The S7-200 series PLC integrates an RS485 interface, which enables freeport communication, MODBUS RTU communication, and USS protocol communication. Among these, freeport communication is the most basic and important. To master the serial communication function of the S7-200 series PLC, it is essential to understand its application. Although Siemens provides corresponding MODBUS and USS protocol library instructions to help users quickly master MODBUS and USS protocol communication programming, those familiar with library instructions will find that these instructions are also function block programs created using the freeport method.
In the following articles, I will share my experiences in learning freeport communication, hoping to provide some help to everyone learning freeport communication.
Before learning how to write freeport programs, we must first understand the following basic knowledge.
(1) Communication protocol
A communication protocol is an agreement between devices for exchanging information, much like a language between people. There are many types of communication protocols. Commonly heard protocols include MODBUS, USS, PPI, and MPI.
Communication protocols are essential to master when learning freeport communication. The core of freeport communication is using transmit commands (XMT) and receive commands (RCV) to send and receive data. What data is sent out, and which data is valid in the received data, are all information derived from the communication protocol. The communication protocol is provided by the communication device acting as the slave station, such as when a PLC communicates with an instrument or a frequency converter. The instruction manual for the instrument or frequency converter will then provide the communication protocols it supports. (The following diagram shows the communication protocol of an instrument.)
(2) Information frame format or message message
When it comes to information frame formats, many people don't understand what they are. Some call them data counts or message messages; regardless of the terminology, the meaning is the same. An information frame format indicates the format of the data that our PLC needs to send out and the data that is received. This information frame format is based on the communication protocol, as described above. Assuming that communication uses ASCII characters, if we need to read the process value data from an instrument with station number 01, and the process parameter name is "M1" (note: these parameter names will be given in the instrument's manual), then according to this communication protocol, we can obtain two information frame formats: one is the information frame format that the PLC needs to send out, and the other is the information frame format that the PLC receives back.
(3) The difference between start character, end character and start bit and stop bit
In freeport communication, you will often hear about start bit, stop bit, start character, and end character. So what are the differences between them?
First, from a literal understanding, you can see that a stop bit or start bit is a single bit. A start character and an end character are single characters. A single information frame format contains multiple characters, including a start character (EOT) and an end character (ENQ), as described in the protocol above. Each character in this information frame format consists of a start bit, a stop bit, data bits, and a parity bit. (See the diagram.)
