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How does SmartStruxure check the communication status of BACnet devices.

Issue

Need information on how exactly SmartStruxure checks whether BACnet devices are still online or have gone offline.

Environment

Enterprise Server - ES

Automation Server - AS

BACnet devices.

Cause

Documentation

Resolution

After a device is discovered via the WHO-IS/I-AM service and moved into the SmartStruxure database, SmartStruxure uses the following scheme to check on the communication status of the device.

FOR IP DEVICE

The server will send a Read Property message to read the devices's system status***  every minute, as long as a reply is received the device is deemed to be ONLINE.

If no reply is received by the APDU Timeout the server will retry up to 3 times, if there is no response the device will be considered to be OFFLINE.

FOR MSTP DEVICE

The server listens as the token is being passed around the bus and if a device fails to get the token it will be tagged OFFLINE. 

FOR BOTH IP AND MSTP DEVICES

Once the device is tagged OFFLINE, the server will check on the device once a minute by sending a WHO-IS message with the ID of the device, if an I am is received the device will be considered back ONLINE and the ReadProperty 'probe' check (for IP) or token pass check (for MSTP) will resume once a minute.

 ***In version 1.4 and below the ReadProperty will read the name property of the device.

See sample Wireshark capture below, NOTE that in the sample below 10.169.90.31 is the Broadcast address of the sub net where the ES is located.

NOTE: For efficiency  the BACnet engine may combine the reading of the system status property with other read property requests into a single Read Property Multiple.

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