H D 2 - D M - A / D M - A M
Measured Values / Parameters
Subject to reasonable modifications due to technical advances.
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erational reliability. The transmitted bit cell jitter shall not exceed 10 %
of one bit time. E. g. at 31.25 kbits/s one bit time is 32 µs long. So the
maximum bit cell jitter shall not exceed 3.2 µs. Indeed, your system may
be able to run with a higher jitter level but with a reduced level of immu-
nity against EMC influences.
H1 Active Field Devices
Provides the number of current active field devices and the associated
field device addresses as well as a target configuration for the field de-
vices.
DC Unbalance Detection (DCUD)
Detects DC unbalance between signal wire and ground (shield). This
measurement signals an earth unbalance if any segment belonging to
the same isolation group has a DC earth unbalance, e. g. short circuit
from one signal line to shield.
Definition
A DC unbalance is the result of a capacitive or resistive connection be-
tween the Fieldbus signal wires and the ground (cable shield).
Causes
Miswiring/incorrect installation: In the installation scenario illustrated in
figure 9.3 several devices have been incorrectly installed. One device
on each of two independent segments has been wired with the negative
data line tied to the shield and the shield is tied to earth ground.
Device influence: To increase EMC stability some facilities modify their
Fieldbus devices with asymmetric capacitive connections between
shield and their + or - fieldbus line. If such Fieldbus devices are connect-
ed to the plant they influence the balance of the specific field device or
the entire segment.
Wire damage: a wire damaged by external influences is also able to cre-
ate unbalance if there is short-circuit between a Fieldbus line and the
cable shield.
Type
Value
Precision
1 %
Measurement range
-100 % (short against - wire)
to
+100 % (short a wire)
