Modbus, Modbus Plus, and Modbus TCP/IP are different variants of the Modbus communication protocol used in industrial automation and control systems. While they share the same basic principles and heritage, there are significant differences between them in terms of architecture, communication medium, and network topology. Here's a breakdown of the main differences:

1. Modbus:

   • Modbus is the original serial communication protocol developed by Modicon (now Schneider Electric) in 1979. It is a master-slave protocol commonly used for communication between programmable logic controllers (PLCs) and other industrial devices such as sensors, actuators, and HMIs.
   • Modbus operates over serial communication interfaces such as RS-232 and RS-485, using a simple, ASCII or binary-based packet format for data exchange.
   • Modbus supports various serial communication modes, including Modbus RTU (Remote Terminal Unit) and Modbus ASCII, with Modbus RTU being the most common variant.

2. Modbus Plus:

   • Modbus Plus is an enhanced version of the Modbus protocol developed by Modicon for high-speed, deterministic communication in industrial automation systems.
   • Modbus Plus uses a token-passing network topology and a proprietary physical layer based on a shielded twisted-pair cable (RG-6U) to achieve high-speed communication rates of up to 10 Mbps.
   • Unlike Modbus, which uses a master-slave communication model, Modbus Plus operates in a peer-to-peer fashion, allowing any device on the network to communicate directly with any other device.

3. Modbus TCP/IP:

   • Modbus TCP/IP is a variant of the Modbus protocol adapted for communication over Ethernet networks using the TCP/IP protocol stack.
   • Modbus TCP/IP uses standard Ethernet hardware (such as Ethernet switches and routers) and TCP/IP networking infrastructure, making it suitable for connecting industrial devices to standard IT networks and the internet.
   • Unlike Modbus RTU and Modbus Plus, which use serial communication interfaces and proprietary physical layers, Modbus TCP/IP relies on standard Ethernet cabling and TCP/IP networking protocols for data transmission.

In summary, while Modbus, Modbus Plus, and Modbus TCP/IP share the same basic protocol principles, they differ significantly in terms of communication medium, network topology, and physical layer. Modbus is a serial communication protocol commonly used for PLC-to-device communication over RS-232 or RS-485 interfaces. Modbus Plus is a high-speed, peer-to-peer protocol designed for deterministic communication over a token-passing network. Modbus TCP/IP is an Ethernet-based variant of the protocol used for communication over standard TCP/IP networks. The choice between Modbus variants depends on factors such as communication requirements, network infrastructure, and compatibility with existing systems.