Why can't I find my BACnet devices when I scan?
Almost always because the Who-Is broadcast is not reaching the devices. The usual causes, in order of how often they turn out to be the problem: you are on a different IP subnet and there is no BBMD forwarding broadcasts; the Wi-Fi access point is dropping or filtering broadcast traffic; client isolation is on; the devices use a non-standard UDP port; or the devices sit on an MS/TP trunk behind a BACnet router. Finding nothing at all points at the network, not at the devices.
Work through these in order
1. Are you on the same IP subnet?
BACnet/IP discovery is a broadcast, and broadcasts do not cross routers. If the controllers are on 10.20.30.0/24 and your phone got a DHCP address on the guest network at 192.168.1.0/24, you will find nothing and the system is working correctly. This is the single most common cause.
The fix is either to get onto the controls subnet, or to have a BBMD (BACnet Broadcast Management Device) configured to forward broadcasts between subnets. Many sites have one; you need its address and, for a client on a foreign subnet, foreign device registration support in your tool.
2. Is Wi-Fi eating the broadcast?
Access points routinely rate-limit or drop broadcast frames, and a single Who-Is can vanish without trace. A tool that sends Who-Is once and gives up will intermittently find nothing on Wi-Fi even when everything is correct. Re-broadcasting throughout the discovery window fixes this — Easy BACnet re-sends every 1.3 seconds for this reason.
If you can, test on a wired connection to rule this out.
3. Is client isolation enabled?
Guest and corporate wireless networks frequently enable client isolation (also called AP isolation), which blocks traffic between wireless clients and often between wireless clients and parts of the wired network. Nothing you do in a BACnet tool will work around it.
4. Are the devices on a non-standard port?
The standard is UDP 47808 (0xBAC0). Sites that run multiple logical BACnet networks over one physical wire often move some onto 47809, 47810, and upward. A scanner that only checks 47808 will miss them entirely.
5. Are the devices behind a BACnet router?
MS/TP devices — most VAV boxes, many unitary controllers — are not on the IP network at all. They sit on a twisted-pair trunk behind a router that forwards for them. They should still answer a Who-Is, but replies come back with a network number and MAC address rather than a plain IP, and token-passing on the trunk makes them slow: allow several seconds per request rather than one.
6. Is a firewall in the way?
Host firewalls and VLAN access lists both block UDP 47808 readily. On a phone this is rarely the issue; on a laptop it often is.
If you find some devices but not others
That rules out most of the above and points at either a non-standard port for the missing ones, an MS/TP trunk whose router is not forwarding, or devices that are genuinely offline. It is also worth checking for duplicate device IDs — two controllers claiming the same instance number can make one appear to vanish.
If devices appear with no name or no points
The device answered discovery but is refusing or failing property reads. Common causes: the device is busy, it only supports segmented responses for its object list and your client does not, or it is an unconfigured unit still sitting on the factory-default device ID.