In-Depth Analysis of MODBUS TCP Communication Protocol and Free Format Communication Protocol

This chapter mainly introduces the MODBUS TCP communication protocol and free format communication protocol based on Ethernet.

MODBUS TCP combines the physical Ethernet network and the network standard TCP/IP with MODBUS as the application protocol standard for data representation. The MODBUS TCP communication message is encapsulated in Ethernet TCP/IP packets, with the maximum length of a MODBUS protocol frame being 256 bytes.

In the MODBUS TCP/IP communication system, there are two types of devices: MODBUS TCP/IP clients and server devices.

The client (TCP Client) actively initiates a connection request to the server (TCP Server). Once the connection is successfully established, only the client can actively initiate communication requests.

When the Ethernet model acts as a MODBUS TCP client, it establishes a TCP connection via the S_OPEN command and initiates MODBUS requests through the M_TCP command.

The server actively listens on port 502, waiting for client connection requests. Once the connection is successfully established, it responds to data communication requests that comply with the Modbus TCP protocol specifications.

The Ethernet model defaults to enabling this service upon power-up, with a maximum response of no more than 4 TCP connections.

Taking the Xinjie PLC as an example, the Ethernet model supports the following Modbus communication function codes:

Free communication based on Ethernet is divided into two major categories: TCP and UDP. When the Ethernet model uses TCP for communication, it can act as a TCP client or a TCP server.

1. As a TCP client, actively establish a TCP connection with the TCP server and bind the socket ID.

2. As a TCP server, wait for the TCP client to establish a TCP connection and bind the socket ID.

3. Using UDP, listen on a specified local port and bind the socket ID.

Based on the above three forms, free communication over Ethernet can be achieved. Free format communication transmits data in the form of data blocks, limited by the PLC cache, with a maximum of 1000 bytes of data sent and received at one time.

Data Buffering Method: 8-bit, 16-bit

1. When selecting the 8-bit buffer format for communication, the high byte of the register is invalid during the communication process, and the PLC only uses the low byte of the register to send and receive data.

2. When selecting the 16-bit buffer format for communication, the PLC stores received data first in low byte and then in high byte; when sending data, it sends the low byte first and then the high byte.

3. If the length of the received data packet exceeds the set receiving length, the data is stored in 16-bit storage format.

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