User Manual

BACnet netw
orks with MS/TP trunks
Designing BACnet networks with MS/TP trunks
5
46 | 71 A6V11159798_en--_03
5 BACnet networks with MS/TP trunks
5.1 Designing BACnet networks with MS/TP trunks
MS/TP is Master-Slave/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:
● Master devices
● Slave devices
A BACnet slave can be read/written, but never receives the token. The disadvantage of slave devices is
that slave devices cannot actually send out messages and always requires a master on the network to
establish communications. On the other hand, it has the advantage of the transferring less tokens the more
slaves 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 master devices.
Each mast on the network periodically outputs a poll for the next master, i.e. it checks its successor ability
to forward the token. Each master sends out a query between the own address and the next known master
address. For example, if the current address of the master device is 12 and the next known master 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 master
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.