Concept
Expanded Unit Load with Network of Isolated Devices Only (MNB and Generic RS-485 Devices)
If the network is comprised exclusively of devices with isolated RS-485 interfaces with the only exception being the automation server, it is recommended that the maximum unit load limit can be stretched higher. It is recommended that a maximum load extension should be 16UL (50% overload) giving a total expanded unit load limit of 48UL. Using a maximum network load of 48UL and subtracting the 24UL for the bias network and the automation server leaves 24UL available for the RS-485 devices. With the example device load of 0.18UL each, it is suggested that the isolated bus arrangement could support the full collection of up to 127 devices.
The extra unit load accommodation is unique to the isolated interface configuration. The configuration is operating with a single non-isolated node (the automation server) which acts as the single point reference for the CMV of the network pair as imposed by the bias arrangement. If this configuration of isolated device nodes is intermixed with any other products that are not isolated, the configuration rules on the network node count must fall back to the limits produced with the standard 32UL maximum total unit load.
If the standard specified unit load limit of 32 is applied, the node count calculation is as follows:
Subtracting 24UL from the starting budget of 32 gives a node budget of 8UL.
Each of the example devices (with isolated RS-485 interfaces) presents a RS-485 network load of 0.18UL.
The calculated node count that consumes the remainder of the budget is: 8UL / 0.18UL = 44 nodes.
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
Generic RS-485 Network Devices
MNB BACnet Devices