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BACnet MS/TP INTERFACE
The BACnet Master-Slave/Token-Passing (MS/TP) data link protocol uses EIA-485 as a two-wire, daisy chain network. A branch is a
discrete chain of devices connected to a controller. The max number of devices per segment is (32), as per the BACnet
specifications. 4000 ft (1219.2 m) is the maximum recommended length for a segment, which includes all devices from the
controller to the last device in the daisy chain. ACI’s BACnet sensors are master devices. Only master nodes are allowed to send
and receive tokens on the MSTP network.
Each branch must have all devices connected with (+) connected to (+) and (-) connected to (-). If a shielded cable is used, this is
not to be connected to the devices. The shield cable should only be connected on one end to earth ground, usually at the
controller. The start and end of each branch should have a termination resistor at the device level or at the controller.
Each device must be configured for the correct baud rate and have a unique address in each branch. The baud rate for the branch
is set by the controller. This product has auto-baud for ease of network configuration but setting the baud rate using the DIP
switches is recommended.
ADDRESS SELECTION
Switches 1-7 are used to set the BACnet address. Refer to
TABLE 2
for switch settings. Each device in a network branch must
have a unique address. The value of each position is printed on the board. By default, the address is (0). If the device is
powered when a change is made, the device must be power cycled or reset for changes in address to be made.
ADDRESS
0
1
12
SW 1 (64)
0
0
0
SW 2 (32)
0
0
0
SW 3 (16)
0
0
0
SW 4 (8)
0
0
1
SW 5 (4)
0
0
1
SW 6 (2)
0
0
0
SW 7 (1)
0
1
0
TABLE 2:
ADDRESS SELECTION
BAUD RATE
Auto-Baud
9600
19200
38400
57600
76800
115200
SW 8
0
0
0
0
1
1
1
SW 9
0
0
1
1
0
0
1
SW 10
0
1
0
1
0
1
0
BAUD RATE SELECTION
Switches 8-10 are used to set the BACnet baud rate.
Refer to
TABLE 3
for switch settings. Where (0) is low
and (1) is high. By default, the device is in auto-baud. If
the system’s baud rate is known, it is recommended to
set the specific baud rate to match the system. If the
device is powered when a change is made, the device
must be power cycled or reset for changes in baud rate
to be made.
TABLE 3:
BAUD RATE SELECTION
EOL TERMINATION RESISTANCE
SELECTION
RS-485 requires that the last device in a chain have a
termination resistor. This is controlled using a jumper
in the EN (enabled) position marked on
FIGURE 4
.
When the jumper is set to EN (enabled), a 120 Ω
resistance is added in parallel to the data line. When
the jumper is set to DIS (disabled), the resistance is not added. By default, the jumper is placed the DIS (disabled) position.
FIGURE 4:
EOL TERMINATION JUMPERS
END OF LINE
TERMINATION ENABLED
END OF LINE
TERMINATION DISABLED
EN
DIS
EN
DIS
