primary or secondary) were trying to
communicate with the same slave device, both would try to share their “half” of the bandwidth. The basic pattern
would still be “dot dot dash ... dot dot dash...” as if there were only one master but there might be some distortion
or extra characters as the messages overlap. The pause in the middle would still be present. The master device
might suffer communications errors or stop updating its readback values while in conflict. Corrective action would
be to have only one primary and/or one secondary HART master communicating at a time.
3. Current loop might need a FIELDVUE HART filter—To determine if a current loop needs a HART filter, configure
ValveLink software as “primary” master and connect both it, a Field Communicator, and the audio monitor to a
FIELDVUE instrument powered by the current loop in question. Start both masters communicating to the FIELDVUE
instrument (something that keeps asking for information, such as displaying the status of the instrument condition
information). Listen to the waveforms on the audio monitor. If there are no “Communication Errors” displayed on
the masters and the pattern sounds like “dot dot dash dot dot dash dot dot dash dot dot dash”, things are probably
OK without a filter. If the pattern sounds like “dot dot dash DOT DOT DASH dot dot dash DOT DOT DASH” where
one series is noticeably louder than the other or like “DOT DOT DASH...DOT DOT DASH...” where it sounds as if
there is only one master, a HART filter should be tried and the test repeated. The louder sound or presence of only
one master's messages where two should be present indicates that the Field Communicator is operating in the
“shout” mode to overcome a poor HART circuit.
4. Noise on the HART circuit—If noise is present on the HART circuit, depending on the frequency, it can sound like
clicks or static, tones which last for a period of time, or a wavering of the existing HART signal. Listen both while the
HART messaging is occurring and while there is no HART communication (to sense the background noise). The
main thing about listening to noise is that it is most likely random in nature but may be due to system layout,
construction methods, or the unshielded operation of nearby equipment. For trouble shooting purposes, use the
audio monitor before and after corrective action to reduce noise as a way to detect whether the change is
beneficial.
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Instruction Manual Supplement
D103265X012
FIELDUVE Instruments
May 2019
Construction
Figure 2 is the circuit diagram for the audio monitor. Parts for constructing the audio monitor are listed under Parts
List.
Figure 2. HART Communications Audio Monitor Circuit Diagram
INPUT JACK
RED WIRE
BLACK WIRE
1/8” PHONE PLUG
OUTPUT TO SPEAKER
MINI AMPLIFIER/SPEAKER
0.1uF
CAPACITOR
470K OHM
RESISTOR
RED BANANA PLUG
BLACK BANANA PLUG
TO FIELDVUE DIGITAL
VALVE CONTROLLER
Cut the circuit board material down to approximate dimensions listed in the parts list. Solder the capacitor, resistor
and input and output wires together on the circuit board. Secure the input and output wires to the circuit board with
the wire ties. Cover the whole assembly with electrician's tape.
Allow about one foot of wire on the output side and attach the 1/8” phone plug (red wire to the center post, black wire
to shield). On the input side allow about 3 feet of wire and attach the red banana plug to the red wire and the black
banana plug to the black wire.
The 1/8” phone plug is connected to the input jack of the amplifier/speaker box. The banana plugs are either
connected to the Field Communicator cable or are used with alligator clips or mini‐grabber‐style clips and connected
to the “TALK +/-” terminals of the FIELDVUE instrument.
Parts List
Quantity
DescriptionRequired
Mini Amplifier/Speaker 1
0.1uF ceramic disc capacitor 1
470K ohm carbon resistor 1
General‐purpose
Component PC Board 0.5” x 1.75”
Quantity
DescriptionRequired
18AWG two‐conductor
stranded wire 4 feet approx
Banana plugs 1 pair
1/8” Phone Plug 1
Nylon wire ties, 4 inch long 2
Electrician's tape as needed
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FIELDVUE Instruments
May 2019
Instruction Manual Supplement
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Documents are available from your Emerson sales office, Local Business Partner, or at Fisher.com.
Neither Emerson, Emerson Automation Solutions, nor any of their affiliated entities assumes responsibility for the selection, use or maintenance
of any product. Responsibility for proper selection, use, and maintenance of any product remains solely with the purchaser and end user.
Fisher, FIELDVUE, and ValveLink are marks owned by one of the companies in the Emerson Automation Solutions business unit of Emerson Electric Co.
Emerson Automation Solutions, Emerson, and the Emerson logo are trademarks and service marks of Emerson Electric Co. HART is a registered trademark of
FieldComm Group. All other marks are the property of their respective owners.
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. All sales are
governed by our terms and conditions, which are available upon request. We reserve the right to modify or improve the designs or specifications of such
products at any time without notice.
Emerson Automation Solutions
Marshalltown, Iowa 50158 USA
Sorocaba, 18087 Brazil
Cernay, 68700 France
Dubai, United Arab Emirates
Singapore 128461 Singapore
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E 2006, 2019 Fisher Controls International LLC. All rights reserved.
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