Application Note

As noted, an H1 Fieldbus
network operates at a speed of
31.25 kb/s, which equals a clock
cycle of 32 µs. So, when there
is a cable anomaly, one should
expect to see reflections of pulses
delayed by an amount of time up
to 19 µs. The actual time of the
reflection depends upon the dis-
tance between the pulse source
and the anomaly.
While a complete short circuit
will cause a full-amplitude reflec-
tion, any disruption or deviation
from the homogeneous nature of
the line will generate a reflection.
The amplitude of the reflection
depends upon the nature of the
anomaly.
For proper network com-
munications, reflections should
be avoided and proper cable
termination must be main-
tained. Again, proper termina-
tion requires one and only one
terminator at each end of a trunk
section.
Encoding
With Fieldbus, digital data is
transferred using Manchester
encoding. That is, a digital 1 is
transmitted as a rising edge in
the middle of a clock cycle (bit
center), whereas a digital 0 is
transmitted as a falling edge. This
encoding mechanism has sev-
eral benefits over the transfer of
straight binary data. One impor-
tant benefit is that it allows for
an easy recovery of the clock at
the receiving end (see Figure 3).
Another consequence is, that
pulses are generated that have
duration of either a half or a full
clock-cycle, whereas the original
bit stream comes with pulses that
are one or more full clock cycles
wide. The resulting bus voltage is
schematically depicted in Figure
2. A true waveform recording of
a data package is seen in Figure
4, where the DC bias has been
filtered out.
digital data (input)
clock
Manchester
encoded data
resultant
waveform
transmitted signal (bus)
Negative going
edges
Positive going
edges
Recovered
clock
Recovered
data (output)
bias voltage +0.5 V
Transmitting side
Receiving side
bias voltage
bias voltage -0.5 V
Figure 3: Manchester encoding, transmission, and decoding
800.00
600.00
400.00
200.00
0.00 mV
-200.00
-400.00
-600.00
-800.00
Figure 4: Basic pulse-train, measured with an oscilloscope on a Fieldbus system