Application Note

Bus
Signal
Data Signal
Amplitude ( V
pp
)
DC-supply voltage
or
Bias Voltage ( V
DC
)
0 V
WN
e.g. 800 mV
pp
e.g. 24 V
DC
Figure 2: The voltage on the Fieldbus includes a DC-supply voltage and the actual bus signal.
work. The maximum length of
all the wiring in a trunk and its
spurs added together is 1900 m
(approx. 6250 ft) per section. If
more length is required, one can
add a section using a repeater.
A repeater takes the place of a
device, but adding it allows for
another 1900 m of cable. A net-
work may use as many as four
repeaters for a total length of
9500 m (31000 ft.).
Note that the shielding is con-
nected to earth-ground at only
one point in the entire system,
and that is important. Grounding
the shielding at multiple places
can induce stray voltages and
currents in the shielding, which
can interfere with data commu-
nications.
The maximum number of
connected field bus devices per
section is 32.
As shown in Figure 1, a DC
power source is required to pro-
vide DC supply or bias voltage. If
the DC power source were con-
nected directly to the trunk, it
would create a short circuit for
the AC signals. Consequently, a
network must have a Fieldbus
compliant power supply, which
is a DC source plus a dedicated
filter arrangement. The filter lets
DC current pass with minimal
losses but creates high imped-
ance for the AC signal coming
from the network side.
The trunk, then, is a transmis-
sion line, on which the propa-
gated speed of AC signals plays
an important role. Thus, the trunk
must be properly terminated at
each end (and only there) for AC
signals. Termination is accom-
plished using a resistor with
impedance equal to the charac-
teristic impedance of the cable,
usually 100±20 Ω. Given that the
network also carries a DC sup-
ply voltage, the terminators must
have a series capacitor to prevent
any DC current from flowing
there.
Diagnostic basics
Certain basic diagnostic and
troubleshooting procedures can
be performed on an H1 Fieldbus
network using a Fluke Scopeme-
ter. In the following section, we’ll
discuss the basics of some of
these. More details can be found
in a dedicated Application Note
‘Using a Fluke ScopeMeter 125
to troubleshoot Fieldbus Installa-
tions’.
Detecting reflections
So-called reflections on a net-
work affect communications. In
the following example, a reflec-
tion is explained for a network
that is short circuited at one end.
However, it is important to under-
stand that any anomaly, including
short circuits and poor termina-
tions, will create reflections.
Consider what will happen
when a step voltage is applied to
one end of a long cable in which
the other end is short-circuited.
Initially, the applied voltage will
encounter the cable’s impedance
and will build up a voltage level
between the conductors. This step
voltage will travel through the
cable at a speed determined by
the type and construction of the
cable. For cables used in H1 Field-
bus networks, that speed is about
two-thirds the speed of light in a
vacuum: 2/3 x 3 x 10
8
m/s = 2 x
10
8
m/s, or approx. 660 x 10
6
ft/s.
When the step voltage reaches
the short circuit, the voltage level
will suddenly change to zero.
This change may be viewed as
a step-voltage of opposite polar-
ity, back to zero, because there
can be no voltage across a short
circuit. At that point, the voltage
level anywhere else along the line
is still the voltage level originally
applied.
Next, this new opposite-
polarity step voltage travels back
toward the voltage source. Only
once it has made the round trip
(has been reflected back), will the
short circuit at the other end be
apparent on the input side. But
the reflecting process does take a
certain amount of time. How much
time it takes will depend upon the
length of the cable. Travel time
in one direction will be the cable
length divided by the speed of the
signal.
The amount of time it would
take a step voltage to travel down
a maximum-length trunk and back
is thus 2 x 9.5 µs = 19 µs.
For the maximum length of an H1 Fieldbus
section, the time, t, is
t = {1900 m ÷ (2 x 108m/s)} = 9.5 µs.