Issue
What is the unit load of the SpaceLogic SLP Room or Plant Room Air Quality Sensor Units that will help me to calculate how many devices that can be added on ComA and ComB of a SmartX Server.
Product Line
EcoStruxure Building Operation, Field Devices
Environment
- SpaceLogic SLP Room Unit
- SpaceLogic Plant Room Air Quality (PRAQ) SCD2 / SHD2 unit
- SmartX Server
Cause
Documentation does not state the Unit Load (UL) of the SpaceLogic SLP Room Unit or Plant Room Air Quality sensor.
Resolution
Both SLP and PRAQ sensors will be refereed as Sensor Unit here bellow.
The Generic application note AN01 can be applied with answers to the transceiver interface attributes below. The Sensor Unit range use the SN65HVD1781DR transceivers which are a failsafe transceiver (not requiring bias) and reduced 1/10 UL. In addition there are no local bias resistors and therefore the 1/10 UL of the transceiver would be the total UL presented to the bus by the Sensors Units.
| Transceiver | Xcvr Unit Load | Xcvr Failsafe | Adaptor Circuit Bias | Circuit Bias Load | Total Unit Load | Isolated 485 Bus |
|---|---|---|---|---|---|---|
| SN65HVD1781DR | 0.1 | Yes | None | None | 0.1 | No |
Based on the above the total number of devices can be calculated as 32/0.1 = 320.
The BACnet MSTP standard specifies that the address range for BACnet master devices is 0-127 and therefore a SmartX Server is limited to 127 devices per com port (254 total per Server) so this should effectively be the limiting factor.
Since the Sensors Units does not have an isolated RS-485 interface it conforms to Generic RS-485 Network Device Configuration 1 in EBO Webhelp. A network consisting of only Sensors Units should only require a 120 ohm termination resistor at each end of the bus and not require any bias resistors.
Also check out the SmartStruxure-RS485-Controller Unit Load Quick-Help video on the Exchange.
