PLC has several functional modules: input, output, communication and display.

There are analog and digital inputs, in which the analog may be floating-point or signed/unsigned integer, and the digital is binary 0 or 1, so there is no string for input and output. Communication mainly includes tcp/ip communication and serial communication. Serial communication is usually based on modbus-rtu protocol of RS485 bus, which is used for external sensors or industrial control panels and adopts binary format data. Some manufacturers can customize the communication protocol of ASCII coding. When PLC interfaces with devices or sensors through this customized ASCII coding communication protocol, character strings are needed.

In addition to modbus-tcp, tcp/ip communication also uses json format data to report data to the cloud server through http protocol, and establishes a long link with the cloud server through mqtt protocol to realize the function of Internet of Things, thus remotely controlling plc. Here, both http and mqtt are text format protocols, and strings are required.

Usually, the display interface of plc is realized by industrial control screen. For the plc all-in-one with its own display, if the display interface language can be configured, then strings are needed. Generally speaking, strings may be used in the following situations: 1. Use a customized text format communication protocol to communicate with devices or sensors. 2. Communicate with the server through a text format protocol such as http/mqtt (usually json data format). 3. Interface configuration of some plc all-in-one machines The following picture shows a PLC designed by myself, which is wirelessly connected to a remote server through a WiFi module. PLC establishes a long TCP link with the server, and uses json data in text format for data interaction.