Modbus

Modbus TCP, UDP, and serial RTU. Still the everyday protocol on meters, drives, and small PLCs. Register meaning is vendor-specific: an address without a map is just a number.

Open these 4 in the directory

IndustriConnect Modbus MCPOpen sourceWrites gatedOpen-source Modbus TCP, UDP, and RTU client for MCP hosts, with a mock device and writes off by default.
kukapay/modbus-mcpOpen sourceWrites exposedPython Modbus MCP server for TCP, UDP, or serial, including write tools in the default set.
plctapOpen sourceWrites gatedOne MCP server for Modbus, FINS, MELSEC, S7, IEC 104, EtherNet/IP, and OPC UA, with protocol detection.
Proxus Industrial MCP ServerFreeRead-onlyRead-only Modbus TCP and OPC UA server with named register maps and a connection diagnostic.