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6.2 oneM2M resource mapping structure

6.2.1 Introduction

In this clause, the overall resource mapping structure for exposing services between Modbus devices and oneM2M entities is introduced. Firstly, Modbus devices are modelled according to the oneM2M SDT described in oneM2M TS0023 [3]. The oneM2M SDT offers a generic and flexible modeling structure for describing functionalities of nononeM2M devices including Modbus devices. After the SDT schemas of the Modbus devices are created, they are mapped to oneM2M resources.

6.2.2 Mapping Modbus devices into SDT schemas

Each Modbus device shall be modelled as a Device component. The Modules of the Device shall be created according to the functionality of the Modbus device as defined in oneM2M TS0023 [3].

For representing data objects of a Modbus device, the mapping between a Modbus device's registers [i.2], [i.3] and SDT DataPoints is defined. Every Modbus register has the following properties: slave id, register type, address, length. The information of these registers is typically provided by a manufacturer in a device's datasheet. Register type and length are used to define the following SDT DataPoint attributes: DataType, writable, readable, and optional. The rules to perform the mapping are shown in Table 6.2.2-1. A holding register and input register of length 2 can be mapped into either xs:integer or xs:float DataType depending on data context. As an example mapping, a coil register can be mapped to a DataPoint with DataType (xs:boolean), Readable (True), and Writable (True). The optional attribute depends on a Modbus device and application logic and is supposed to be defined by the system integrator.

Table 6.2.2-1: Mapping between Modbus register types and SDT Data points

Modbus Register Mapping SDT Data points
Modbus register type Length
DataType Readable Writable
Coil (1 bit, Read-Write) 1 (1 bit) =>
 xs:boolean True True
Discrete Input (1 bit, Read-Only) 1 (1 bit) xs:boolean True False
Holding Register
(16-bit, Read-Write)		 2
(4 bytes)		 xs:integer / xs:float
 True
 True
Input Register
(16-bit, Read-Only)		 2
(4 bytes)		 xs:integer / xs:float
 True
 False
Holding Register
(16-bit, Read-Write)		 1
(2 bytes)		 xs:integer
 True
 True
Input Register
(16-bit, Read-Only)		 1
(2 bytes)		 xs:integer
 True
 False
Holding Register
(16-bit, Read-Write)		 4
(8 bytes)		 xs:double
 True
 True
Input Register
(16-bit, Read-Only)		 4
(8 bytes)		 xs:double
 True
 False

6.2.3 Mapping SDT schemas into oneM2M resources

The mapping of all SDT components follows the mapping procedure defined in clause 6.2 of oneM2M TS0023 [3]. For example, the ModuleClass models shall be mapped to the specializations of <flexContainer> resource and their DataPoints to customAttributes of the corresponding <flexContainer> specializations. However, the SDT schemas do not consider interworking options with non-oneM2M Device Nodes (noDN) such as Modbus devices. For that reason, a nodnProperties attribute shall be added as a customAttribute of a <flexContainer> resource specialization which is mapped from an associated ModuleClass model.

The nodnProperties attribute stores one-to-one mappings in CSV string format [i.6] between each customAttribute of <flexContainer> resource specialization and a Modbus register with which it is associated. Each line in the nodnProperties shall contain the name of a customAttribute and associated Modbus register properties (slave id, register type, address, length). The order they are aligned is the following: customAttribute name, slave id, register type, address, length. The nodnProperties shall have one record per line and each property separated by a comma. The header line for this CSV string is mandatory and shall contain the names corresponding to the fields in the string as defined in the section 2.3 of the CSV format specification [i.6]. Table 6.2.3-1 shows the detailed information on the fields of the nodnProperties attribute.

An example oneM2M resource schema including nodnProperties is provided in Annex B, Figure B.2.2-2.

Table 6.2.3-1: Fields of nodnProperties attribute

Field name Type Description
customAttribute name String Name of customAttribute
slave id Integer Slave id of Modbus device
register type Enumeration One of 4 register types (see Table 6.2.3-2)
address Integer Address of the first register associated with a variable
length Integer Number of registers an associated variable occupies

Table 6.2.3-2: Interpretation of register type

Value Interpretation
1 Coil
2 Discrete input
3 Holding register
4 Input register