Concept
Expanded Unit Load with Network of Isolated Devices Only (Legrand Power Meters)
If the network is comprised exclusively of devices with isolated RS-485 interfaces with the only exception being the SmartStruxure server device, it is recommended that the maximum unit load limit can be stretched higher. With a network comprised exclusively of isolated interfaces connected to the SmartStruxure server device, the standard unit load budget of 32UL can be increased by 50% to have an expanded budget of 48UL. Using a maximum network load of 48UL and subtracting the 24UL for the bias network resistors and the load of the SmartStruxure server device leaves 24UL available for the nodes to be connected on the bus. With the example device load of 1.32UL each, the expanded budget of 48UL supports 18 nodes (meters) on the bus.
The excess unit load accommodation is unique to the isolated interface configuration. Excluding the SmartStruxure server device, if a device with a non-isolated RS-485 interface is connected to the bus, then the complete network must fall back to comply with the RS-485 standard specified maximum budget of 32UL.
The recommended limits on RS-485 bus node counts discussed here pertain to hardware bias and unit load considerations only. The recommended maximum node count may be further limited based on product and system version.
RS-485 Communications
Legrand Power Meters