When the control computer is located far from each CNC machine tool, serial communication is typically preferred over parallel communication. This is due to the high cost of parallel systems and the numerous connections that are prone to interference and line failures. In most control systems, RS-232C is used as the standard for serial communication. It is an asynchronous communication protocol recommended by the Electronic Industries Association (EIA) and widely used in connecting peripheral devices with host computers.
However, when applied in computer systems, RS-232C has several limitations:
- Short transmission distance: Due to the limit on load capacitance, it generally does not exceed 15 meters.
- Signal level shift: When used for long-distance communication, a large current can flow through the signal ground, causing signal distortion and errors.
- Potential grounding issues: The signal and shielded grounds must be connected, but this can create potential differences and reduce system immunity to interference.
To address these challenges, the EIA developed the RS-422/485 standard. These standards use differential signaling instead of single-ended signals, eliminating shared grounds and improving noise immunity. RS-422 supports full-duplex communication, while RS-485 supports half-duplex, making them ideal for long-distance and industrial environments.
In the DNC (Direct Numerical Control) system described in this paper, MOXA’s C320Turbo/PCI multi-port serial card is used to expand the number of available serial ports. Additionally, the FC232 optical isolation transceiver is employed to extend communication distances beyond 15 meters and protect the interface from electrical damage caused by grounding issues or power surges.
This paper outlines a network architecture tailored for enterprise production needs, focusing on efficient data transfer between CNC machines and the central control system. The system includes both management and equipment layers, ensuring seamless communication and program control.
Key components include:
- Management Layer: Comprising the host computer, expansion cards, and peripheral devices responsible for program management and data transmission.
- Equipment Layer: Consists of CNC machines and their corresponding communication interfaces, enabling bidirectional data exchange.
The system also addresses practical challenges such as limited serial ports and long-distance communication. By using multi-port expansion cards and optical transceivers, the system ensures reliable and stable operation even under complex conditions.
Finally, the paper concludes that serial communication remains a vital technology in DNC systems. Future developments will focus on increasing transmission speed, reducing errors, and enhancing overall reliability. The VC-based DNC system presented here demonstrates simplicity, efficiency, and stability, offering real-world value in modern manufacturing environments.
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