Honeywell CENTRA LINE MERLIN N Series Manual page 16

MERLIN NX IP AND MST/TP VAV CONTROLLER - INSTALLATION INSTRUCTIONS
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 for 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 7. 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.
EN1Z-1076GE51 R1021
; 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)
Wiring Method
NOTE:
When attaching two or more wires to the same termi-
nal, other than 14 AWG (2.0 mm2), be sure to twist
them together. Deviation from this rule can result in
improper electrical contact, see
Each terminal can accommodate the following gauge of
wire:
• : From 22 AWG (0.3 mm2) to 18 AWG (1
Single wire
mm2) solid or stranded
• : Up to two 18 AWG (1 mm2) stranded, with
Multiple wires
1/4 watt wire-wound resistor
— Prepare wiring for the terminal blocks, as follows:
— Strip 1/2 in. (13 mm) insulation from the conductor.
— Cut a single wire to 3/16 in. (5 mm). Insert the wire in
the required terminal location and tighten the screw.
— If two or more wires are being inserted into one ter-
minal location, twist the wires together a minimum of
three turns before inserting them, see
— Cut the twisted end of the wires to 3/16 in. (5 mm)
before inserting them into the terminal and tightening
the screw.
— Pull on each wire in all terminals to check for good
mechanical connection.
NOTE:
Do not over-tighten the terminal screws to avoid
deformation and damage of the terminal block. The
maximum torque for the terminal screws 4.4 lb.-in.
(0.5 Nm).
Fig. 16. Attaching two or more wires at terminal block
16
below.
Fig. 16
below.
Fig. 16