BACnet MSTP – An Introduction

The MS stands for Master – Slave although in practice there are not many slaves out there.
The TP stands for Token Passing.

This flavor of BAcnet is most commonly used to connect field devices to controllers / routers / control applications.

The physical layer uses RS485 which allows up to 128 devices to be installed on a single network with a max physical length of 4000ft and speeds up to 115k baud. Using repeaters allows the length to be increased. Compare to Ethernet where the spec allows a max of 100 meters (330ft) on a single unrepeated segment.

Common baud rates are 19200, 38400 and 76800. All devices must operate at the same baud rate. More and more devices can auto sense the baud rate and configure themselves correctly.

We divide the messages on a MSTP network into two categories:
1. Overhead (token, poll for master…)
2. Application. These carry payloads that we have an interest in.

Only a device with the token can initiate an application layer message. It can send the message to any device on the network. Some messages demand an instantaneous reply, some don’t. The receiving device doesn’t need the token to respond. There is a limit to how many application layer messages a device can send before it must pass the token on.

The benefits of token passing networks are the following:
1. They are self healing
2. They can discover new devices
3. They ensure each device gets its chance
4. They avoid collisions making network performance (somewhat) deterministic

A disadvantage of the token system is that any one device gets a limited use of the bandwidth. Thus a device may need to keep an internal queue of application layer messages it wants to send, waiting to use the token. We have encountered some vendor systems which fill there queue and then drop subsequent messages without notifying the user of the problem. Limited access combined with the overhead makes it easy to use up all the bandwidth on the network if there are many devices with many objects and many properties of interest.