Dealing With Multi-function Devices : The DSA.
Whereas the "Instance" concept allows multiple nodes on the network to have the same function, another challenge for any protocol is dealing with multiple functions being served by the same node. A simple example is the "inverter/charger", which obviously serves two different functions. But there are far more complex examples, and there are nodes on the market today that serve over a dozen different functions.
RV-C's mechanism for dealing with the identification of a node's function is the Default Source Address, or DSA. Ironically, the least important role of the DSA is determining the node's source address. Whereas some nodes are statically addressed and thus will use the DSA as its source address, there is no particular meaning attached to the source address. In fact, with the exception of proprietary messages and a few related messages, the source address is ignored. The key purpose of the source address is in bus arbitration, not in identifying the node itself.
That's the DSA's role. The DSA is transmitted in DM1-RVC messages, which serve both as a diagnostic warning and an indicator of the general node condition. A device that has multiple functions reports a separate DM1-RVC for each function. Any other device or service tool can simply watch the DM1-RVC messages being broadcast to obtain a "census" of the devices on the network.
If the device has a fault condition, the DSA also helps identify the code within the DM1-RVC. The SPN within the DM1-RVC indicates the "suspect parameter" for the fault. To interpret the SPN requires the DSA. For example, the SPN 260 may mean something completely different if the DSA is 64 (Generator) or 129 (Waste Dump).
To summarize, every device shall send a different DM1-RVC for each function that the device performs. The DM1-RVC is sent at least every five seconds - more often if a fault condition exists. If a device contains multiple instances of a single type - e.g. a tank monitor likely supports three tank instances - only one DM1-RVC is sent unless a fault exists.
With this scheme it is easy to determine the full range of products on the network and obtain general operating information on all of them.