Modbus Rtu; Modbus Defaults; Wiring Topology; Modbus Master Specifications - Honeywell Centra Line MERLIN NX Series Installation Instructions Manual

MODBUS RTU

The controller features a removable 2-wire non-isolated
RS485 interface suitable for Modbus communication
(terminal 16, 17, and 18). The terminal block containing it is
grey. The controller can function only as a Modbus Master.
In general, the TIA/EIA 485 wiring rules must be followed.

Modbus Defaults

Baudrate
19200
Parity Even parity
Bytesize
8 bit
Stopbits
1 stop bit

Wiring Topology

Only daisy-chain wiring topology is allowed.
Fig. 31. Allowed Wiring Topology
Other wiring topologies (such as, star wiring, mixed star
wiring, and daisy chain wiring) are prohibited. This is to
avoid communication problems of the physical layer.
Fig. 32. Prohibited Wiring Topology (example)
Cables and Shielding
See "TIA/EIA 485 Cable Specifications" on page 19.
Use 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.
MERLIN NX IP AND MST/TP VAV CONTROLLER - INSTALLATION INSTRUCTIONS
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.
RS485 Repeaters
RS485 repeaters are possible but have not been tested by
Honeywell. Therefore it is the responsibility of the installing
/ commissioning person to ensure proper function.
NOTE: Each Modbus segment will require its own line

Modbus Master Specifications

Physical Layer
Communication
rates:
Maximum
number of
devices:
Cable and wiring
specifications:
Communication
Mode
Transmission
Mode
Address Range
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
communication rates of 1.2, 2.4, and 4.8 kbps (because
these communication rates are not market-relevant).
Modbus Considerations
The RS485 interface suitable for Modbus communication is
2-wire non-isolated, hence the following considerations
apply:
— Maximum Modbus length ("L"): 4000 ft (9.6 – 78.8
— Use only shielded, twisted-pair of cables and daisy-
25
polarization and line termination.
2-wire serial line (TIA/EIA-485) (with
additional common)
9.6, 19.2, 38.4, 57.6, and 76.8 kbps
supported.
32, It is recommended to connect a
smaller number of devices for better
Modbus performance.
See section TIA-EIA 485 Cable
Specifications on pg. 13
Always Modbus Master.
RTU (Remote Terminal Unit).
Modbus master can have an address
between 1 and 247.
Discrete Inputs, Coils, Input Registers
and Holding Registers can have an
address between 1 and 65534.
kbps) or 2600 ft (115.2 kbps)
Cable Specifications" on page 19.
mended that you select a low baud rate (for exam-
ple, 19.2 kbps) for reliable operation.
chain topology.
See "TIA/EIA 485
It is recom-
EN1Z-1076GE51 R0722