Rockwell Automation 1770 DH,DHII,DH485 CABLE - Manual (Page 104)

Planning a DHĆ485 Cable System
Chapter 8
8-3
The trunkline is constructed of Belden 9842 cable. This cable is jacketed and
shielded, and consists of two twisted wire pairs and a drain wire. One pair of
twisted wires provides a balanced signal line, and one wire of the other pair
serves as a common reference line between all nodes on the link. The shield
reduces the effect of electromagnetic noise from the industrial environment on
the link communications. The drain wire provides a shield connection point.
Refer to Chapter 9 for information on installing the trunkline.
Link Couplers
Link couplers connect devices to the DH-485 network. You must install a link
coupler for each non-isolated node on the network. The type of coupler depends
upon the device that you are connecting to the DH-485 network.
Refer to Chapter 9 for information on installing link couplers.
To install a DH-485 network, you need the following tools for connecting the
data link components together. These tools are used to strip the cable and to
attach the cable to the link couplers
Wire stripper
Wire cutter
The trunkline on your network is divided into several connected segments that
form a daisy chain configuration. The cable length of the trunkline is equal to
the total cable length of the trunkline segments.
Important: When determining the cable length of trunkline segments, be sure
to measure the actual cable path as it is routed in your network. Be sure to
consider vertical dimensions as well as horizontal dimensions. You should
always calculate the three-dimensional path distance when determining
cable lengths.
How you configure your DH-485 network determines the lengths of all
trunkline segments.
Important: Remember that the maximum total cable length of the trunkline is
4,000 feet (1220 meters).
Selecting the shortest path for routing the cable will help minimize the amount
of cable you will need. The specific details of planning such a cable route
depends upon the needs of your network.
Necessary Tools
Guidelines for Determining
Cable Length