MAC36NL - BACnet IP client - Problems with communication

Issue

After adding the BACnetNetwork component to the station, no communication with other devices over the BACnet IP protocol is possible.

Possible causes and solutions

Differing device subnet mask

If it is impossible to find the device on the network with the discover function, it may result from different subnet masks in devices. For example:

• if the eth0 in the MAC36NL controller is configured as below and it is selected in the Adapter slot of the BACnetNetwork component in BacnetComm->Network->IpPort->Link:

  • IPv4 Adress → 192.168.1.12,

  • IPv4 Subnet Mask → 255.255.0.0,

• and the 8I-IP I/O module is configured as follows:

  • IPv4 Adress → 192.168.1.13,

  • IPv4 Subnet Mask → 255.255.255.0,

the devices will see each other in the network and it will be possible to manually add the 8I-IP module in the MAC36NL controller’s station, but the discover function will not work.

The problem is rooted in the fact that the BACnet discover function uses the Broadcast address, which differs for both devices:

• for iSMA-B-MAC36NL - 192.168.255.255

• for iSMA-B-8I-IP - 192.168.1.255

The 8I-IP module will not recognize the frame sent by the MAC36NL controller, because it is out of the module’s address range.

Solution

The solution is to set the same device subnet masks in all devices communicating over BACnet within one network.