Table of Contents
Advertisement
BACNET™ MS/TP WIRING
The MS/TP models of the Honeywell Unitary controllers
use the BACnet™ MS/TP communication protocol. The
controller's data is presented to other controllers over a
twisted-pair MS/TP network, using the TIA/EIA 485
signal standard capable of the following baud rates: 9.6,
19.2, 38.4, 57.6, and 76.8 kbps. The Honeywell Unitary
controller BACnet™ MS/TP are server devices on the
MS/TP network. Each Honeywell Unitary controller
BACnet™ controller uses a high-quality TIA/EIA 485
transceiver and exerts 1/8-unit load on the MS/TP
network. The controller features a 2-wire non-isolated
RS-485 interface (terminals 45, 46, and 47) suitable for
BACnet™ MS/TP communication. The terminal block
containing it is grey. The cable length affects the baud
rate.
Table 17 Baud Rate vs Maximum Cable Length
Baud Rate
9.6, 19.2, 38.4, 57.6, and 76.8 kbps
NOTE:
The maximum length of a BACnet™ MS/TP
network bus segment with recommended wiring is
4,000 ft (1200 m). Repeaters must be used when
making runs longer than 4,000 ft (1200 m).
Between any two devices, a maximum of three
repeaters can be used.
Auto Baud rate functionality
The MS/TP network is listened up to 4 minutes each
time the supply voltage to the controller is turned on to
establish a baud rate. Once the proper baud rate has
been identified, the auto baud detection is stopped, and
the new baud rate is used and saved in the controller as
a successful baud rate.
If no baud rate is determined after 4 minutes, the
controller will switch to the baud rate successfully used
before the controller was powered up. However, if the
controller is new from the factory and has yet to
communicate successfully, then a default baud rate is
used but not stored as a successful baud rate in the
controller. This causes the same process to start again
next time the supply voltage is switched on.
HONEYWELL UNITARY CONTROLLER - 230 VAC - INSTALLATION INSTRUCTIONS
Termination Resistors
Matched terminating resistors are required at each end
of a segment bus wired across (+) and (-). Use matched
precision resistors rated ¼ W ±1 % / 80 = 130 Ω.
Ideally, the value of the terminating resistors should
match the rated characteristic impedance of the
installed cable. For example, if the installed MS/TP
cable has a listed characteristic impedance of 120 Ω,
install a 120 Ω resistor.
Shield Termination
Following proper MS/TP cabling shield grounding
procedures is important to minimize the risk of
communication problems and equipment damage
caused by capacitive coupling. Capacitive coupling is
caused by placing MS/TP cabling close to higher
voltage lines. If shielding is used, the shielding of each
bus segment should be separately connected at one
end to a verified earth ground.
Maximum Cable
Length (L)
4000 ft (1200 m)
The controller communicates via its BACnet™ MS/TP
interface with other BACnet™ MS/TP capable devices.
In doing so, the following considerations should be
considered.
• Maximum BACnet™ MS/TP bus length.
• Twisted-pair cable, for example,
25
NOTE:
If any of the devices are electrically isolated, it is
recommended that those devices be connected to
a single ground.
2
1. AWG 18 (1 mm
)
2. J-Y(ST)Y 4 x 2 x 0.8 mm2 or a special RS-485
cable.
3. CAT5,6,7 cable: use only one single pair for one
bus.
4. Belden 9842 or 9842NH
5. Daisy chain topology.
6. Must conform to TIA/EIA RS-485 cabling guide-
lines and ANSI/ASHRAE Standard 135-2010.
31-00610-01
Advertisement
Table of Contents
