Industry Term of the week: Protocol

bas networking

Intro to common protocols in Building Automation Systems

In building automation systems, a protocol is the language used by the HVAC, lighting, and control devices to communicate.?

A protocol defines how data is formatted, how devices identify themselves, and how commands/values are exchanged across a network.?

Protocols matter because they determine:

Interoperability between different manufacturers
Speed and reliability of communication
Cost to install and maintain
Flexibility when expanding a building system

The Most Common HVAC Protocols

In the commercial HVAC world, these are the most common:

BACnet
Modbus
LonWorks (LON / LonTalk)
Proprietary protocols (Johnson N2, Carrier CCN, Trane Comm5, etc.)

Below is what each does and how they differ.

1.) BACnet What it is:?

BACnet (Building Automation Control Network) is the dominant open protocol for HVAC today, standardized by ASHRAE.

Where it’s used:?
AHUs, VAVs, chillers, boilers, VRF systems, fans, meters, lighting, access control.

How it works:
Devices communicate using BACnet objects (analog input, binary output, schedule, trend, etc.)

Runs on several physical layers:

BACnet/IP (Ethernet – most common today)
BACnet MS/TP (RS485 – still common for VAVs)
Devices announce themselves on the network using Who-Is / I-Am messaging.

Pros:

Widely supported
Easy interoperability
Strong device discovery tools
Scales well on IP

Cons:

MS/TP networks can be slow and wiring-sensitive
Implementation quality varies by manufacturer

Key differentiator:?
Object-based, highly standardized, and the most open and universal protocol in HVAC today.

2.) Modbus What it is:?

Oldest industrial protocol still widely used; simple and reliable.

Where it’s used:?
Boilers, VFDs, pumps, meters, generators, packaged equipment.

How it works:
Uses a register structure (40001, 30001, etc.).

Can run on:

Modbus RTU (RS485 – most common)
Modbus TCP (Ethernet)
The BAS polls each device for data; devices do not announce themselves.
Pros:
Very simple
Extremely common in industrial equipment
Works over long distances

Cons:

No device discovery
Requires point maps
No standard naming or object definitions

Key differentiator:?
Register-based and master-slave, which means easier for equipment manufacturers but requires more integration work.

3.) LonWorks (LON / LonTalk)

What it is:?
Previously very popular before BACnet overtook the market; still used in older buildings and by certain manufacturers.

Where it’s used:?
VAVs, terminal units, lighting, distributed control networks.

How it works:
Uses network variables (SNVTs) as standardized data types
Typically runs on TP/FT-10 twisted-pair networks
Devices require binding (mapping variables between devices)

Pros:
Excellent peer-to-peer performance
Very stable distributed architectures
Good for large, decentralized networks

Cons:
Specialized tools and proprietary chip sets needed
Declining market share
Fewer contractors trained on it today

Key differentiator:?
True peer-to-peer control without a master—devices communicate directly with each other.

4.) Proprietary Protocols

Examples:
Johnson Controls N2
Carrier CCN
Trane Comm4/Comm5
Daikin, Mitsubishi, LG VRF protocols

Where used:?
OEM equipment, legacy systems, or internal communications within a manufacturer’s ecosystem.

How they work:?
Every manufacturer defines its own data structures, commands, and addressing scheme.

Pros: