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The DSA has a very important purpose, besides its role in address assignment. The DSA identifies the kind of product the node is, for purposes of diagnostics. A product may in fact have multiple DSAs, and report multiple DM1s accordingly.

This is a critical difference between RV-C and other J1939-based protocols. The RV-C DM1 includes the DSA as a date field. This DSA is used to parse the SPN. Whereas in J1939, a specific SPN will always have the same meaning regardless of the source (e.g. Coolant Temperature), in RV-C the same SPN may have different meanings depending on the DSA of the source.

A product with more than one function - say, a combination Inverter/Charger - must send multiple DM1 messages, each with a different DSA. Of course, it might be functioning perfectly as an inverter, but report a fault as a charger. The status of both must be reported independently, as though the two devices were in completely separate boxes.

Why do we handle DM1's differently? Because of the Instance concept, absent from similar protocols. Since the Instance of an item is often embedded in the SPN, we have far fewer SPNs to play with. The J1939 SPN is 19 bits, but we allocated eight of those for the Instance in many situations. Thus we only have 11 bits left to use in the general case - a mere 2047 different faults.