Engineering Documentation

Chapter 2 – Network Electrical Systems
RS-485 ALN (P2/P3) and FLN (P1) Trunk Communications Wiring
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Multi-Drop Trunk Cabling Limits
The following table provides the maximum wiring distances per 2-wire RS-485 trunk
section. At bit rates over 9600 bps, no stubs or tees are permitted in the trunk cabling.
A Trunk Terminator is required at each end of the trunk section at speeds over 9600
bps. See Figure Multi-Drop Trunk Terminator [➙ 49].
Distance per 2-wire Trunk Section.
1,2
(Using Recommended Cabling—Based on Cable Wire to Wire Capacitance.)
Speed and Maximum Distance
Trunk Type 4800 bps Max.
Distance
9600 bps Max.
Distance
19.2K –
57.6K bps
Max. Distance > 57.6K bps Max.
Distance
ALN Trunk 18 AWG 10,000 ft
(3048 m)
18 AWG 4,000 ft
(1219 m)
N/A
ALN Trunk 20 AWG
4,000 ft
(1219 m)
20 AWG
4,000 ft
(1219 m)
ALN Trunk 24 AWG 4,000 ft
(1219 m)
24 AWG 4,000 ft
(1219 m)
24 AWG
(Low Cap)
4,000 ft
(1219 m)
24 AWG
(Low Cap)
3,280 ft
(1 km)
FLN Trunk 18 AWG 5,000 ft
(1524 m)
N/A
FLN Trunk 20 AWG
4,000 ft
(1219 m)
FLN Trunk
3
24 AWG 4,000 ft
(1219 m)
24 AWG 4,000 ft
(1219 m)
24 AWG
(Low Cap)
4,000 ft
(1219 m)
24 AWG
(Low Cap)
3,280 ft
(1 km)
1)
A trunk section is referenced as a length of cable that is electrically isolated from another cable. Electrical isolation is obtained with
network devices such as HSTIEs, TI2s, and Fiber Optic TIs.
2)
The maximum amount of cable per logical trunk may be extended beyond the maximum physical trunk segments limits shown in
this table via network devices, such as the HSTIE or TIE, that function as Trunk Extenders. See HSTIE Usage in this chapter for
more information.
3)
Reduce the FLN trunk length by 20 feet (6 m) for every BACnet TEC on the FLN above 150 devices.