Untangling the Wires: Building Automation System IP vs. MS/TP Network Topologies Explained

By: Douglas Frey, PE, LEED AP, Vice President

In the world of building automation systems (BAS), the network topology you choose can make or break your system’s performance, scalability, and ease of maintenance. Whether you’re working with legacy systems or designing a future-proof smart building, understanding the differences between IP and MS/TP topologies is key.

Let’s break it down.

What Are IP and MS/TP Networks?

IP (internet protocol) networks are Ethernet-based and use standard IT infrastructure. Think switches, routers, and CAT5/6 cabling.

MS/TP (master-slave/token-passing) is a BACnet protocol that runs over RS-485 twisted pair wiring. It’s slower but still widely used in legacy and cost-sensitive installations.

Common Network Topologies

1. Daisy Chain (Linear Bus)
   A network setup where all devices are connected to a single central cable, known as the bus or backbone. This cable carries data in both directions, and each device listens for data addressed to its unique IP address. The networks are cost effective and easy to implement but have limited scalability, and the wiring is a single point of failure. You won’t find these networks used in an IP setting anymore, but they are common for MS/TP.
2. Star
   A network setup where all devices are connected to a central device – typically a switch, hub, or router – using IP for communication. All data traffic passes through the central device. Devices do not communicate directly with each other; instead, they send data to the central device, which then forwards it to the destination. These networks are easy to manage and scale; they’re reliable, and it’s easy to isolate faults.  However, these SYSTEMS involve a single point of failure (the central device) and more cabling. You won’t find this topology for MS/TP, as it can cause reflections and communication issues.
3. Ring
   A network setup where each device (node) is connected to exactly two other devices, forming a closed loop or ring. Data travels around the ring in one direction (or sometimes both, in a dual-ring setup), passing through each device until it reaches its destination. These networks are cost effective, requiring much less cabling than other topologies. However, data must pass through multiple hops, increasing latency. In addition, failure of one device in a single-ring network can disrupt the entire network downstream.
4. Tree/Hierarchical
   This network type is a hierarchical structure that combines characteristics of star and daisy chain topologies. It resembles a tree, with a root node (often a central router or switch) branching out to other nodes, which may themselves branch further. Routing or switching is handled hierarchically:
   • Data from a leaf node travels up to its parent node.
   • The parent forwards it up or down the tree depending on the destination IP.

   These networks are easily scalable and have a clear hierarchy that simplifies administration and troubleshooting, but the cabling can be complex and failures in higher-level nodes can affect large portions of the network.

5. Stub
   Found in MS/TP networks, a stub topology comprises a main trunk line (the primary RS-485 bus) connected to devices via short branches called stub lines. These stubs are typically very short (often less than 1 foot) to avoid signal reflections and maintain network integrity. This topology allows flexibility in device placement and simplified wiring, but networks must remain compact to avoid signal degradation.

IP vs. MS/TP: When to Use What?

Hybrid Networks: The Real-World Scenario

Most modern BAS deployments use a hybrid approach:

• IP for supervisory controllers and integration devices.
• MS/TP for field-level devices like VAVs and thermostats.

This strategy makes it possible to leverage existing infrastructure while preparing for future upgrades.

Final Thoughts

Choosing the right topology isn’t just about tech specs – it’s about your building’s needs, budget, and long-term strategy. As BAS continues to converge with IT, IP networks are becoming the backbone of smart buildings, but MS/TP still has its place in retrofit and cost-sensitive projects.

Image Source: iStock.com/eravau

Doug Frey is responsible for supporting company growth and success in the healthcare, hospitality, and education sectors. He has extensive experience in environmentally friendly HVAC, plumbing, and electrical system design for all types of buildings, as well as existing condition assessments and master planning. He is a licensed Professional Engineer in Illinois, Michigan, North Carolina, Texas, and Wisconsin, and has been a LEED Accredited Professional since 2009. He is a member of ASHRAE.