Automatic alarm device composed of MODEM chip and 89C51 single chip

　　The single-chip automatic alarm device composed of MODEM (modem) chip can transmit alarm information over long distances with the help of the telephone exchange network within factories and enterprises, or even the public telephone exchange network, without being restricted by location and time, and truly achieves safety, speed and accuracy.
　　
　　SSI73K222AL is a product launched by TDK, which is a highly integrated single-chip MODEM chip. The main features of this chip are as follows:
　　
　　● It can be connected to 8048 or 89C51 single-chip microcomputers, and the interface circuit is simple;
　　
　　● Serial port data transmission;
　　
　　● It can work both synchronously and asynchronously, including V. 22 expansion overspeed;
　　
　　● It is compatible with CCITTV. 22, V. 21, BELL 212A, 103 standards;
　　
　　● It has the functions of call progress, carrier, answer tone, and long loop detection;
　　
　　● It can generate DTMF signals and 550 Hz, 1800 Hz defense tone signals through programming;
　　
　　● It has automatic gain control, and the dynamic range is up to 45 dB;
　　
　　● It uses CMOS technology, low power consumption, and single power supply.
　　
　　SSI73K222AL has a 28 DIP package, and its pins are shown in Figure 1-102.

　　
　　There are 4 registers inside the SSI73K222AL, which can be used for control and status monitoring. The control register CRO is used to control the data transmission mode on the telephone line; the control register CR1 is used to control the interface between the internal state of the SSI73K222AL and the single-chip microcomputer; the detection register DR is a read-only register that provides the conditions for monitoring the working status of the MODEM; the tone register TR is used to control the generation of audio signals. Under the control of TR, the MODEM can generate DTMF signals, answer tone signals and defense tone signals. In addition, TR can also control the RxD pin during the MODEM startup and contact with the other party. The functions of each status bit of the register and the use of each register are briefly described in Table 1-27 (for detailed information, please refer to the TDK company's 1997 MODEM data sheet).
　　
　　When constructing a single-chip microcomputer automatic alarm device, there are three options for users to choose from.
　　
　　Table 1-27 Functions and usage of each status bit of the SS173K222AL register

1. Call the police directly

      Directly dialing a mobile phone number to sound an alarm is the simplest solution. The hardware circuit is shown in Figure 1-103.

　　Working principle: First, the single-chip microcomputer monitors the alarm signal. In Figure 1-103, the low potential of port Pl.3 is used as the alarm signal. If there is an alarm, the single-chip microcomputer immediately outputs a low level , attracts relay J1, and connects the device to the telephone line; then, the single-chip microcomputer sends a DTMF signal according to the pre-given mobile phone number - starts dialing, and automatically hangs up when receiving the echo signal from the mobile station (disconnects the contact of relay J1).

2. Alarm combined with voice circuit
　　
　　In the scheme combined with voice circuit, a voice circuit should be added. The 1SD-1110 voice circuit is used in the experiment in this section. This circuit has the function of random recording and loop playback, and the playback time is 10 seconds each time. The pin-out of this circuit is shown in Figure l-104.

　　
　　When recording, press the AN button, the LED in the circuit lights up, and the person speaks into the microphone, and the voice is recorded into the chip. The recorded content will not be lost even after power failure. When playing in a loop, just connect PL to a low potential, and
　　
　　the previously recorded voice will be played back through the speaker. Now use the Pl.4 pin of 89C51 to control it, so that it can be put into operation when needed. When wiring, connect the output terminal - SP+ (or SP-) to point A in Figure 1-103, and connect other pins as prompted. The
　　
　　microcontroller programming scheme is basically the same as the above example, except that: here the DTMF signal should be sent according to the pre-given telephone number (such as 110). After the dialing process is completed, after a certain delay, the voice circuit can be started to play the same pre-recorded words repeatedly, such as: "I am someone in a certain place, the situation is urgent, please help", etc. The attendant who answers the phone, no matter who it is, can understand it immediately.
　　
　　In terms of microcontroller control, only two instructions need to be added to control the P1.4 port.

3. Alarm using MODEM and single-chip
　　
display device at the receiving 　　end. Using MODEM and single-chip display device at the receiving end can automatically monitor the alarm information sent from various places in unattended places, store it and display it with digital tubes
　　
, and add other facilities such as alarm sound when necessary. 　　Figure 1-105 shows a simple circuit schematic diagram of MODEM and single-chip receiving device. This device can work with the above alarm device through telephone lines to communicate data.

　　Since the receiving end does not need DTMF dialing and other functions, the low-speed MODEM MSM6946 of OKI Company is used in Figure 1-105. It has a simple structure, low price, and is easy to control and use; it is suitable for 300 b/s, FSK working mode; it can meet the BELL103 standard (for detailed data on MSM6946, please refer to OKI's 1996 M()DEM data manual).
　　
　　In Figure 1-105, the receiving end MODEM is wired in answering mode, and the single-chip microcomputer 89C51 is usually in the state of patrolling and detecting telephone ringing signals. Once the signal is detected, J2 can be pulled in, and after about 2 seconds of silence, the MODEM is started to send the answering tone. After a brief handshake process between the two parties, the 89C51 will display the code sent by the other party through the digital tube. In order to make the circuit simple, Figure 1-105 uses the CMOS
　　
circuit CD4511 　　with BCD conversion, latching, 7-segment decoding and driving functions . When 89C51 outputs BCD codes of O to 9 at ports Pl.7 to Pl.4, the digital tube can display them directly.

　　From this, it can be seen that, depending on the user's programming, this circuit can distinguish the alarm information sent from at least 9 alarm points.
　　
　　In this scheme, the hardware circuit of the alarm device at the sending end shown in Figure 1-103 remains unchanged, but the control software should be supplemented accordingly, that is, after sending the DTMF (dial signal), the program should also add the detection of answering tone, sending and receiving handshake signals, and cyclically sending the local code.
　　
　　Figure 1-106 is a flowchart of the communication program used by both parties in this experiment.