6.4 Modbus service mapping

The Modbus devices can accept either read or write requests from the Master. The operation to be executed is identified from the function code of a Modbus message. Therefore, the IPE needs to be able to map the oneM2M messages to Modbus messages with the appropriate function code. The function code is identified from register type of the register to be read for the read requests and from a tuple of register type and number of registers to be written (length) for the write requests.

For the read requests, the IPE shall map the register type of the register to be read to the function code according to Table 6.4-1. For the write requests, the IPE shall map the tuple of register type and the number of registers to be written (length) to the function code according to Table 6.4-2. Both the register type and the length along with other Modbus data needed to construct the Modbus message can be retrieved from the nodnProperties customAttribute of a <flexContainer> specialization derived from a ModuleClass.

Table 6.4-1: Register type to function code mapping for Modbus read request

Register type	Function code
Coil	01
Discrete input	02
Holding register	03
Input register	04

Table 6.4-2: Register type and length to function code mapping for Modbus write request

Register type	Length > 1	Function code
Coil	false	05
Coil	true	0F
Holding register	false	06
Holding register	true	10