31-XXXX-XX
16
Prelim Draft - Brand Neutral
BACnet IP/MSTP VAV Controller - INSTALLATION
INSTRUCTION
NOTE:
L*
C
MSTP VAV
#1
ONTROLLER
C
MSTP VAV
#2
ONTROLLER
C ONTROLLER #..N
MSTP VAV
C ONTROLLER
PLANT
ϭϮϬɏ
ϭϮϬɏ
Termination Resistors
Matched terminating resistors are required at each end of a
segment bus wired across (+) and (-). Use matched precision
resistors rated ¼ W ±1% / 80 - 130
Ω
.
Ideally, the value of the terminating resistors should match the
rated characteristic impedance of the installed cable. For
example, if the installed MS/TP cable has a listed characteristic
impedance of 120
Ω
, install 120
Ω
matched precision resistors.
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.
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
Figure. 12 .
See
Table 6,
“Baud
rate vs. Maximum cable
length,”
on
page 15
.
—
Twisted-pair cable, for example,
•
AWG 18
•
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.
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 BACnet 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, “MAC Address,” on page 16
for default
maxmaster value. It can be modified by the user. Refer IRM
Engineering Tool User Guide - 31-00282 for more information
on how to do this.
A maxMaster of 127 means we can support a maximum of 125
BACnet MSTP VAV controllers, one supervisor, and one BACnet
client (tool) per BACnet MS/TP channel.
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.The
recommendation is not to have more than 62 controllers on
single MSTP channel.
Table 8. MAC Address
Brand
Default
maxmaster
Default
MinMAC
Default
MaxMAC
Default
Baud rate
CentraLine
64
1
60
38400
Webs
127
1
127
76800
HBS
35
1
30
38400
*for Maximum BACnet MS/TP bus length refer table Baud rate vs. Maximum cable length
Figure 12. Connection to a BACnet MS/TP Bus
NOTE:
If any of the devices are electrically isolated, it is
recommended that those devices be connected to signal
ground.
GND
RS
4
85
-
RS
48
5
+
4
8
5
+
485
-
SH
D
4
8
5
+
485
-
SH
D
4
8
5
+
485
-
SH
D
