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20
Prelim Draft - Brand Neutral
BACnet IP/MSTP VAV Controller - INSTALLATION
INSTRUCTION
NOTE:
NOTE:
Each Modbus segment will require its own line polar- ization
and line termination.
MODBUS
The controller features a removable 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.
Wiring Topology
Only daisy-chain wiring topology is allowed.
Figure 1. Allowed Modbus 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.
Figure 2. Prohibited Modbus wiring topology (example)
Cables and Shielding
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.
You must use three wires:
—
One wire for
—
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.
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
communication rates of 1.2, 2.4, and 4.8 kbps (because these
communication rates are not market- relevant).
Physical Layer
2-wire serial line (TIA/EIA-485) (with
additional common)
Communication
rates:
9.6, 19.2, 38.4, 57.6, 76.8, and 115.2
kbps supported.
Maximum number of
devices:
32 It is recommended to connect a
smaller number of devices for better
Modbus performance.
Cable and wiring
specifications:
See section TIA-EIA 485 Cable
Specifications on pg. 13
Communication
Mode
Always Modbus Master.
Transmission
Mode
RTU (Remote Terminal Unit).
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 Considerations
The RS485 interface suitable for Modbus communication is
non-isolated, hence the following considerations apply:
—
Maximum Modbus length (“L”): 4000 ft (9.6 – 78.8 kbps) or
2600 ft (115.2 kbps) See
It is recommended that you select a low baud
rate (for example, 19.2 kbps) for reliable operation.
—
Use only shielded, twisted-pair of cables and daisy- chain
topology.
—
Ground noise should not exceed the EIA-485 common mode
voltage limit.
