Issue
What is the maximum number of devices that can be placed on the MS/TP trunk of the ENC-520, UNC-520 and MNB-1000?
Product Line
TAC IA Series
Environment
- Niagara R2
- Niagara G3
- UNC-520
- ENC-520
- MNB-1000
Cause
Unit Load is a measure of the electrical loading of an EIA-485 (MS/TP) network. An EIA-485 network may have 32 unit loads. One unit load consists of one full-load transceiver, two half-load transceivers, four quarter-load transceivers, or eight eighth-load transceivers.
Resolution
All MNB-xxxx, UNC-xxx, and ENC-xxx devices use quarter load transceivers, which mean that a MS/TP network comprised solely of MNB devices can have up to 128 total devices (1 router plus 127 controllers). The maximum number of BACnet devices allowed per MS/TP trunk, if all devices are quarter-load transceiver, is 127 plus the ENC/UNC or MNB-1000 controller.
Note: 32 unit loads times 4 transceivers per unit load equals 128 maximum transceivers per network (32 X 4 = 128).
UNC/ENC Resource Limit
Although the 128 BACnet devices is the stated maximum for a MS/TP trunk, this maximum is normally not achieved. This is because the number of MS/TP devices that can be placed on a MS/TP bus is limited by the resource limit of the UNC/ENC controller. A shortage of resources will limit the devices on an MS/TP network to a number less than the physical limit. Exceeding the resource limit will negatively affect the UNC/ENC-520s, possibly causing poor performance and resets (of the UNC/ENCs).
Reference Appendix A of BACnet Wiring and Networking Practices Guide for additional information.
