MERLIN NX ROOM CONTROLLER – INSTALLATION & COMMISSIONING INSTRUCTIONS
Overview of Interfaces
e.g., LIGHT I/O or WALL MODULE
Fig. 23. System overview (example: RLxN)
The TIA/EIA-485 Standard
According to the TIA/EIA-485 standard ("Electrical Charac-
teristics 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. E.g., Honeywell /
CentraLine devices have as little as ¼ unit load each, so that
up to 128 devices can be connected.
BACnet and Modbus connections to the RS485 interfaces
must comply with the aforementioned TIA/EIA-485 standard.
Thus, it is recommended that each end of every bus be
equipped with a termination resistor (not included in ship-
ment) having a resistance equal to the cable impedance
(120 Ω; the 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
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 all EIA 485 buses
(incl. Modbus and BACnet MS/TP).
Table 9. TIA/EIA 485 cable specifications
distributed cap. between
conductors and shield
*A higher communication rate (necessitating a shorter max.
length) is possible for Modbus. See section "Modbus
Considerations" on pg. 20.
The following cables fulfill this requirement:
shielded, twisted pair cable J-Y-(St)-Y 4 x 2 x 0.8;
CAT 5,6,7 cable (use only one single pair for one bus);
Belden 9842 or 9842NH.
BACnet MS/TP Interface
The controller features an isolated RS485 interface (RS485-
1) suitable for BACnet MS/TP communication:
RLxN: terminals 62, 63, and 64;
RSxN: terminals 40, 41, and 42.
The terminal block containing it is gray. The cable length
affects the communication rate. Table 10 provides a few
Table 10. Baud rate vs. max. cable length
9.6, 19.2, 38.4, 57.6, and 76.8 kbps
The controller supports auto-baud rate adaption for BACnet
MS/TP communication at all of the aforementioned baud
rates (default: 38.4 kbps).
For information on wire gauge, max. permissible cable length,
possible shielding and grounding requirements, and the max.
number of devices which can be connected to a bus, refer to
BACnet MS/TP Considerations
The controller communicates via its isolated RS482-1 inter-
face with other BACnet MS/TP-capable devices (e.g., other
room controllers or plant controllers like the EAGLE / Excel
Web II or EAGLEHAWK NX). In doing so, the following con-
siderations should be taken into account.
Max. BACnet MS/TP bus length (= "L" in Fig. 24): See
Twisted-pair cable, e.g.:
J-Y-(St)-Y 4 x 2 x 0.8;
CAT 5,6,7 cable – use only one single pair for one
Belden 9842 or 9842NH);
and daisy-chain topology.
1200 meters (9.6 – 78.8 kbps)*
twisted pair, shielded (foil or
braided shields are acceptable)
Less than 100 pF per meter
(30 pF per foot)
Less than 200 pF per meter
(60 pF per foot)
max. cable length (L)