The Importance of the Probe in BACnet
When setting up 485 communication to a BACnet device like an AHU, it is important that you Probe it. This gets you some important information:
- Instance number is a good thing because if you add it to the program screen of the controller, it connects much faster when CBAS is restarted. Instance is usually provided in the Probe file but can sometimes have a different name.
- A Probe gets you the list of points that are on the AHU controller. Then you can add them from the Probe file easily by clicking the controller then click Add Points From Probe File.
- Perhaps the most important advantage to probing is that it automatically sets a few parameters to be used by CBAS that can improve performance. Note that you can alternatively set these parameters manually.if probing isn’t possible The most useful of these settings are are COV and Read Multiple. COV is Change of Value and if it is available, it sends any changes in value to the host. Read Multiple reads up to 14 points at a time. By default, CBAS only reads one point at a time unless you have Read Multiple set.
CBAS does not support Write Multiple. It only commands one point at a time and commands take precedence over statuses. So if you have a few or a bunch of commands going out, it will take a while to get status changes. If the commands are going out constantly and you have a slow scan rate, you might not get ANY status updates. This is why it’s important to use the Probe to enable COV and Read Multiple if they are available. Do not set them manually unless they are available. It is also advisable to keep commands to a minimum by slowing down the Logic Evaluate time.
To Probe a BACnet controller in CBAS, first program the BACnet 485 channel and add a BACnet Generic Controller to the Channel at the correct address.
Also make sure the Channel is set to the correct baud rate. This is done on the Program screen of the Channel.
Restart CBAS.
When the Host controller goes normal on the IP Controller Channel, look at the BACnet Channel and wait for the BACnet Generic Controller to go normal. Click the controller then go to Program and click the Probe button. This will only work if you have Normal (blue) Communication to the controller and it will only stay normal for about 1 minute unless you have a correct point programmed. If there are no points, it will go lost after the timeout because it has no points to poll.
If all goes well, the Probe should complete in a few minutes and a popup will tell you where it is saved. (CBASxx\Data or CBAS17\Sites\[Database]\Data)
Using the Probe File
Find the Probe file and open it. Find the Instance number then enter it on the Program screen of the Generic BACnet controller.
Go up one level and click the Controller again then click Import Probe File.
In the Import screen, click the Browse button and select your Probe file then click Load.
You will see the list of points. If there are any points that you don’t need, uncheck them.
Click the Import All Selected Points button or select the desired points.
Restart CBAS and it should start polling the points and giving values.
