Application Note

2 Fluke Corporation Using a Fluke ScopeMeter 125 to Troubleshoot FOUNDATION™ Fieldbus Installations
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. If there is a
pattern, is the cause of the pat-
tern evident?
If modifications have been
made or devices added recently,
make measurements at these
places, too. Try to determine
which segments, if any, of the
network have problems and
which are problem free.
A number of measurements
will help find discrepancies
and thereby help us identify
problems. Such measurements
include:
•
Capacitances and resistances
in and between conductors
•
Improper shield and conductor
contacts
•
DC voltages
•
AC signal levels
•
Noise and signal quality.
These measurements using the
Fluke 125 ScopeMeter test tool
are discussed in more detail in
what follows.
The Fluke 125 is a compact,
portable combination oscil-
loscope and digital multimeter
(DMM) that offers dedicated
analysis capabilities for trouble-
shooting industrial bus systems.
Being a battery-operated
instrument, the 125 can make
so-called floating measurements
in which neither point of the
measurement instrument is at
ground (earth) potential. This
capability ensures that the float-
ing nature of the network will
be maintained, whereas other
oscilloscopes may introduce
unwanted connections to earth-
ground through their safety,
ground-referenced input contacts
or through large capacitors in
their power supplies. Such con-
nections in themselves could
disrupt a network’s integrity and
easily block communications.
The Fluke 125 supports the
storage of scope screens in
internal memory. These screens
can be copied into reports just
as has been done (see later) in
this application note. Instrument
settings used are also stored
with the stored screens, and the
instrument also allows names to
be added to screen copies.
Test connections
Most of the measurements car-
ried out along the trunk of a
Fieldbus network require that
the hot input of the instrument’s
channel A (marked “A”), and the
ground reference contact (“COM”)
get connected to the positive and
negative conductors of the bus
wiring. The most common color
coding of Fieldbus compliant
cables is orange for positive and
blue for negative.
Occasionally, troubleshooting
requires measuring the voltage
of one or the other conductors
over ground. For such measure-
ments, the shield can be taken
as the ground-reference contact.
However, the shield of the cable
should not be connected to the
chassis or earth ground at the
location of the device.
Bear in mind that the structure
of the Fieldbus assumes a float-
ing operation over ground. So,
neither of the wires should be
connected to earth-ground any-
where in the system. Should an
investigation uncover a ground
contact, then it should be treated
as a likely source of network
trouble. According to wiring and
installation specification from
the Fieldbus Foundation, cable
shielding is to be grounded only
once in a trunk section at the
control room side of the trunk.
Access to the trunk wires can
be gained easily at the junction
boxes where spurs connect to
the trunk, or at the terminals of
devices. The junction boxes typi-
cally used in Fieldbus networks
are built around screw terminals.
(See Figure 1). Measuring at
junction box terminals means not
having to make any changes to
the cable structure. Furthermore,
the schematic and accompany-
ing text on many of these boxes
permit unambiguous conductor
identification.
The screws of the terminals
make a good place to apply a
STL120 shielded test-lead tip and
a TL75 reference lead. Both are
standard items with the Fluke
125 ScopeMeter. (See Figure 2.)
In case of heavy noise from
the environment, a shorter
ground lead with an alligator
clip (Figure 2, center item) will
help reduce the amount of noise
recorded. When using this lead,
connect the alligator clip to the
negative trunk wire. This shorter
lead is also a standard item with
the Fluke 125.
One alternative for making
test connections is the optional
HC120 hook clip (Fgure 3), which
allows one to hook the STL120
tip to the actual conductor of the
cable. Another alternative con-
nection method uses the optional
TP88 back probe-pins (Figure 4),
which can be used to probe the
screw terminals at wire entry
points. These long, thin needles
allow easy access to points
crowded with wires that are
difficult to reach using standard
test pins.
Cable hardware
verification
When a network is down and the
problem is difficult to identify, it
often pays to start with verifying
the cable installation. A problem
may stem from the environment
Figure 2: The STL120 (red) and TL75 (black) test leads are the
primary tools for making test connections onto a FOUNDATION
Fieldbus. When encountering heavy noise on the bus, try the shorter
lead with the alligator clip (center Item). All three items are standard
with the Fluke 125 Scopemeter.