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Supplement to HART

Communicating Fisher

FIELDVUE

Instrument Instruction Manuals

Audio Monitor for HART

Communications

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.

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 either a 375 Field

Communicator or a PC running AMS



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

BANANA PLUGS

MINI AMPLIFIER/SPEAKER

INPUT JACK

0.1 μF DISC

CERAMIC CAPACITOR

470K OHM RESISTOR

WIRE TIE

PC BOARD

1/8-INCH

PHONE PLUG

W6943/IL

Figure 1. Audio Monitor for HART



Communications

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 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 375 Field

Communicator has a broadcasting mode called the

Instruction Manual Supplement

D103265X012

February 2009

FIELDVUE



Instruments

Summary of content (4 pages)

PAGE 1
Instruction Manual Supplement D103265X012 February 2009 FIELDVUE Instruments Supplement to HARTR Communicating FisherR FIELDVUER Instrument Instruction Manuals Audio Monitor for HARTR Communications 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.
PAGE 2
Instruction Manual Supplement FIELDVUE Instruments “shout” mode. If it doesn’t receive an answer to its initial “question”, the 375 will ask again with a larger signal strength. Demonstrate this by connecting the audio monitor to the 375 without a FIELDVUE digital valve controller attached. Power on the 375 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.
PAGE 3
Instruction Manual Supplement FIELDVUE Instruments February 2009 INPUT JACK 0.1uF CAPACITOR RED WIRE 470K OHM RESISTOR RED BANANA PLUG BLACK WIRE 1/8” PHONE PLUG BLACK BANANA PLUG TO FIELDVUE DIGITAL VALVE CONTROLLER OUTPUT TO SPEAKER MINI AMPLIFIER/SPEAKER Figure 2. HART Communications Audio Monitor Circuit Diagram 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).
PAGE 4
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