Analysis of Modbus Protocol in TCP communication

1.TCP
Application layer direction TCP Layer send is used for inter network transmission 、 use 8 Data stream represented by bit bytes , then TCP Partition the data stream into message segments of appropriate length （ The largest transmission unit of the data link layer of the network usually connected by the computer （MTU） The limitation of ）. after TCP Pass the result package on to IP layer , It transmits the packet to the receiving entity through the network TCP layer .TCP In order to ensure no packet loss , Just give each package a serial number , At the same time, the serial number also ensures the sequential reception of packets sent to the receiving entity .
TCP/IP The hierarchical model is divided into four layers ： application layer 、 Transport layer 、 The network layer 、 Data link layer .

2.TCP And modbus difference
The effect is different
1、modbus agreement ：modbus The protocol enables the communication between controllers , And the controller communicates between the network and other devices .
2、tcp/ip agreement ：modbus tcp/ip The protocol provides services between the transport layer and the network layer .
Transmission is different
1、modbus agreement ：modbus The protocol transmission distance is short 、 Slow speed , The application is limited .
2、tcp/ip agreement ：modbus tcp/ip Long protocol transmission distance 、 Fast transmission speed , So that it can be widely used .
TCP transmission modbus The data of , by modbus service ,TCP Different services are distinguished by port numbers , The port is equivalent to the house number of entering a building ,tcp Support modbus The port used is 502
3.modbus
encapsulation ： Through encapsulation, the upper protocol uses the services provided by the lower protocol
TCP message ： after TCP Encapsulated data
frame ： Data encapsulated in the data link layer
modbus The four components of communication protocol ：