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Honeywell CENTRA LINE MERLIN NX Series Installation & Commissioning Instructions page 20

Room controller
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MERLIN NX ROOM CONTROLLER – INSTALLATION & COMMISSIONING INSTRUCTIONS
Modbus Interface
The controller features a non-isolated RS485 interface
(RS485-2) suitable for Modbus communication:
RLxN: terminals 26, 27, and 28 (GND);
RSxN: terminals 23, 24, and 25 (GND).
The terminal block containing it is gray. The controller can
function only as a Modbus Master. In general, the TIA/EIA-
485 wiring rules must be followed.
NOTE: The GND is internally connected with 24V-0 (small
controllers: terminal 4; large 24-V controllers: ter-
minals 4 and 6; large 230-V controllers: terminal 6).
For more information about Modbus, see Niagara IRM
Engineering Tool – User Guide (EN2B-0414GE51).
Wiring Topology
Only daisy-chain wiring topology is allowed.
MODBUS
MODBUS
MASTER
SLAVE
MODBUS
SLAVE
Fig. 26. Allowed Modbus wiring topology
Other wiring topologies (e.g., star wiring, or mixed star wiring
and daisy chain wiring) are prohibited; this is to avoid com-
munication problems of the physical layer.
MODBUS
MODBUS
MASTER
SLAVE
MODBUS
MODBUS
SLAVE
SLAVE
MODBUS
SLAVE
Fig. 27. Prohibited Modbus wiring topology (example)
Cables and Shielding
See also section "TIA-EIA 485 Cable Specifications" on pg.
18.
Use shielded twisted pair cable shielded twisted pair cable J-
Y-(St)-Y 4 x 2 x 0.8 and connect the Modbus shield to a
noise-free earth ground – only once per Modbus connection.
Shielding is especially recommended when the Modbus cable
is installed in areas with expected or actual electromagnetic
noise. Avoiding such areas is to be preferred.
You must use three wires:
One wire for Modbus +
One wire for Modbus –
One wire for the signal common
When using one pair for Modbus (+) and Modbus (-) and one
wire of another pair for the signal common, CAT5 cable may
also be used.
EN1Z-1035GE51 R0420
RS485 Repeaters
RS485 repeaters are possible, but have not been tested by
Honeywell. Hence it is within responsibility of the installing /
commissioning person to ensure proper function.
NOTE: Each Modbus segment will require its own line
Modbus Master Specifications
Modbus Compliance
As per the Modbus standard, the controller is a conditionally
compliant "regular" Modbus device.
The controller differs from an unconditionally compliant
"regular" Modbus device in that it does not support communi-
cation rates of 1.2, 2.4, and 4.8 kbps (because these com-
munication rates are not market-relevant).
Physical Layer
MODBUS
2-wire serial line (TIA/EIA-485) (with additional common)
SLAVE
Communication rates:
Max. number of devices:
Cable and wiring specifications: See section "TIA-EIA 485
MODBUS
Cable Specifications" on pg. 18.
SLAVE
Communication Mode
Always: Modbus Master.
Transmission Mode
RTU (Remote Terminal Unit).
MODBUS
SLAVE
Address Range
Modbus slaves can have an address between 1 and 247.
Discrete Inputs, Coils, Input Registers and Holding Registers
can have an address between 1 and 65534.
MODBUS
Further Information
SLAVE
For further information, please refer to the Modbus Driver
documentation (docModbus.pdf).
Modbus Considerations
The RS485 interface suitable for Modbus communication is
non-isolated, hence the following considerations apply:
Max. Modbus length ("L"): 1200 meters (9.6 – 78.8 kbps)
or 800 meters (115.2 kbps) (see also section "The
TIA/EIA-485 Standard" on pg. 18). It is recommended that
you select a low baud rate (e.g., 19.2 kbps) for reliable
operation.
Use only shielded, twisted-pair cable and daisy-chain
topology.
Ground noise should not exceed the EIA-485 common
mode voltage limit.
Must conform to TIA/EIA-485 cabling guidelines (see
section "TIA-EIA 485 Cable Specifications" on pg. 18).
Should not extend beyond a single building.
Max. no of Modbus devices per interface: 32.
20
polarization and line termination.
9.6, 19.2, 38.4, 57.6, 76.8, and
115.2 kbps supported.
32

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