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MERLIN NX IP AND MST/TP VAV CONTROLLER - INSTALLATION INSTRUCTIONS
Shied Termination
Following proper MS/TP cabling shield grounding
procedures is important to minimize the risk of
communication problems and equipment damage caused
by capacitive coupling. Capacitive coupling is caused by
placing MS/TP cabling close to lines carrying higher
voltage. If shielding is used, the shielding of each individual
bus segment should be separately connected at one end to
earth.
NOTE:
If any of the devices are electrically isolated, it is rec-
ommended that those devices be connected to signal
ground.
Connecting to BACnet MS/TP Buses
The controller communicates via its BACnet MS/TP
interface with other BACnet MS/TP-capable devices (for
example, other room controllers or MS/TP controllers). In
doing so, the following considerations should be taken into
account.
— Maximum BACnet MS/TP bus length = "L" on
Fig. 18 See Table 8, "Baud rate vs. Maximum cable
length," on page 17.
— Twisted-pair cable, for example,
• AWG 18 (1 mm2)
• J-Y(ST)Y 4 x 2 x 0.8 mm2 or a special RS485 cable.
• CAT 5,6,7 cable – use only one single pair for one bus
• Belden 9842 or 9842NH and
• Daisy-chain topology.
• Must conform to TIA/EIA RS485 cabling guidelines and
ANSI/ASHRAE Standard 135-2010.
PLANT
CONTROLLER
NOTE:
1) If any of the devices are not electrically isolated, it is recommended that those devices be connected to the ground terminal (V0).
2) The 120-Ohm termination resistor must be inserted directly into the terminals of both end devices.
3) If shielding is used, the shielding of each individual bus segment should be separately connected at one end to earth.
4) Always power each controller and the connected slaves via separate transformers.
5) Between devices equipped with non-isolated RS485 bus interfaces, potential differences of max. ±7 V are allowed. Further, this bus should not extend beyond a single building.
EN1Z-1076GE51 R1021
L*
MSTP VAV
CONTROLLER #1
*for Maximum BACnet MS/TP bus length refer table Baud rate vs. Maximum cable length
Fig. 18. Connection to a BACnet MS/TP Bus
18
BACnet MS/TP Limitataions
There are two limitations regarding the number of
controllers per BACnet MS/TP channel:
PHYSICAL LIMITATION:
32 loads as per TIA/EIA-485 standard. One MERLIN NX
MSTP VAV controller represents 1/8 load. The physical
limitation is important in case 3rd party devices
representing a full load are connected.
AUTOMAC LIMITATION:
See Table 8, "Baud rate vs. Maximum cable length," on
page 17 for default maxmaster value. It can be modified by
the user. Refer IRM Engineering Tool User Guide for more
information on how to do this?
A maxMaster of 64 means we can support a maximum of
62 BACnet MSTP VAV controllers, one supervisor, and one
BACnet client (tool) per BACnet MS/TP channel.
Default
Default
maxmaster
MinMAC
64
1
Thus, depending upon the actual performance needs and
required communication rates, it is recommended to
connect a smaller number of BACnet MS/TP devices per
channel.
NOTE:
It is recommended not to have more than 62 control-
lers on single MSTP channel.
MSTP VAV
CONTROLLER #2
Default
Default Baud
MaxMAC
rate
60
38400
MSTP VAV
CONTROLLER #..N
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