Introduction to Modbus TCP IP
Modbus TCP IP protocol based communication architectures (TCP / IP) client / server designed for Ethernet communication Industrial controllers PLCs.
Advantages over other Industrial Protocols
1. It is public.
2. Its implementation is easy and requires little development.
3. Manages data blocks without supposing restrictions.
More Information Modbus .. Wikipedia.

There are a lot of implementations of this protocol in the variety of languages Java, Python, C programming, etc.

it would seem that a lot of Industrial controllers, OPC, HMI and Scada They use Modbus TCP IP.

Important:

To make the whole project that is powerful regarding their interaction with SCADA HMI OPC and PLC correctly you should see Before this Post:

Arduino Mega 2560 Oled Display I2C Master Modbus RTU Scada Industrial Connection

Application Project
I have made the search for examples or functional applications Module ESP8266 - 01
in the industrial field, After testing with PLC controllers via serial Modbus RTU,

Perform module implementation ESP8266 as IP and TCP slave Arduino with SCADA Industrial .

Explaining project

ESP8266 module configuration as slave Modbus TCP / IP is performed, a Modbus TCP IP simulator is used as the master network.

1 module ESP8266 send in Holding Register [0] one Random value generated by the module internally, it will be displayed from the Modbus Master Simulator.

2 The serial port is used ESP8266 with Serial FTDI USB to display the Random value