Networking Concepts; The Rs485 Standard; Network Topologies; Daisy Chain - Honeywell Centra Line MERLIN NX Series Installation Instructions Manual

NETWORKING CONCEPTS

The RS485 Standard

According to the RS485 standard (TIA/EIA 485: "Electrical
Characteristics of Generators and Receivers for Use in
Balanced Digital Multipoint Systems"), only one driver
communicating via an RS485 interface may transmit data at
a time. Further, according to U.L. requirements, each
RS485 interface may be loaded with a max. of 32 unit
loads. For example, if a controller utilizes as little as 1/8 unit
load each, up to 256 devices can be connected.
BACnet connections to the RS485 interfaces must comply
with the RS485 standard. Thus, it is recommended that
each end of every bus should be equipped with a
termination resistor (not included in shipment) having a
resistance equal to the cable impedance (120
wattage should be in the range of 0.25 – 0.5 W).
RS485 systems frequently lack a separate signal ground
wire. However, the laws of physics still require that a solid
ground connection be provided in order to ensure error-free
communication between drivers and receivers – unless all
of the devices are electrically isolated, and no earth
grounding exists.
NOTE:
A separate signal ground wire must be used. Failing
to obey this requirement can lead to unpredictable
behavior if other electrically non-isolated devices are
connected and the potential difference is too high.
TIA/EIA 485 Cable Specifications
The following cable specification is valid for BACnet MS/TP
EIA 485 buses.
Table 5. TIA/EIA 485 cable specifications
Maximum length
Cable type
Characteristic
impedance
Distributed
capacitance between
conductors
Distributed cap.
between conductors
and shield
The following cables fulfill this requirement:
— AWG 18 (1 mm2)
— Shielded, twisted pair cable J-Y-(St)-Y 4 x 2 x 0.8 mm2.
— CAT 5,6,7 cable - use only one single pair for one bus
— Belden 9842 or 9842NH.
MERLIN NX IP AND MST/TP VAV CONTROLLER - INSTALLATION INSTRUCTIONS
; the
Ω
4000 ft (9.6–76.8 kbps)
Twisted pair shielded (foil or braided
shields are acceptable)
100-130 Ω
Less than 100 pF per meter (30 pF
per foot)
Less than 200 pF per meter (60 pF
per foot)

Network Topologies

— Recommended cable: Cat5/Cat6
— Maximum distance between 2 controllers or control-
ler and switch should be less than 328 ft. (100
meters)

Daisy Chain

In the daisy chain connection type, if any of the device in
the network fails, the devices next to the failed device also
fail.
Suppose there are 10 devices in a network, and device
number 1 is the client, connected to device number 2, and
device number 2 is connected to device number 3, and so
on. If device 5 fails to function, device 6, 7, 8, 9, and 10 also
fails to communicate with the client device.
Maximum number of controller that can be connected in a
daisy chain is 100.
Daisy Chain Topology
Supervisor/
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Fig. 21. Daisy chain topology

Spanning Tree Protocol (RSTP)

If the MERLIN NX IP VAV controllers are to be connected in
a redundant ring, then you must have one spanning tree
protocol supported Ethernet switch as a part of the ring.
MERLIN NX IPVAV supports Ethernet switch for 10/100
Mbps IP connection.
The switch will connect MERLIN NX IP VAV ring with the
backbone network.The loop-free topology ensures that
there is no broadcast storms and duplicate frame
transmission. Maximum number of controllers that can be
connected in STP loop is 39. A switch manages the
connection of a loop.
Ring Topology (39 controllers recommended)
Supervisor /
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Fig. 22. RSTP wiring
19
#2 #3
#4
#N
#2 #3
Multi Port (RSTP)
Ethernet Switch
EN1Z-1076GE51 R0722