6-4
Section 6 F
OUNDATION
Fieldbus Guidelines
Installation and Operation Manual
X-DPT-FF-MT3809G-Alarms-eng
Part Number: 541B218AAG
December, 2017
Model MT3809G F
OUNDATION
TM
Fieldbus
For new installations, ask cable vendors for "fieldbus cable".
When using shielded cable, connect each spur's shield to the trunk
shield and connect the overall shield to ground at one point.
The grounding point has been chosen as the junction of the field
instruments. For intrinsically safe (I.S.) installations, a specific location may
be required for the ground.
DC Power for Two-Wire Field Devices
Some field devices draw operating power fromthe fielbus network in much
the same way as 2-wire analog field devices. The power supply is
connected to the network in the same way as a field device )or other
communicating device). All of the same rules apply, except that you don't
have to count the power supply as one of the 32 field devices. If we add a
power supply to the network of Figure 6-3, it might look something like that
of Figure 6-6. Another spur near the FFI has been created to add the
power supply. Of course we could have put it toward the field end of the
trunk.
We can't use just any off-the-shelf power supply, because it would
short circuit the (digital) fieldbus signals.
The power supply is specially designed for fieldbus. Some fieldbus
equipment will have a built-in power supply so that you don't need to add
one. You should consult manufacturers' specifications.
If you have 2-wire field devices in your network, you have to make sure
they have enough voltage to operate. Each device should have at least 9
volts.
You need to know:
1. The current consumption of each device.
2. Its location on the network.
3. The location of the power supply on the network.
4. The resistance of each cable section.
5. The power supply voltage.
The voltage at each field device is determined through straightforward DC
circuit analysis.
Refer to F
OUNDATION
TM
Fieldbus installtion guides for details on
determining power requirements.
Figure 6-6 Adding a Power Supply to the Fieldbus Network
F
OUNDATION
Fieldbus
Interface
FFI
Wire Pair
Signals
Terminator
Terminator
T
T
FD
FD
FD
FD
FD = Field Device
Fieldbus
Power
Supply
+
-
