Install Instructions

BACNET® FIXED FUNCTION THERMOSTAT
3 31-00093—03
WARNING
Do not apply line voltage to source pins.
A jumper is pre-installed between pins 1 and 2. The
jumper supplies 24 VAC to BO-1, BO-3, and BO-4 relays. It
can be removed if those BOs are powered from an external
source. (Note: BO-0, BO-2, and BO-5 relays are powered
from terminal 1.)
See BACnet Fixed Function Thermostat System
Engineering Guide (31-00098) for typical wiring
examples.
MS/TP LAN Wiring
The BACnet FF communicates on the site-wide BACnet
system over a twisted-pair MS/TP LAN, which uses the
EIA–485 signaling standard. The BACnet FFs are master
devices on the MS/TP LAN.
Each BACnet FF employs a high-quality EIA–485
transceiver and exerts 1/4 unit load on the MS/TP LAN.
Terminating MS/TP LAN Cabling
MS/TP terminations are located on the lower left of the
BACnet FF wallplate.
Maintain polarity of the MS/TP wire run throughout the
MS/TP LAN.
Grounding the MS/TP LAN Shield
Proper shield grounding of the MS/TP cabling can help
minimize the risk of communications problems and
damage to equipment because of transient voltage spikes
(for example, lightning strikes).
Follow these guidelines for grounding MS/TP cable
shields:
• Each MS/TP segment should have a single point of
shield ground, preferably as close to the middle of the
cabling run as possible.
• Do not ground the MS/TP shield using a BACnet FF
terminal.
Table 1. Terminal Identification.
Terminal Label Connection
1 24VAC 24 VAC Power
2 RELAY 24VAC 24 VAC Power
3 BO0 Relay Output
4 BO1 Relay Output
5 GND Ground
6 AI0 Universal Input
7 COM Common
8 AI1 Universal Input
9 COM Common
10 AI2 Universal Input
11 MS/TP - BACnet ® Communications
12 MS/TP + BACnet ® Communications
13 AO1 Analog Output
14 COM Common
15 AO0 Analog Output
16 GND Ground
17 BO2 Relay Output
18 BO3 Relay Output
19 BO4 Relay Output
20 BO5 Relay Output
Table 2. MS/TP LAN Facts.
Transmission
speed
9.6, 19.2, 38.4, 76.8Kbps (configured at
global controller).
Layout Bus (daisy chain).
Cabling BACnet specifies the following. Shielded,
twisted-pair cabling with characteristic
impedance between 100 and 130W.
Distributed capacitance between
conductors must be less than 30 pF/foot
(100 pF/m). Distributed capacitance
between conductor and shield must be
less than 60 pF/foot (200 pF/m). Foil or
braided shield acceptable.
Segment
length
4000 ft. (1071 m.) per segment using
recommended wire.
Maximum
devices
overall
Depends on classification of devices as
master or slave. Maximum number of
master devices is 128. Maximum number
of slave devices or devices overall (mixed
master and slave) is 255. This includes
BACnet FFs, BACnet global controllers (all
are considered masters) and any other
devices, regardless of their relative unit
loads.
Maximum
devices per
segment
Depends on relative unit load of devices
(see “Terminating MS/TP LAN Cabling” on
page 3).
Repeaters Required when making runs longer than
4000 ft. Three repeaters maximum
between any two devices.
Terminating
resistors
Matched resistors required at each end of
segment bus wired across (+) and (–). Use
matched precision resistors rated |
1/4 W ±1% / 80 - 130 Ohms.
Shield
grounding
Ground shield drain wire at single point
earth (panel) ground, not BACnet FF
ground. Tape off shield drain wire at other
end. Tie shield drain wire through at each
BACnet FF.