In Modbus you need a data sheet to know what data is inside a field device. In BACnet you don’t. You can go-online and discover the devices on a network and then interrogate the devices so they report what data objects they contain and what properties each object supports and what the current state of each property is.
The ability to discover is an almost universal truth in BACnet but there are some limitations:
1. An obscure technicality may limit what you can learn about the object properties.
2. Devices configured as MSTP slaves cannot be discovered but most vendors don’t implement their devices as slave devices for this reason.
The only way to communicate with a slave device is to know about it in advance and configure the client appropriately.
We should also mention that most ‘Discoverers’ (or clients as we like to call them) cannot discover Vendor created proprietary properties.
There are two important practical implications of discovery:
1. If your client software is half decent you do not have to type object/properties into the configuration screens. You simply discover and then drag and drop. Unfortunately more than half the BACnet software out there is not half decent.
2. You would think you can get away without data sheets but again you are then dependent on how decent a job the device Vendor did in naming and describing their points. Bad naming, missing object descriptions, un-implemented properties like max and min values make your job harder and force you to use Vendor docs.