IM702-3
13
A network segment
is any part of the free topology network in which each conductor is electrically continuous. Each of the
four diagrams in Figure 9 is an illustration of a network segment. Some applications may require two or more segments (see
Free Topology Restrictions section). If necessary, segments can be joined with FTT-10A-to-FTT-10A physical layer
repeaters. See Figure 10. Refer to Echelon LonWorks FTT-10A Transceiver User’s Guide. See Reference Documents for
part number.
Figure 10. Combining Network Segments With a Repeater
Termination
Termination
FTT-
10A
FTT-
10A
Free Topology Restrictions
Although free topology wiring is very flexible, there are restrictions. Refer to the Echelon FTT-10A User’s Guide for
details. See Reference Documents for part number.
1.
The maximum number of nodes per segment is 64.
2.
The maximum total bus length depends on the wire size (see Qualified Cables for details)
Wire Size
Maximum Node-to-Node Length
Maximum Cable Length
24 AWG
820 ft (250 m)
1476 ft (450 m)
22 AWG
1312 ft (400 m)
1640 ft (500 m)
16 AWG
1640 ft (500 m)
1640 ft (500 m)
The longest cable path between any possible pair of nodes on a segment must not exceed the maximum
node-to-node
distance. If two or more paths exist between a pair of nodes (e.g., a loop topology), the longest path should be
considered. Note that in a bus topology, the longest node-to-node distance is equal to the total cable length.
The total length of all cable in a segment must not exceed the maximum total cable length.
3. One termination is required in each segment. It may be located anywhere along the segment.
Doubly Terminated Networks
You can extend the maximum total cable length without using a repeater by using doubly-terminated network topology. See
Figure 11. The trade-offs are (1) this network topology must be rigorously followed during the installation and subsequent
retrofits and (2) two terminations must be installed at the ends of the bus for proper transmission performance. Refer to
Echelon LonWorks FTT-10A Transceiver User’s Guide. See Reference Documents for part number.
Note:
Limitations to wire lengths apply and must be observed.
Figure 11.
Doubly Terminated Network Topology
Termination
Termination
