Contemporary Control Systems BASrouter Application Guide - Manual (Page 9)

AG-BASRTB00-BD0 Page 9
Application Guide — BASrouters
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ONTR LS O
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CONTEMPORARY
Application #7 — Connecting Field Devices on Multiple Floors
In a typical HVAC installation, many Fan Coil Units (FCU)
or Variable-Air-Volume (VAV) terminal units will be
installed in the ceiling on each floor of a multistory office
building. Frequently, these units contain BACnet MS/TP
controllers all sharing one EIA-485 bus. Depending on
the rating of the EIA-485 transceivers, 32, 64 or 128
devices are allowed on the same bus. Having a large
number of devices on the same bus over several floors
with concealed wiring makes troubleshooting much
more difficult. Adding EIA-485 repeaters compounds
the problem.
A better approach has all the terminal control units on
each floor bused together with one end connected to
the MS/TP port on the BASrouter and the far-end
terminated with a 120 ohm resistor. The BASrouter is
shipped with EIA-485 bias and termination applied so
there is no need for an external terminator on the near
side. The BASrouter Ethernet port is then tied to a
10/100 Mbps Ethernet Switch and then a cable connection
is brought up through risers to the centrally-located
building or automation controllers that operate over
BACnet/IP. On each floor will be installed a BASrouter
dedicated to the field devices on that floor. If the building
supports structured wiring, the BASrouter Ethernet
wiring can share this system. With this approach, there
is no need to carry EIA-485 wiring between floors. By
having a fewer number of field devices on one MS/TP
segment, network traffic on that segment is reduced.
Many MS/TP field controllers support auto-bauding so it
is only necessary to set the baud rate on the BASrouter
to the desired setting and all the field controllers will
follow suit. The maximum number of MS/TP devices
on each segment is often arbitrary. Assuming that
half-load EIA-485 transceivers exist on the field
controllers, a reasonable limit would be about 40. Each
MS/TP segment is treated as a separate BACnet
network so each segment must be assigned a unique
network number including the BACnet/IP network.
Network 1
Network 2004
Network 2003
Network 2002
Network 2001