6-2
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
In Figure 6-2 more field devices have been added to the network of Figure
6-1. These new devices are simply connected in parallel with the first field
device. The new devices are shown connected in a "star" fashion.
However, they could also be chained from the first device as in Figure 6-3.
All of the field devices and the FFI in Figures 6-2 and 6-3 are said to be "on
the same network". A detailed figure showing the wiring of the junction of
Figure 6-3 is given later.
You can see that new devices are always added in parallel to existing
ones. Notice that the number of terminators in Figures 6-2 and 6-3 stays at
two, regardless of what else we add to the network.
Speaking of terminators, how did we know where to put them?
To answer this we need to define a trunk. A trunk is the longest cable path
between any two devices on the network. Once we've identified this stretch
of cable, all other connections to it are called spurs.
The terminators should be placed at the ends of the trunk. We've assumed
in Figures 6-2 and 6-3 that the FFI is further from the group of field devices
(in cable length) than they are from each other.
This rull on location of terminators is one that can be bent. In Figure 6-2,
for example, we haven't bothered to find the longest cable path. Instead,
the terminator was placed at the junction of the group of field devices.
We've assumed that all of the field devices were about the same distance
(in cable length) from the junction. Had one of them been a lot longer than
the others, then we would move the terminator out to that device.
In Figure 6-3 we kept moving the terminator out to the farthest field device
each time we added a device. However, if we are adding to an existing
network and all of the field devices to be added are located on one short
stretch of cable (100 m (328 ft.) or less), then the terminator could have
been left in its original position at the first device.
Spurs: Shorter is Better!
Repeaters
What if you need a lot more than 1900 m (6232 ft.) of cable? You can do it
by using a repeater. The repeater takes the place of one of the field
devices. But it means you get to start fresh. You can add another 1900 m
(6232 ft.) of cable, as illustrated in Figure 6-4. Notice that a new trunk has
been created so that we have to add more terminators. The first trunk has
Figure 6-3 Fieldbus Network with Chained Devices
F
OUNDATION
Fieldbus
Interface
FFI
T
T
FD
FD
FD
FD
Terminator
Terminator
FD = Field Device
