Daqing Petroleum Pipeline Co., Ltd. is responsible for maintaining oil pipelines and the power systems that support oil transportation, which means oil pump workstations are spread across vast and remote areas. Due to the varying distances and workload intensities, accidents can occur suddenly and unpredictably, making it challenging for employees to manage their attendance and salary tracking when working at different locations. To address this issue, a centralized control system with remote attendance terminals has been implemented. Each employee uses a radio frequency (RF) IC card as an identification card, and the communication network relies on GPRS technology, which is well-suited for the company's operational environment.
GPRS is built upon the existing GSM network and requires additional components such as the Gateway GPRS Support Node (GGSN) and the Serving GPRS Support Node (SGSN). The GGSN acts as a gateway between the GPRS network and external data networks, enabling connections to various types of networks. The SGSN tracks the location of mobile devices and manages the transmission of data packets between the device and different networks, ensuring efficient and real-time communication. This network structure is shown in Figure 1.
Figure 1: GPRS Network Block Diagram
The use of the GPRS network in this system offers several advantages. It is cost-effective, with options like monthly subscriptions available from China Mobile, providing affordable service. It supports high transmission speeds, up to 115 kbit/s, and offers fast access times, less than one second. It also allows for always-on connectivity, strong reliability, and good resistance to interference, making it ideal for managing attendance across remote locations.
Overall System Design
Based on the company’s requirements, the system workflow is structured as follows: After an employee joins the company, the master control center software communicates with a card issuer via a serial port, reading the 26-bit Wiegand protocol code from the RF card and assigning an employee number. This information is then sent over the Internet to remote terminals where it is stored along with the employee’s RF IC card number. When an employee clocks in at a terminal, the time and employee number are recorded. The timing is managed by the GPRS module, and login status along with terminal information is transmitted back to the central control system. The system stores this data and assigns different salary weights based on the terminal used, allowing for accurate attendance and payroll management. The system architecture is illustrated in Figure 2.
Figure 2: System Architecture Diagram
Hardware Terminal Composition
A key challenge in designing the hardware terminals is ensuring reliable communication through the GPRS network. The SIM300-3j module from SIMCOM is used for wireless communication due to its low cost and powerful features. It includes an embedded TCP/IP protocol stack, eliminating the need for complex programming on the microcontroller. It also provides a standard asynchronous serial port with TTL-level signals, simplifying the connection between the MCU and the module. No level conversion chips are needed, and only a three-wire connection is required for stable communication. The module also offers various peripherals, including A/D, SPI, audio, and microphone interfaces, and supports AT commands for easy development.
Power Supply: The SIM300 requires a voltage between 3.4V and 4.5V. If the voltage drops below 3.4V, the module will shut down. During data transmission, it consumes significant current, so the power supply must provide a peak current of 2A. To meet this requirement, the MIC29302bu adjustable voltage regulator chip was selected. It delivers 3A output and has a low voltage drop of 350mV, making it suitable for the system’s power needs.
To protect the SIM300 module from electrostatic discharge, a transient voltage suppression diode is placed near the SIM card slot, and a 20Ω resistor is added to each signal line for impedance matching.
Antenna selection is crucial for the SIM300. The RF cable should have minimal signal loss, ideally less than 1dB in the GSM900 band. The antenna impedance should be 50Ω, with a VSWR of less than 2. For this, Murata’s MM9329-2700B RF adapter and a universal rod antenna were chosen.
Other components include a liquid crystal display module, real-time clock module, EPROM storage module, and a serial communication module. The card reader uses a standard Wiegand26 interface, outputting 26-bit encoded data. The data signal waveform is shown in Figure 3.
Figure 3: Data Signal Waveform
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