Control Network Newsletter

August, 2009: BACnet Discovery and the BAS Remote

BACnet supports the discovery of devices and objects within these devices. Using the BACnet discovery function with the BAS Remote (BR) from Contemporary Controls is normally quite simple. But as you change BR channel definitions on its webpage, its BACnet objects also change – that is, the BR has configurable I/O. Therefore, its BACnet objects are not fixed. If your client supports object discovery, this is not an issue, but for others this can be confusing. For example, if you select 0–10V Analog Input on channel 1, then channel 1 will be represented by an Analog-Input instance 1.

Each channel must be configured as an Analog Input, Analog Output, Binary Output or Binary Input – except for relay channels 7 and 8 which are only Binary Outputs. If your BACnet application supports discovery, you should first configure the BR channelsthen select the matching object instance types in your software.

The BR object instances are identified by channel number. For example, channel one will always be instance one – but depending on how you have configured channel one via the BR webpage, it can be:

      Analog Output instance 1
      Analog Input instance 1
      Binary Input instance 1

Channels 2–6 work in the same manner, but channels 7 and 8 can only be Binary-Output instance 7 and Binary-Output instance 8.

When you connect expansion units to the BR, more objects within the BR become available.  With 32 channels (BR with 3 expansion units) you will have 32 object instances of the types which have been configured.  When using three expansion units, channels 7–8, 15–16, 23–24 and 31–32 are all Binary-Outputs.

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