The mechanical manufacturing industry has evolved from stand-alone automation to distributed DNC control systems. The first thing to be solved in technology is the information exchange and interconnection of CNC machine tools (host computers), which is the core issue of DNC control. The author developed a RS422 communication board for PCs and defined a set of reliable and effective communication protocols. 2 Convention of the communication procedure of this DNC control system Stainless Steel One-piece Ball Valve WENZHOU FOREVER CLASSIC TECHNOLOGY CO.,LTD , https://www.fosicvalve.com
1 Network structure of DNC system
The topology of the DNC control system adopts a two-stage master-slave bus network structure, and the communication network adopts a three-layer structure: a physical layer, a data link layer, and an application layer.
The physical layer uses a balanced standard RS422 interface to improve the reliability of data transmission. In the new balanced standard RS422 (see figure), both the transmitter and the receiver operate in a differential manner, each signal is transmitted with two wires, and the entire interface can operate normally without a common signal line. The level of the signal is represented by the difference between the signals on the two wires. The voltage range is -6 to +6V. For one of the wires, the difference is +4 to +6V, indicating a binary "0". The difference is -4 to -6V indicating a binary "1". For the new balanced standard RS422, when the transmission distance reaches 1000m, the signal transmission rate is below 100kb/s, and in the short-distance transmission within 10m, the rate can reach 10Mb/s. Since each signal is transmitted in two lines, this differential balanced structure can separate the effective signal from the ground interference, and the differential receiver can distinguish the potential difference above 0.20V, so it can be free from the ground potential of the ground reference system. The effects of fluctuations and common mode electromagnetic interference.
Link layer The DNC system control system uses asynchronous serial communication. System convention: Data transmission is verified by positive and negative code: when sending the command word and status word, you must first send two BREAKs, then send the command word or status word, and then send the reverse code: 32 words during file transfer. Sections (front and back) form a packet.
The application layer is user-oriented and is primarily a description of the command input and transmission results. The input of the command includes the input of the real-time control command of the lower computer to the lower computer and the input of the file transfer command. There are two descriptions of the transmission result: when the command or file is correctly transmitted, the correct transmission flag is displayed on the screen: When the command or file is transmitted incorrectly, the error transmission flag is displayed on the screen. 
Figure new balanced RS422
In order to enable data communication between different nodes in the network, the two parties must have a set of rules and conventions that can understand each other and comply with each other. These formats, conventions and response relationships are summarized as communication procedures.
The DNC control system is mainly a bus-type network structure, and the upper computer performs the organization and management of the entire system. Therefore, the communication procedure of the system is unbalanced: the upper computer sends commands, receives responses, and is responsible for error recovery at the link level: the lower computer sends a response and receives a command.