Application of RS422 Communication in DNC Control System

The evolution of mechanical manufacturing, from single-machine automation to distributed DNC (Direct Numerical Control) systems, marked a significant step in solving the challenge of information exchange and interconnection among CNC machines. This was the core issue addressed by DNC control systems. To tackle this, the author developed a PC-based RS422 communication board and established a set of reliable and efficient communication protocols. The network architecture of the DNC system is based on a two-level master-slave bus structure, with a three-layer communication model: physical, data link, and application layers. At the physical layer, the balanced RS422 interface was used to enhance the reliability of data transmission. In this new standard, both the transmitter and receiver operate differentially, using two wires for each signal. This eliminates the need for a common signal line, and the signal level is determined by the voltage difference between the two conductors, ranging from -6V to +6V. A difference of +4V to +6V represents a binary '0', while -4V to -6V indicates a binary '1'. The RS422 interface can support transmission rates up to 10 Mb/s over short distances (under 10m), and down to 100 kb/s at 1,000 meters. This differential structure helps isolate effective signals from ground interference and common-mode EMI. At the data link layer, asynchronous serial communication is employed. Data transmission follows a positive and negative code check mechanism. When sending command or status words, two BREAK signals are first transmitted, followed by the actual word, then its inverse code. For file transfers, data is divided into 32-word sections, each containing a positive and negative code. The application layer is user-focused, handling real-time control commands from the lower computer and file transfer commands from the host. Upon successful transmission, a correct flag is displayed; otherwise, an error flag appears. The communication protocol of the DNC system is unbalanced, with the host computer managing the entire system. Command words are sent from the host to the lower computers, while status words come back from the lower to the upper. Each byte contains address information in the lower four bits (coded as 1–3) and command/status information in the upper four bits. For example, if the host sends a command word of 041, it instructs the machine tool with address 1 to stop, while others ignore it. Similarly, a received status word like 02 indicates that the machine tool with address 2 is in edit mode. To ensure reliable communication, the system uses special start characters (BREAK, defined as 0x80 with the lower nibble zero) and forward/reverse code checking. This prevents misinterpretation of random file characters and ensures accurate frame detection. Both the host and lower computer must send two consecutive BREAKs before transmitting a command or status word, followed by its inverse code, forming a complete frame. Only after receiving a properly structured frame does the lower computer respond, ensuring correct command execution. The author implemented the communication modules using Borland C++ 3.1 and Z80 assembly language. The system has been successfully deployed in the three-side punching machine system at the Zhejiang Huaneng CNC Research Institute and has been widely adopted at the Ningbo Metallurgical Machinery Spare Parts Factory. It has demonstrated stable and reliable performance, achieving the intended control objectives and proving the effectiveness of RS422 communication.

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