2.3. Address mapping

2.3.1. Types of address
2.3.2. Defining address ranges
2.3.3. Overlaying address ranges
2.3.4. Example address mappings

As explained in section 4.3 of the current Modbus specification, the Modbus protocol uses four different address ranges (called "tables" in the specification): discrete inputs, coils, input registers, and holding registers. The function code of a message determines to which address range the address in the message belongs. The address ranges may be separate, or they may be overlaid so that a register may appear in more than one address range. Note that the specification says nothing about how the overlaying is done - in particular, there is no mention of the byte-ordering issues involved.

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