MS/TP is Multidrop Serial bus/Token Passing and stands for the BACnet data link protocol with services based on EIA-485 as the physical layer. It permits simple cabling on site with 3 wires up to a max. cable length of 1200 meters.

The logical principle behind MS/TP is the token passing, i.e. a token is transmitted in the system from one device to another over the EIA-485 bus. A device can only communicate after receive this token. MS/TP distinguishes between two types of devices:

  • Manager devices
  • Subordinate devices

A BACnet subordinate can be read/written, but never receives the token. The disadvantage of subordinate devices is that subordinate devices cannot actually send out messages and always requires a manager on the network to establish communications. On the other hand, it has the advantage of the manager transferring less tokens the more subordinates that exist on the EIA-485 bus, which then accelerates overall communication. Overall, however, the disadvantages outweigh the advantages so that most MS/TP devices are set up as manager devices.

Each mast on the network periodically outputs a poll for the next manager, i.e. it checks its successor ability to forward the token. Each manager sends out a query between the own address and the next known manager address. For example, if the current address of the manager device is 12 and the next known manager address thus far is 22, the addresses 13...21 is polled periodically in ascending order to see whether a new mast device was added to the network with an address in this range. Unused gaps in addresses should be avoided since the system must wait for a timeout error to determine that there is no corresponding manager under the queried address and this would negatively impact performance of a BACnet MS/TP system.

BACnet MS/TP has less bandwidth than BACnet/IP. MS/TP networks typically operated at 76.8 kBit/s, while IP networks operate at 100..1000 kBit/s.

All MS/TP devices must have the same baud rate in general with the slowest MS/TP device specifying the setting limit. You must ensure that no single MS/TP device slows down the system to achieve solid performance.

Even if MS/TP cannot be used as a general BACnet backbone, MS/TP devices still represent the correct choice for lots of applications due to the following benefits:

  • Simple, cost-effective cabling based on proven EIA-485 technology
  • MS/TP generally supports cable lengths of max. 1200m without additional network devices, thus supporting distributed control
  • BACnet MS/TP is officially supported in the BACnet standard resulting in a seamless BACnet system at all levels
  • BACnet MS/TP devices are cost-efficient on applications that do not require fast update rates (such as VAV and heat pump controllers) and where BACnet/IP devices cannot be used for cost reasons.

It is nevertheless important to correctly use and install the MS/TP networks to take advantage of the benefits. The following outlines aspects that are important for achieving optimum system performance.