BACNET® FIXED FUNCTION THERMOSTAT
3
31-00093—01
Fig. 4. Fixed Function Thermostat terminal assignments
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. It can
be removed if those BOs are powered from an external
source. (Note: BO-0, BO-2, and BO-5 are powered from the
board.)
Refer to BACnet Fixed Function Thermostat System
Engineering Guide (31-00098) for complete application
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.
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.
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.
