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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RS-485 ALN Trunk Shield Connection Using 2-Wire
Cabling
NOTE:
ALN trunk terminal "S" is grounded or connected to the field panel case. It is used
on
ly to provide a shield connection for the ALN trunk cable. NEC Article 800 does not
allow a communication cable to provide a ground path between equipment chassis.
The Figure
RS-485 ALN Trunk Shield Connection shows how the ALN trunk shield is
connected to only one field panel marked "OUT" and is tied back at the field panel
marked "IN".
Fig. 22: RS-485 ALN Trunk Shield Connection.
1. The “S” pin of the PXC-C and PXC-M must be left open, see NOTE.
2. The “E” pin of the MEC and the pin of the PXC Compact and PXC Modular
must be tied to earth ground.
3. The “S” pin of the MEC, MBC, SCU, and FLNC is earth grounded so the shield
conductor can be connected there.
NOTE:
The equipotential symbol ( ), is the symbol being used to represent RS-485
communications common reference.
Early versions of some 3-Wire controllers (PXC Compact, PXC Modular, P1-BIM,
Long Platform BACnet TECs and BACnet Equipment Controllers) may show the
earth ground symbol ( ) or the "S" designation.