Engineering Documentation

Table Of Contents
Chapter 2 Network Electrical Systems
RS-485 ALN (P2/P3) and FLN (P1) Trunk Communications Wiring
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Communications Ground
See Grounding [ 16], National Electric Code (NEC) Communications Requirements
[ 18], and Controlling Transients [ 23], in Chapter 1 for definitions of NEC Articles
and Local Building Ground.
CAUTION
Buildings with unbonded electrical services should be considered separate buildings
for communications purposes. All field panel, equipment controller, and network
devices on one service should be isolated from those on another service. Failure to
isolate will result in loss of communication.
All RS-485 ALN and FLN (P1) trunks must share the same electrical service and
single building ground point. Wherever the electrical services are not bonded, as
described in NEC Article 250 or by local authorities, appropriate network devices such
as the HSTIE, Fiber Optic Trunk Interface or the Trunk Interface II should be used.
Only one side of the network device should be grounded to the single building ground
point. Network devices plugged into the field panel may be grounded to the field panel
chassis as shown in the Installation Instructions. The third wire (green or green/yellow)
from the field panel enclosure is tied to the single building ground point. Either all RS-
485 FLN (P1) equipment controller power trunk neutrals must be tied to the single
building ground point or network isolation devices must be used.
Multi-Drop Trunk Terminator
The Multi-Drop Trunk Terminator (P/N 538-664) consists of a 120-ohm resistor in
series with two opposing polarity diodes placed in parallel. See Figure Multi-Drop
Trunk Terminator.
The Multi-Drop Trunk Terminator is required at each end of a 19.2K bps ALN or FLN
(P1) trunk segment. See Figure ALN Trunk Terminator Requirements.
CAUTION
No more than two trunk terminators should be used on a single trunk segment. Using
more than two can cause unpredictable results.
NOTE:
While Trunk Terminators are required only on RS-485 ALN or FLN trunks running
over 19.2K bps, due to accumulated cable distortion, Trunk Terminators are
recommended on any RS-485 ALN or FLN trunk at 9600 bps if old style TIEs are
installed (silver enclosures) or if the trunks are over 4000 ft (1219 m) in total length.