1 Introduction
Fieldbus technology has become a significant focus in the modern industrial landscape. The all-digital control systems based on fieldbus are shaping the future of automation, offering more reliable and efficient communication between devices. Among various fieldbus standards, PROFIBUS-DP stands out as a high-speed, cost-effective solution for communication between control systems and distributed I/O devices at the field level. This protocol is designed to replace traditional analog signals such as 24V or 0-20mA with digital communication, enabling faster and more accurate data exchange in distributed control systems.
PROFIBUS-DP operates at speeds ranging from 9.6 kbit/s to 12 Mbit/s, with a message size of up to 244 bytes. Its response time typically falls between several hundred milliseconds, making it ideal for real-time applications. The communication medium can be either shielded twisted pair cables or optical fibers, which ensures robust signal integrity in industrial environments. It is widely used in building automation, hydropower systems, and process control applications where reliability and performance are crucial.
2 Hardware Design of PROFIBUS-DP Communication Interface in Soft Start Controller
In the design of the PROFIBUS-DP communication interface for a soft start controller, a combination of a single-chip microcontroller and a dedicated integrated chip is employed. Specifically, the SCM3 + RS485 drive architecture is used, where SPC3 (SIEMENS PROFIBUS CONTROLLER) serves as the core component for handling the PROFIBUS-DP protocol.
2.1 SPC3 Overview
The SPC3 is an intelligent PROFIBUS-DP slave controller that integrates the physical layer functions of data transmission and reception. It can independently process the PROFIBUS-DP protocol, reducing the burden on the main microcontroller. Inside, it includes RAM, mode registers, status registers, interrupt registers, and multiple buffer pointers. With 8 data lines and 11 address lines, it supports connections to various microcontrollers like the 80C32, 80C166, 80C196, and HC196. Additionally, it features 1.5KB of dual-port RAM, acting as an interface between hardware and software, and automatically adjusts the baud rate from 9.6K to 12M.
2.2 PROFIBUS-DP Communication Interface Hardware Design
The connection between the PIC16F877 and the PROFIBUS-DP network involves the SPC3 protocol chip and an RS-485 driver circuit. The interface consists of two key parts: the processor interface and the serial bus interface.
The processor interface circuit, as shown in Figure 1, uses the P0 and P2 ports of the 80C32 to expand external memory, while the SPC3’s internal dual-port RAM acts as external memory. By reading and writing to this memory, the SPC3 is initialized and communicates with the system. The PIC16F877, used as a communication slave, includes an SPI interface and can be connected with the SPC3 and MAX485ESA to link to the PROFIBUS-DP network.
Figure 1: Processor Interface Circuit
Figure 2: Serial Bus Interface Circuit
The SPC3 transmits data via TXD and receives it via RXD. These signals are routed through a high-speed optocoupler (HCPL7720) to the ADM1485 bus transceiver, forming the serial bus interface. As shown in Figure 2, although the SPC3 handles the physical layer, it lacks an integrated RS-485 driver, so an external driver is added. The ADM1485 is selected due to its compatibility with high baud rates up to 12M. To isolate the signals and protect the circuits, optical isolation is implemented using HCPL7720 for TXD/RXD and HCPL0601 for RTS.
3 Software Design of Communication Interface
The SPC3 integrates the entire PROFIBUS-DP protocol, allowing the 80C32 to focus on configuration, initialization, and packet processing. The software operation mainly consists of two phases: SPC3 initialization and interrupt handling. The flowcharts for the main program and interrupt routine are presented in Figures 3 and 4 respectively.
Figure 3: Main Program Flow Chart
Figure 4: Interrupt Program Flow Chart
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