What is the BACnet MSTP unit load of the SE8000 room controllers

Issue

What is the unit load of SE8300 that will help me to count how many SE8300 controllers that can be added on AS ComA and ComB

Product Line

Field Devices, SmartStruxure Solution, SmartStruxure Lite

Environment

SER8300 room controller

SE8300 room controller

Automation Server

Cause

Documentation does not state the unit load of the SE8000 range

Resolution

The Generic application note AN01 can be applied with answers to the transceiver interface attributes below. The SE8000 range use the ADM4850A transceivers which are a failsafe transceiver (not requiring bias) and reduced 1/8 UL load. In addition the 100K local bias resistors have been eliminated and therefore the 1/8 UL of the transceiver would be the total UL presented to the bus by the SE8000.
 

Transceiver Xcvr Unit Load Xcvr Failsafe Adaptor Circuit Bias Circuit Bias Load Total Unit Load Isolated 485 Bus
ADM 4850A0 0.125 Yes None None 0.125 Yes**


** Unlike the normally recommended interconnection of the communication common/reference (ground) terminal on all bus devices such as documented in the generic application note for the isolated RS485 bus interfaces, the Viconics bus should be operated with the communications REF (reference) terminal open on each SE8000 and not connected to the bus shield wire, or to any other circuit.

Based on the above the total number of devices can be calculated as 32/0.125 = 256

The BACnet MSTP standard specifies that the address range for BACnet master devices is 0-127 and therefore an Automation Server is limited to 127 devices per com port (254 total per AS) so this should effectively be the limiting factor.

 

Also check out the SmartStruxure-RS485-Controller Unit Load Quick-Help video on the Exchange.