6.5 Modbus interworking procedures

6.5.1 Retrieve data from a Modbus device

Suppose a scenario when current readings of a Modbus device need to be displayed at an AE application and Modbus-IPE continuously monitors a Modbus device and uploads that data to a CSE hosted on a server in the network. Initially, the AE shall be subscribed to the <flexContainer> resource, which is a specialization of some SDT module for a Modbus device, using a <subscription> resource (notificationEventType A, see clause 9.6.8 in oneM2M TS0001 [1]). The following steps described in Figure 6.5.1-1 shall be performed for this scenario:

1. The Modbus IPE shall send a retrieve <flexContainer> request to the hosting CSE. This <flexContainer> resource is a specialization of some Modbus module and contains nodnProperties attribute.
2. The Hosting CSE shall respond to the retrieve request with <flexContainer> data that includes nodnProperties .
3. The Modbus IPE shall use information stored in nodnProperties to compose Modbus read request. The function code can be identified from a register type as in Table 6.4-1. Slave id, address and length should be written in corresponding message fields. After the Modbus message is composed, the Modbus IPE sends this message to Modbus device.
4. The Modbus device responds with requested data.
5. The Modbus IPE shall send an update <flexContainer> request (see clause 7.4.37.2.3 in oneM2M TS0004 [2]). The request body specifies the customAttributes to be updated and their new values read from Modbus device.
6. After verifying the privileges and the given attributes, the hosting CSE shall update <flexContainer> resource.
7. The hosting CSE shall respond with updated <flexContainer> data after successful update to the Modbus IPE, otherwise it responds with an error.
8. The hosting CSE shall send a notification for <flexContainer> resource update to the AE (see clause 7.5.1.2.2 in oneM2M TS0004 [2]).
9. The AE sends a confirmation message about notification receiving to the hosting CSE (see clause 7.5.1.2.2 in oneM2M TS0004 [2]).

Figure 6.5.1-1: Modbus Slave Device monitoring call flow

6.5.2 Write data to a Modbus device

Suppose a scenario when it is required to update some value in a Modbus device through an AE application registered to a CSE. Initially, the Modbus IPE shall be subscribed to the <flexContainer> resource, which is a specialization of some SDT module for a Modbus device, using a blocking type of <subscription> resource (notificationEventType G, see clause 9.6.8 in oneM2M TS0001 [1]). The following steps described in Figure 6.5.2-1 shall be performed for this scenario:

1. In order to write data to a Modbus device from the AE, the AE sends a request to update specified customAttributes of the < flexContainer > resource which map to the Modbus Device (see clause 7.4.37.2.3 in oneM2M TS0004 [2]).
2. After verifying the privileges and the given attributes, the hosting CSE shall send a notification for the received write request to the Modbus IPE (notification shall include nodnProperties ) and temporarily blocks the <flexContainer> resource for any UPDATE operations (see clause 7.5.1.2.2 in oneM2M TS0004 [2]).
3. The Modbus IPE shall use information stored in nodnProperties to compose Modbus write request. The function code to be used can be identified from a register type and length as in Table 6.4-2. Slave id, address, and length should be written in corresponding message fields. After the Modbus message is composed the Modbus IPE shall send this message to Modbus device.
4. The Modbus device responds with written data to the Modbus IPE.
5. The Modbus IPE shall respond to the hosting CSE with successful device update message, otherwise respond with an error (see clause 7.5.1.2.2 in oneM2M TS0004 [2]).
6. If the device was updated successfully, the hosting CSE shall update the <flexContainer> resource internally, otherwise discard the changes. The resource is unlocked for UPDATE operations.
7. The hosting CSE shall respond to the AE with the result of the UPDATE request.

Figure 6.5.2-1: Writing to a Modbus Slave Device call flow