Bacnet Ms/Tp Variant; Termination Resistors; Rs-485 Bias Switches; Bacnet Mstp Wiring Example - Honeywell Centra Line MERLIN NX Series Installation Instructions Manual

MERLIN NX IP AND MST/TP VAV CONTROLLER - INSTALLATION INSTRUCTIONS

BACNET MS/TP VARIANT

Each MERLIN NX IP and MSTP VAV controller uses the
BACnet MSTP communications. The controller's data is
presented to other controllers over a twisted-pair MSTP
network, using the TIA/EIA 485 signaling standard capable
of the following baud rates: 9.6, 19.2, 38.4, 57.6, and 76.8
kb/s. The MERLIN NX BACnet MSTP controllers are server
devices on the MSTP network. Each MERLIN NX BACnet
controller uses a high-quality TIA/EIA 485 transceiver and
exerts 1/8 unit load on the MSTP network.
According to U.L. requirements, each RS-485 interface
may be loaded with a maximum of 32 unit loads.
Depending on the actual performance and connection
speed, connecting fewer BACnet MSTP devices per
network is recommended. It is recommended to have less
than 62 controllers on a single MSTP network.
The controller features a 2-wire non-isolated RS-485
interface (terminals 32, 33, and 34) suitable for BACnet
MSTP communication. The terminal block containing it is
gray. The cable length affects the baud rate. See
below.
Table 7. Baud rate vs. Maximum cable length
Baud rate
9.6, 19.2, 38.4, 57.6, and 76.8 kbps
NOTE:
The maximum BACnet MS/TP network Bus segment
length is 4,000 ft. (1200 m) using recommended wir-
ing. Repeaters must be used when making runs lon-
ger than 4,000 ft. (1200 m). A maximum of three
repeaters can be used between any two devices.
EN1Z-1076GE51 R0722
Table 7
Maximum cable
length (L)
4000 ft (1200 m)
Fig. 28. BACnet MSTP Wiring Example with WEB-8000 or CIPer Model 50

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 MSTP cable has a listed
characteristic impedance of 120 Ω, install a 120 Ω resistor.

RS-485 Bias Switches

Each RS-485 port has an adjacent 3-position biasing
switch, with these settings:
• BIA - (Default, middle) Controller provides RS-485
biasing, but without a termination resistor.
• END - Both RS-485 biasing and a termination resistor
are provided by the controller.
• MID - No RS-485 biasing or termination resistor is
provided by the controller.
Often, adding RS-485 biasing can improve communications
by eliminating indeterminate idle states.

BACnet MSTP Wiring Example

Following proper MSTP 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 MSTP cabling close to lines carrying higher voltage.
The shield should be grounded on only one end of the
MSTP segment, for example, on the earth ground terminal
on the HAWK 8000 and to earth ground.
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