Application Note

Fluke Corporation
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©2008 Fluke Corporation. Specifications subject
to change without notice. Printed in U.S.A.
1/2008 3209610 A-EN-N Rev A
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Electrical signal
generation
If one were to open up the cable
at any point along the trunk and
access the two wires, he or she
would be looking into two sec-
tions of cable, going in opposite
directions away from the access
point. Electrically, those two
sections would be seen as con-
nected in parallel. The imped-
ance at any point along the line
therefore is equal to the imped-
ance of two sections of cable in
parallel. Therefore, the imped-
ance that one observes at any
junction box is 50 Ω or one-half
the characteristic impedance of
the cable.
The bus signal is created
electrically by applying a dif-
ferential current into the two-
wire bus system. By doing so,
a differential voltage of 800 or
900 mV
pp
(where “pp” stands for
“peak-to-peak”) is generated on
the bus. This is then also the
nominal peak-to-peak amplitude
(V
pp
) of the signal that any Field-
bus device generates. According
to the Fieldbus specifications,
devices should be able to gener-
ate an output signal with ampli-
tude of at least 750 mV
pp
.
Ideally, this would then also
be the amplitude of the signal
that Fieldbus devices receive.
However, because signal attenu-
ation occurs along the network,
an incoming signal will usually
have lower amplitude. Field-
bus specifications require that
a device keeps on working
properly with an input signal as
low as 150 mV
pp
. If amplitude
exceeds 1000 mV
pp
, it usually
indicates a network error, e.g. a
missing terminator.
Anomalies
If anywhere along the network
an additional low-impedance
device is connected to the bus,
the overall impedance seen at
any junction box will be lower
because the additional load is
connected in parallel to the cable
impedance. Lower bus imped-
ance automatically means lower
amplitude of the bus signals.
Since the bus signals are more
or less random pulses, such an
additional load will manifest
itself as a discontinuity in the
transmission line and will cause
reflections of the original pulses,
where the load was added. Such
reflections will lead to distortion
of the pulse’s wave shape, which,
in turn, may lead to incorrect sig-
nal detection.
If, for instance, a third termi-
nator is connected, the overall
network impedance and the sig-
nal amplitude will drop to two-
thirds of nominal. This signal loss
will lead to more distorted pulses,
making proper signal detection
more difficult. Field experience
shows that one of the most com-
mon sources of error in industrial
networks is the result of having
either too few or too many net-
work terminators connected.
“Foundation Fieldbus” is a trademark of
Fieldbus Foundation.