1. Introduction
Fieldbus technology has become a central topic in modern industrial automation. The all-digital control system based on fieldbus is considered the standard for automation systems in the 21st century. PROFIBUS-DP, as an optimized and cost-effective communication solution, is specifically designed for fast and reliable data exchange between control systems and distributed I/O devices at the field level. It replaces traditional 24V or 0-20mA analog signals with digital communication, offering high-speed data transmission in distributed control environments. PROFIBUS-DP operates at speeds ranging from 9.6 kbit/s to 12 Mbit/s, with response times between hundreds of milliseconds. Each message can carry up to 244 bytes of data, making it ideal for real-time applications. The communication medium typically includes shielded twisted pair cables or fiber optics, and it is widely used in building automation, hydropower systems, and industrial process control.
2. Hardware Design of PROFIBUS-DP Communication Interface in Soft Start Controller
The hardware design of the PROFIBUS-DP interface in a soft start controller uses a combination of a single-chip microcontroller and a dedicated integrated chip, specifically the SCM3 + RS485 drive scheme. This setup ensures reliable and efficient communication between the controller and the fieldbus network.
2.1 SPC3 Overview
SPC3 (SIEMENS PROFIBUS CONTROLLER) is an intelligent PROFIBUS-DP slave controller that integrates both the physical layer and protocol functions. It can handle data transmission and reception independently, reducing the load on the main processor. Inside the SPC3, there are RAM, mode registers, status registers, interrupt registers, and multiple buffer pointers. It features 8 data lines and 11 address lines, with the data and address lines multiplexed. It supports various microcontrollers such as 80C32, 80C166, 80C196, and HC196. Additionally, it includes 1.5 KB of dual-port RAM for communication with software or programs, and it automatically adjusts the baud rate from 9.6 Kbps to 12 Mbps.
2.2 Hardware Design of PROFIBUS-DP Communication Interface
The connection between the PIC16F877 and the PROFIBUS-DP network involves a protocol chip SPC3 and an RS-485 driver circuit. The PROFIBUS-DP interface comprises a processor interface and a serial bus interface. The processor interface, shown in Figure 1, connects the 80C32 to external memory through the P0 and P2 ports, with the SPC3's internal dual-port RAM acting as external memory. Data exchange is performed through reading and writing to this RAM. In this configuration, the PIC16F877 serves as a communication slave station and integrates an SPI interface, which allows it to work with the SPC3 and MAX485ESA to connect to the PROFIBUS-DP network.
Figure 1: Processor Interface Circuit
Figure 2: Serial Bus Interface Circuit
The SPC3 transmits data via TXD and receives data via RXD. These signals pass through a high-speed optocoupler HCPL7720 and connect to a bus transceiver ADM1485, forming the serial bus interface. Although the SPC3 includes physical layer data transfer functions, it lacks an RS-485 driver interface, so an external RS-485 driver circuit is added. Among the few drivers capable of handling 12 Mbps, ADM1485 is selected for this system. To prevent signal interference, optical isolation is implemented between the SPC3 and the RS-485 bus drivers. The TXD and RXD signals are isolated using the HCPL7720, while the RTS signal is isolated using the HCPL0601.
3. Software Design of Communication Interface
The SPC3 ASIC chip integrates the full PROFIBUS-DP protocol and manages the state mechanism, allowing the 80C32 to focus on configuration, initialization, and packet processing. The software operation of the SPC3 mainly consists of two parts: initialization and interrupt handling. The flowcharts for the main program and the interrupt program are shown in Figures 3 and 4, respectively.
Figure 3: Main Program Flow Chart
Figure 4: Interrupt Program Flow Chart
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