How many BACnet MSTP devices on a trunk?

There are non electrical considerations to determine how many devices you put on an MSTP network.

The chart below illustrates (from one installation) how little of the bandwidth is used to transfer data. The APDU’s are application layer message that poll and respond with property values – they do work for us as data consumers. The rest is used to maintain the network – passing the token around and looking for new devices.

It’s not possible to provide a calculator to work out how many devices to install on a single network but the following list provides some help in assessing bandwidth considerations.

It takes approx 30 bytes to poll for a single property. It takes about 40 bytes to reply. A token is 8 bytes as is a Poll for master.

Assume that 50% of your bandwidth will be used by overhead (token, poll for master).

Divide the baud rate by 10 to get bytes per seconds.

Using a number like 30+40=70 as a best case and 100 as a worst case (obviously reading descriptions will take more) multiply by the number of objects and properties you are going to poll on a regular basis.

Here are some typical numbers assuming the device doesn’t support the ‘multiple’ services (see below).

Baud	38400	divide by 10
Bytes per sec	3840
Overhead use (token and poll for master)	50%
Byte per sec for payload	19200	a
Typical Poll and response for a single property	70	b
Number of properties that can be polled per sec	27.42857	=a/b
Typical number of props that will polled per object (pres value, status_flags, reliability, out of service)	4
Number of objects per sec	6.857143