The Modbus industry consortium has published extensive documentation accessible online from their home page at Modbus.org. Their protocol specifications apply to a broad range of products for different applications and are much more general than most WattNode® meter users will need. Also see their Modbus Technical Resources page. To save time for programmers writing software to communicate with the Modbus WattNode meter, the document sections that are the most relevant are summarized here. Links to articles on the Modbus RTU protocol over an RS-485 serial network:
- Modbus – General Wikipedi introductory overview of the Modbus protocol
- Modbus Application Protocol Specification V1.1b – Describes the Modbus PLC (Programmable Logic Controller) register model that originated with industrial PLCs and gives details of the message structure. Section 6.3 on page 15 and section 6.12 on page 30 describe the two most important commands i.e. “Read Holding Registers” and “Write Multiple Registers”. Section 7 on page 48 describes the slave exception responses.
- Modbus Over Serial Line Specification and Implementation Guide V1.02 – Describes the Modbus RTU serial protocol. Page 8 describes the message frame. This is probably the best place to start reading in order to understand the Modbus frame structure and message exchange for a typical Master/Slave transaction to read the slave registers. Note: The WattNode meter uses the RTU protocol.
Links to documentation on the Modbus TCP/IP protocol for communicating via third-party gateways over Ethernet LANs and WANs:
- Modbus Messaging On TCP/IP Implementation Guide V1.0b Describes how the Modbus RTU protocol is layered on top of TCP/IP
- Modbus Device Directory – Lists some Modbus gateway devices that support connectivity using Modbus over TCP/IP via Ethernet