In most cases the answer is decided for you by the BMS already on site, so the first step is to check what the head end speaks rather than choosing on merit.
BACnet is the dominant protocol in commercial building management and is what most modern BMS platforms use natively. It carries device names, units and object descriptions with the data, so an air conditioning unit appears in the head end already labelled and structured, which makes commissioning faster and reduces the amount of manual point mapping. It splits into two physical variants:
BACnet/IP runs over the building's Ethernet network. Choose it where there is structured cabling or a network point near the AC unit.
BACnet MS/TP runs over a two-wire RS485 daisy chain. Choose it where you are extending an existing MS/TP trunk, or where running network cable to each unit is impractical.
Several of the Intesis interfaces on this page support both BACnet/IP and MS/TP in one unit, so you can decide on site.
Modbus is simpler and more common in energy metering, industrial plant and smaller or older systems. It has no self-description, so every point has to be mapped from the register list at commissioning. It is the right choice when the existing controller or data logger only supports Modbus, and Modbus RTU over RS485 is the usual form here.
KNX is worth mentioning if the building already runs KNX for lighting and blinds, as an AC interface with KNX support keeps everything on one bus.
A few practical pointers:
- If you are unsure and the site is new or being refurbished, specify BACnet. It is the safer long-term choice for commercial buildings.
- If cable runs are the constraint rather than the protocol, the MS/TP and Modbus RTU variants share the same RS485 wiring, and some models support both, which gives you a fallback.
- Check the interface is matched to your AC system type as well as the protocol. The Domestic, Commercial and VRF variants are not interchangeable, even for the same manufacturer.
If you are still unsure, send us the AC model number and the make of your BMS and we will confirm the right part before you order.
