Application Note

ManualsBrandsFluke ManualsPortable OscilloscopesFluke 123

4 Fluke Corporation Using a Fluke ScopeMeter 125 to Troubleshoot FOUNDATION™ Fieldbus Installations

user can enter other limits at set

up limits on the front menu.

For Fieldbus systems, selecting

9 volts as the minimal value and

32 volts as the maximum value

will generally do the job.

During use, the Fluke 125 will

indicate via icons whether the

measured voltage is within the

limits: ✔ = OK; X = not-OK. In

addition, the icon may change

to a warning sign (!) when a

reading is within a certain per-

centage of a limit value.

Figure 5, a screen shot, shows

an actual bus-health test screen

for an H1 Fieldbus system. If the

instrument recognizes that com-

munication is taking place, the

activity indicators will blink.

The first line displays bias

voltage. OK indicates that the

dc-supply voltage (27.7 volts) is

within the limit values: 9.0 and

32.0 volts.

The Fluke 125 differs from a

standard DMM in that it displays

the voltage reading that is clos-

est to a limit over the time span

that the measurement is happen-

ing. 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 instru-

ment displays the value closest

to the lower or upper limit.

To reset the hold function,

press the Hold/Run key twice

to hold and restart the mea-

surements. This action initiates

a new measurement cycle

in which all result fields are

cleared.

The instrument can be used as

a standard DMM or as a standard

oscilloscope. Then, it can record

momentary voltages in order to

isolate instabilities in a power

supply. In addition, the instru-

ment can record supply-voltage

changes on the bus. The function

that allows this is TrendPlot

™

detailed in the instrument’s

users manual.

The hour-glass icon on the

fourth line of Figure 5 indicates

that a rise-time measurement

was in progress at the moment

the screen-copy was made. Next

to the icon is the result of an

earlier measurement and the

limit to which this rise time is

compared. In this application,

rise times up to 8 µs are viewed

as acceptable.

Because of inevitable voltage

drop based on copper resistance

and Ohm’s-law, supply voltage to

devices near the end of a trunk

are lower that for those nearer

the power supply. Making bias

voltage measurements at various

junctions can reveal disruptions,

such as poor connections, along

the trunk. Good notes and knowl-

edge of the trunk layout will help

troubleshooters in finding which

junction/spur is at fault.

In H1 Fieldbus systems, the

maximum cable length in the

trunk is 1900m. For a twisted

pair cable made from AWG#18

(1 mm in diameter or with a

cross section of 0.79 mm

per

conductor), one must take into

account a resistance of 2.26

Ω per 100m of wire, which, of

course, translates to 4.52 Ω per

100 m of dual-conductor cable

and a total resistance of 86 Ω for

the two wires over the maximum

trunk length.

If there is only a single device

drawing 25 mA connected to the

end of the trunk, that device in

itself will cause a voltage drop of

2.2 volts across the trunk cable.

With multiple devices connected

along the line, the voltage drop

due to supply-current consump-

tion can make the difference

between a good and a poor sup-

ply voltage at some devices.

Table 1 (following Figure 6)

shows the calculated supply

voltages for the network in

Figure 6, which depicts a full-

length trunk with a limited

number of devices. In the design

phase of a new network, similar

calculations should be made to

determine the type of cabling

and the power supply needed.

For existing systems, if a

plant’s archives include “as-

builts” with design data and

information about the sys-

tem’s physical layout and cable

Figure 5: Fluke 125 screen showing results of a Bus Health Test.

Figure 6: Calculation of voltage drops resulting from connected devices and their distances from the power supply.

25 mA50 mA75 mA100 mA

25 mA

1900 m

25 mA

1000 m500 m100 m0 m

0 V 0.452 V 1.81 V 2.94 V 3.96 V

Distance

DC Power

Supply

Voltage

Drop

Cable: AWG 18 per conductor (= 1 mm diameter)

Resistance: 2.26 ohm per 100 m per conductor

25 mA