BACnet MSTP Installation, RS485 and Cables

Token passing and looking for new devices on the MSTP trunk consumes a fair amount of bandwidth.

A BACnet slave can be read/written but never gets the token. So it can't initiate any messages because it never gets the token. The more slaves the fewer token passes. Typically you are not able to put a device in slave mode. Most vendors implement their devices as masters (ie token passing devices).

The more you read and the greater the frequency the more bandwidth that will be consumed.

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).

Every few cycles each (master) token passing device on the network must look to see if there are new devices. Max Master determines the biggest address that must be searched for. Each search involves sending a message and waiting for a response or a timeout (if the devices isn't there). Timeouts cost time. The higher the number of Max Master (default is 127) the more potential devices must be searched for. If you use Max master to improve bandwidth then you must adjust it in each device.

Find the answer to this question by reading the BIBs statement for each device or you could explore the device object of the device, find the property called BACnetServicesSupported and then look at the 14th item in the array to see if Read Property Multiple is supported and the 16th for Write Property Multiple. However, we have found that a large number of devices don't display this information.

Obviously, if you can read a chunk of properties in one message you will be better off than if you can only read a single one.

COV stands for Change of Value. When a device supports COV another device/application can subscribe to receive notifications when an object property changes. This means the data client doesn't have to poll for data continuously but can wait passively to be notified of the change. This reduces the number of messages on a network dramatically.

BACnet allows up to 15 msec for a device to use the token. Since most messages on a MSTP network are token passes a device that uses the token in 5 msec will consume much less bandwidth than one that takes 15msec. (A number of vendors relax this requirement to allow for other vendors implementations. The more relaxed the more bandwidth is consumed doing nothing.)

Without a scope and expertise, you won't know this is a factor. It's easy and cheap to eliminate. Look at the cable spec. Find the nominal impedance. Buy two resistors of the same value. At each end of the trunk install the resistors between the Tx and Rx terminals. If you don't have obvious ends of the trunk (because you created a star) then we recommend re-cabling to form a linear trunk or we wish you luck.

Some devices have terminating resistors built into them. If the vendor did a poor job, the default is to have the resistor active and they must be disabled unless they are the terminating devices on the network. Read vendor docs.

There are a whole string of terms uses as synonyms to describe this phenomenon.

To use two wires ( as opposed to full duplex 4 wire) for RS485 each devices transmitter and receiver must be set to an idle state to release the line for others use. Releasing the line means allowing it to float. It must not be allowed to float at any voltage level so devices have pull up/down resistors to pull the line to an allowable floating voltage. (the floating state is also known as the tri-state. ) The load presented by other devices on the network affects this floating so the resistor values may need to be changed depending on the number of devices installed and the values of the pull up/down resistors they are using. (You can imagine how tricky it's going to be to resolve this). If a device floats out of the specified range then to other devices it will look like the floating devices isn't floating at all. The other devices will think that it is transmitting or receiving and thus blocking the line.

The simplest way of knowing if this is a factor Does the device work properly when it is the only device on the network ?. When you install it in the full network other devices or this device stops working properly. This device and/or the pull up/down resistors of other devices are candidates for investigation.

A number of vendors have a range of pull up/down resistors installed and allow you to change the selection using software or jumpers.

To use two wires for RS485 each devices transmitter and receiver must be set to an idle state to release the line for others use. When a device wants to send it must grab the line. When it has finished sending it must release the line. You can see there are potential problems here. What happens if one device waits too long after sending its last bit before releasing the line it's possible that the other devices will miss some bits of data.