Rockwell Automation 1770 DH,DHII,DH485 CABLE - Manual (Page 79)

Planning a DH II Cable System
Chapter 5
5-3
Trunkline
The trunkline is the bus, or the central part of the network cable system. The
length of the trunkline on your network depends upon the number of nodes you
require, however, the maximum length is 8,000 feet (2440 meters).
Each node on the trunkline reduces the 8,000 feet (2440 meters) of available
trunkline length by 75 feet (22.9 meters).
For example, if your network requires 20 nodes, the maximum total length of
your trunkline is:
8,000 - (20 x 75) = 6,500 feet or
2440 - (20 x 22.9) = 458 meters.
The trunkline on your network is divided into several segments by taps. The
cable length of the trunkline is equal to the total cable length of the trunkline
segments. Each cable segment must be at least 30 feet (9.2 meters) in length
between taps.
The trunkline is constructed of RG-11 type coaxial cable (Allen-Bradley part
no. 94082901). This cable has a foil shield and a braided shield. (Figure 5.2).
Figure 5.2
The RGĆ11 type coaxial cable has a foil shield and a braided shield.
Insulating Jacket
Braided Shield Foil Shield
Dielectric
Inner
Conductor
16047