Application Note

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Application Note

Using a Fluke ScopeMeter

125 to Troubleshoot

FOUNDATION

™

Fieldbus Installations

From the Fluke Digital Library @ www.fluke.com/library

1) Disruptive environmental

conditions, which include

mechanical vibrations, extreme

temperature changes, high

humidity levels and poor air

quality caused by chemicals,

dust and other agents. These

conditions can create loose

or intermittent connections,

corrosion in conductors and

junction boxes and/or changes

in impedance.

2) Electrical disturbances come

from a variety of sources.

Breakers turn high-energy

circuits on and off generating

transients. Belt conveyors and

mechanical drives discharge

high-voltage static electricity

into electronic systems. Load

changes on branch circuits

create fluctuations in supply

voltages. And there are still

other possible sources of elec-

trical disturbances.

Both types of disturbances

can temporarily or perma-

nently adversely affect system

components—terminators, input

components and cabling. The

result often is a disruption of the

millivolt signals upon which pro-

duction processes rely. Therefore,

it makes great sense to avoid

potential process communications

problems and pinpoint exist-

ing problems by monitoring and

Industrial fieldbus networks operate under very

different conditions than do office and other commercial

communications networks. In the industrial world,

external influences can play havoc with sensitive

electronic devices such as PLCs, network controllers

and other instruments supporting process control. These

external, industrial influences fall into two categories:

troubleshooting industrial digital

communications systems using an

oscilloscope.

The following discussion

focuses on the monitoring of

a specific type of system, FOUN-

DATION Fieldbus 31.25 kb/s

(H1) networks, using a new

instrument designed and

programmed for such monitor-

ing: the Fluke 125 Industrial

ScopeMeter

test tool.

(Note: The Fluke 125 is not limited to moni-

toring just Fieldbus H1 networks.)

Troubleshooting

procedures

When troubleshooting a Fieldbus

system, first attempt to document

recent changes to the system:

Have any devices or any parts

of the network recently been

disconnected? Was anything

added or modified just before the

trouble began?

Determine what’s working

and what’s not. Make notes

about what is observed versus

what was expected. Investigate

whether certain disturbances can

be traced back to specific events:

a motor starting, a valve opening,

a light being turned on, etc.

Next, make measurements to

“look into” the network in order

to see and understand what’s

going on. Carefully document

each measurement: What was

Figure 1: A typical FOUNDATION Fieldbus junction box

with wiring diagram.

Summary of content (7 pages)

PAGE 1
Using a Fluke ScopeMeter 125 to Troubleshoot FOUNDATION™ Fieldbus Installations Application Note Industrial fieldbus networks operate under very different conditions than do office and other commercial communications networks. In the industrial world, external influences can play havoc with sensitive electronic devices such as PLCs, network controllers and other instruments supporting process control.
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measured? Exactly where was the measurement made? Under what conditions was it made? To start, measure at both ends of the trunk and compare the results. Next, measure at one or more locations along the trunk and compare the results. If only one device has a problem, make measurements near that device. If multiple devices have problems, then try to determine if there is a pattern.
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Figure 3: HC120 Hook Clip When a Fluke 125 is the test instrument, the test involves connecting the TL75 lead to the shield connector in control room and applying the hot tip of the STL120 to the A and the B wires respectively. Depending upon the length of the trunk, it may take a few seconds for a capacitance reading to stabilize. When a reading is stable, record it. contact near a screw terminal, or it might be an indication of a cable defect or the result of moisture in a junction box. 4.
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The Fluke 125 differs from a standard DMM in that it displays the voltage reading that is closest to a limit over the time span that the measurement is happening. The instrument has a hold function that helps installation and maintenance personnel determine if the supply voltage is at risk of going outside of the preset limits. In other words, during load changes, the instrument displays the value closest to the lower or upper limit.
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Device 1 Device 2 Device 3 Device 4 Distance from supply 100m 500m 1000m 1900m Length of section (single conductor) 100m 400m 500m 900m Resistance of section 4.52 Ω 18 Ω 22.6 Ω 40.7 Ω Total copper resistance 4.52 Ω 22.6 Ω 45.2 Ω 85.88 Ω Device current 25 mA 25 mA 25 mA 25 mA Total current 100 mA 75 mA 50 mA 25 mA Voltage drop in section 0.45 V 1.36 V 1.13 V 1.02 V Total voltage drop 0.45 V 1.81 V 2.94 V 3.
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Signal quality and noise In general conversation, signals on a bus are referred to as “digital signals” as though they change state from low to high almost instantaneously. In reality, that is not the case. For some types of networks, the speed of signal transitions is quite critical. For Fieldbus networks, transition speeds, as such, are not so critical.
PAGE 7
munication problems do not have to be close to each other for the former to affect the latter. Figure 9, another screen capture, depicts a somewhat noisy bus signal. In such cases, check the noise level at the middle or baseline level on the left of the screen (just below the “A” trace marking). This segment of the waveform represents the steadystate voltage on the bus just before the capture of any data packets.