Matrix BACNET, User Manual, Page: 23 / 46

W
IRING AND
I
NSTALLATION
C
OMMUNICATION
C
ONNECTIONS
MATRIX BACnet Router User Manual (6/6/2007)
2-7
2.3.2 EIA-485 N
ETWORK
The EIA-485 network connections are used for BACnet MS/TP network communications. MS/TP is a
communications protocol used in BACnet to network multiple devices, such as unitary controllers, together
on one continuous bus. MS/TP uses EIA-485 signaling standards on shielded-twisted pair cabling where
units are networked together in a daisy-chained (multi-drop) format.
Terminals for the MS/TP network are located on the left side of the controller board and are labeled J2 and
J3. Each MS/TP terminal has a connection for Network Positive (N+), Network Negative (N-), and Shield
(SH). The SH terminal is coupled to the housing of the MATRIX BACnet Router and will provide noise
protection for MS/TP communication signals.
The MATRIX BACnet Router must be installed at an physical and electrical end of the EIA-485 network, as
the device is connection intensive. MATRIX BACnet Router should not be wired into a network where it is
“t-tapped” or “starred” into an existing network.
2.3.2.1 C
ABLE
T
YPE AND
L
ENGTH
Because BACnet MS/TP is an EIA-485 based network, the standard specifies that MS/TP networks shall
utilize 18AWG, shielded-twisted pair cabling with characteristic impedances ranging from 100 - 130 ohms.
Distributed capacitance between conductors must be less than 30pF/foot (100 pF/m). Distance of length
for EIA-485 is typically limited to about 4000 feet with 32 electrical nodes.
2.3.2.2 C
ONNECTING THE
MS/TP N
ETWORK
Connect the MS/TP network to the MATRIX BACnet Router’s EIA-485 terminal block. Be sure to maintain
network polarity at all times for MS/TP communications. Failure to do so will result in a loss of network
communications. To prevent against the possibility of ground loops or noise on your 485 network, connect
the drain wire of the 485 to one of the supplied enclosure studs located within the enclosure panel.
Figure 2-6 MS/TP Network Connection
2.3.2.3 N
ETWORK
T
ERMINATION AND
B
IASING
Adjacent to each EIA-485 port is a two-position switch block, used to apply termination and biasing to your
MS/TP network. To enable either termination (TERM) or biasing (BIAS), configure the respective switch
into the ON (Up) position.
When applying termination, be certain that it is applied at both ends of your EIA-485 network at the two
end devices. If termination resistors are used, the network will be more heavily loaded and this will change
idle state voltage. In termination applications, biasing should also be enabled to ensure a proper idle state
voltage.
N+
N-