1、Modbus protocol

(1) Modbus packet type

Modbus protocol is an application-layer message transmission protocol that can be transmitted on various media, and the transmission mode is divided into three types: ASCII, RTU (Remote Terminal Control System), and TCP. 

There are multiple versions of the Modbus protocol for serial ports, but the following three are the most common:

Modbus-RTU: In this mode, RS-485 is often used as the physical layer, and the serial port of the chip is generally used to send and receive data packets, and the message data is communicated with binary data
Modbus-ASCII: packets use ASCII characters. The ASCII format uses vertical redundancy checksums
Modbus-TCP: This is a variant of Modbus that uses a TCP/IP network for communication, connecting via port 502. Packets do not need to be checksumed, as CRC32 data integrity
verification is already implemented at the bottom layer of the Ethernet. 

(2)Differences in the types of Modbus packets
The Modbus protocol can choose RTU or ASCII mode when using serial port transmission, and specifies the message, data structure, command and response mode, and needs to verify the data. The ASCII mode uses LRC check, and the RTU mode uses 16-bit CRC check. TCP is used for

transmission over Ethernet, and this mode does not use checksums, as the TCP protocol is a reliable, connection-oriented protocol.

2、Modbus RTU

(1) Modbus RTU (Remote Terminal Unit) is one of the two transmission modes defined in the original Modbus specification. The two modes are Modbus RTU and ASCII, both of which are designed to be used with serial devices that support RS232, RS485, and RS422 protocols. A distinctive feature of Modbus RTU is that it uses binary encoding and powerful CRC error checking. Modbus RTU is an implementation of the Modbus protocol and is most commonly used in industrial applications and automated production facilities.

(2) RS232：232 is one of the mainstream serial communication interfaces. Due to the early appearance, it is inevitable that there will be shortcomings:
1. The signal level value of the interface is high, which is easy to damage the chip of the interface circuit. 

2. The transmission rate is low. 

3 .Weak anti-noise interference. 

4. The transmission distance is limited, the maximum transmission distance is 50 feet, and in fact, it can only be used for about 15 meters

(3) RS485: The RS-485 interface has good noise immunity, long transmission distance and multi-station capability, and the above advantages make it the preferred serial interface. Because the half-duplex network composed of RS485 interfaces generally only needs two wires, the RS485
interfaces are transmitted by shielded twisted pair cables. The RS485 interface connector adopts the 9-pin plug socket of DB-9, and the RS485 interface with the intelligent terminal adopts DB-9 (hole)

3、Advantages of the Modbus protocol

interfaces, so it is very widely used. 

The Modbus protocol standard is open, publicly published and copyright-free. The Modbus protocol supports a variety of electrical interfaces, including RS232, RS485, TCPIP, etc., and can also be transmitted on various media, such as twisted pair, fiber, infrared, wireless, etc