Schema

Schema concept:

There are modbus devices that (within a single function and set of registers) have defined blocks with registers that belong together.

In most modbus devices a schema is only a single block and any variable can be retrieved independently.

SunSpec is the best example where it works differently.

Sunspec is really a meta-schema where you retrieve the actual set of schema blocks that are present on the device. So the offset of such a block on the actual device varies because each type of device has a different list of blocks.

In Sunspec these blocks are called models and each of those has groups that can have the requirement of being fetched in a single request or can repeat multiple times.

So in this schema standard we have

• Schema:
  • A set of blocks for a single device with the same base parameters (like same base offset and functioncode)
• Block:
  • A group of fields which are based on registers (with optionally its own offset from the schema)
• Field:
  • A single logical value that is based on 1 or more registers within the block.
  • A field can have a ‘fetch group’ value which indicates that it can only be fetched if the entire ‘fetch group’ is fetched.
  • A field can have the immutable flag which means there is no need to read it again on a refresh of the data.
• Register: A modbus register that needs to be fetched. All registers with the same ‘fetch group’ value are always retrieved in a single modbus request.

Remarks about the ‘fetch group’:

• By default, a register does not have a fetch group.
• If a field needs a register then that register gets a fetch group from the field.
  • The normal ‘fetch group’ value from a field is an id based upon the Field itself.
  • If a Field has an explicit ‘fetch group’ then that is used.