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BACnet provides a solution for this by defining services to report events. These services allow a device to be transformed from a passive server to an active server since it is now capable of sending messages reporting the value of an object property based on some event rules. BACnet COV is a sub-set of the 'Alarm and Event Services'.
This article discusses how the technology operates and then provides a discussion on some weaknesses in the COV system.
" Frames with destination address equal to this station address (TS) and with frame type Receive_Data_Expecting_Reply, Test_Request or proprietary type known to this station that expects a reply.
The passive slave node needs to respond, to a frame expecting a reply, within a specified time frame (Not exceed Treply_delay) from when receiving the last octet of the requesting frame. If this time period has expired the slave will not respond to the frame and will return to its Idle mode.
17. BACNET BBMD connects networks
The BACnet discovery uses two services - called 'Who-Is' and 'I-Am'. They rely on the use of broadcasts.
Routers join IP networks together so messages from one network can be sent to another. Most routers do not forward broadcast messages and this means discovery cant discover devices on another network.
To solve this problem BACnet provides a technology called BBMD - BACnet/IP Broadcast Management Device.
In overview the technology is simple. You install a BBMD (might be a physical device or just a software application on a computer) on each network. You can configure the BBMD by specifying the IP Address and mask of the each BBMD. This makes both BBMD configs identical. When the one BBM receives a broadcast, it forwards the messages to the other BBMD which in re-broadcasts on the other network. They are configured by BDT files and these may be modified on the fly using selected Bacnet services.
The technology also p