Design of Fire Alarm Network Monitor Based on GSM Network

Preface

The fire alarm network monitor is an intermediate device between the fire alarm system and the city fire alarm network monitoring center. It completes the information forwarding between the fire alarm system and the city fire alarm network monitoring center. Only through the fire alarm network monitor can the city fire alarm network monitoring center connect the fire alarm system of the entire city into a fire alarm monitoring network to achieve the purpose of quickly grasping the dynamics of the city fire alarm. The composition of the city fire alarm network is shown in Figure 1.

Traditional urban fire alarm networks are generally based on PTSN networks (public telephone networks) for communication through DTMF signals. Fire alarm network monitors use modem chips to send and receive DTMF signals, and network monitoring centers use voice cards and other devices to send and receive DTMF signals. The urban fire alarm network implemented using PTSN networks has the characteristics of low cost and high communication reliability, so it has been widely used. However, some shortcomings have been found in actual use. If the telephone line used by the network monitor is on the same line as the user's telephone line, it will affect the user's normal call. In order to ensure reliable communication, an independent dedicated telephone needs to be opened, which adds additional workload such as laying another telephone line. Moreover, wired telephone lines are more susceptible to external factors such as human factors, and line communication failures may occur, affecting the reliable operation of the system. If the wireless GSM network can be used to realize the communication between the network monitor and the monitoring center, it should be a good choice. However, due to the low reliability of early GSM network communication, the network coverage is relatively narrow, and the communication charges are very expensive, it has not been put into practical use.
Nowadays, with the rapid development of wireless mobile communication technology, wireless mobile communication technology based on GSM network is becoming more and more perfect, communication reliability is further improved, network coverage is expanded, and communication charges are already comparable to wired telephones, or even lower. Therefore, it should be feasible to use GSM network for communication in urban fire alarm network.

1. The working principle of network monitor

1.1 Alarm sending

The network monitor constantly monitors the various alarm signals sent by the fire alarm controller through the RS232/485 communication interface. When receiving the alarm information sent by the fire alarm controller, the monitor will give a sound and indicator light prompt, the display screen will show the current alarm content, and enter the alarm sending delay at the same time.

During the delay phase, the user can confirm the authenticity of the alarm information through keyboard operation. The monitor will then convert the manually confirmed alarm information into the specified communication format and upload it to the city fire alarm network monitoring center. The indicator light and display screen of the monitor will also display the prompt information during the communication process. If the user does not perform any judgment operation during the delay phase, the delay ends and the monitor sends the alarm information that has not been manually confirmed to the network monitoring center.

If the data is not uploaded successfully, the monitor will reselect another monitoring center number and dial, and the screen will display the message of re-uploading. Repeat this process until the data is successfully uploaded or all target numbers are called. If all target numbers are called and the data is not uploaded successfully, the screen will display the message of upload failure.

1.2 Receiving Data

The monitoring center can remotely set the engineering parameters of the monitor: automatic registration time, monitor time, alarm delay, ringing and ringback times. It can also call the monitor by name and check on the on-site duty personnel.

When the monitor detects the calling phone and confirms the legitimacy of the calling phone through "handshake verification", the monitor indicator light and display screen will show the prompt information during the communication process, and the monitor will start to receive data information from the monitoring center. After the data is received, the monitor will process the data and return the processing results to the caller, and the communication ends.

1.3 Monitor Settings

Users can set project parameters through the keyboard and display: number of rings and ringbacks, user code, city code, alarm delay, monitor time, user password, and check historical records.

2. Hardware design of network monitor

It mainly includes GSM module interface circuit, DTMF signal demodulation circuit, host computer interface circuit, LCM circuit, power supply circuit and MCU control circuit. The hardware block diagram is shown in Figure 2.

In the design of the monitor, the GSM module is used to complete the transmission of DTMF signals between the fixed phone and the mobile phone. Since the GSM network supports sending DTMF signals through the GSM module through signaling, but does not support the GSM module to directly decode the DTMF signal, the DTMF decoding circuit must be added in the design of the monitor to decode the signal output by the GSM module audio output interface.

2.1 DTMF signal decoding circuit

DTMF signal decoding is achieved through the DTMF signal encoding and decoding dedicated integrated chip MT8888C. MT8888C has read and write signals suitable for interfacing with the MSC51 series microcontroller. Its signal input interface is connected to the GSM module audio output interface, as shown in Figure 3.

2.2 GSM module interface circuit

The GSM module uses Siemens' TC35i module[1]. The TC35i module has a standard MODEM serial control interface, and the MCU uses AT commands to control its operation. When a call comes in, the module's RING pin will output a corresponding level change signal to the MCU for detection.

2.3 Host computer interface circuit

The monitor communicates with the host computer through the RS232/RS485 interface. In order to prevent the host computer from inputting interference noise through the interface circuit and affecting the work of the DTMF circuit, an optical coupler is used to isolate the interface signal.

2.4 Power supply circuit

The system is powered by 220VAC mains electricity, and the low-voltage AC power is isolated by a transformer. After being stabilized by a voltage stabilizing circuit, it supplies power to the GSM module, the single-chip microcomputer system and the optical coupler isolation part respectively.

2.5 MCU Control Circuit

The WINBOND W77E58P microcontroller is used, which has two built-in USRT interfaces, which are respectively allocated to the GSM module and the host computer for communication. A watchdog circuit is used to ensure the reliable operation of the program, an external RAM is used as a buffer for sending data, and an EEPROM is used to save working parameters.

3. Software design of network monitor

The microcontroller initializes the TC35i module and sends DTMF signals through AT commands[2]. The DTMF signals output by the TC35i module are decoded by the MT8888C. The software workflow of the terminal device is shown in Figure 4.

The monitor control program is implemented using the C51 language [3], which features fast development speed, strong structure and good maintainability.

After the microcontroller is powered on and reset, it is first initialized, including setting the serial port working mode, baud rate, and initial values ​​of variables and flags. Then, the TC35i module is initialized through serial port 1 to check the SIM card status, GSM network signal strength, etc. Then the monitor enters the detection loop to detect whether the alarm signal sent by the fire alarm controller and the external phone call signal are received.

If the alarm signal sent by the fire alarm controller is received, a prompt message will be displayed, waiting for the user to confirm manually. After the manual confirmation or the delay time has expired, the TC35i module will be controlled by the AT command to dial the monitoring center. If the call fails, the call will be repeated. After the call is successful, the DTMF alarm signal composed of the fixed communication format will be sent to the monitoring center. If the monitoring center fails to receive and does not send back the receiving response signal, it will dial the monitoring center again for uploading. After completion, it will return to the detection cycle.

If an external call is detected, the module will count the ringing signal output by the RING pin, wait for a fixed number of rings before answering the call, verify the legitimacy of the incoming call, and display the corresponding prompt information. If it is not a call from the monitoring center, it will hang up and return to the detection loop; if it is confirmed that it is a call from the monitoring center, it will wait for the data command sent by the monitoring center, process the command accordingly, and return to the detection loop after completion.

4. Results and analysis

Three prototypes were placed at three sites for a one-month trial run. During the test, the on-duty personnel manually created and uploaded alarm data at irregular intervals every day. The test results are shown in Table 1.

The test results were relatively good. Only the No. 2 and No. 3 prototypes had a total of 3 upload failures during the test run. The main reason was that too few target phone numbers were set during the run, only 2 target phone numbers were set (all setting was afraid of affecting normal communication), and the network monitoring center phone was busy. The run test shows that the actual run should be able to get satisfactory results.

5. Conclusion

The fire alarm network monitor based on the GSM network uses the voice channel of the GSM network to complete the communication with the network monitoring center, which is figuratively speaking, the voice communication between mobile phones and fixed phones. The design of the fire alarm network monitor based on the GSM network takes into account both the compatibility with the existing urban fire alarm network (considering that some old users will not immediately replace the original PTSN equipment), and the real-time and reliability of the communication. In this way, the new fire alarm network monitor based on the GSM network can be seamlessly compatible with the original urban fire alarm network, and at the same time complete the upgrade to the GSM network, and the monitoring center does not need to be upgraded, saving the transformation cost.