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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING DTMF standard number identifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony Phones with automatic number identification (CAL) have been popular since their inception. Recently, in connection with the installation of modern equipment at the automatic telephone exchange, the old AONs stop working. In this article, the author talks about a variant of a number identification device that works with the CLIP (Calling Line Identification Presentation, which literally means "caller identification") service at digital stations. We have been using the opportunity to determine subscriber numbers for a long time thanks to an amateur radio enthusiast who was the first to think of assembling a device capable of receiving a subscriber number from an automatic telephone exchange. All this worked well on old Soviet PBXs, but with the commissioning of modern telephone exchanges of foreign manufacturers, the good old AON turned into just a jukebox - music plays, alarm clocks ring, speaks in a pleasant female voice, but does not fulfill its main function - number identification. This was to be expected because in Soviet automatic telephone exchanges it was not supposed to provide such a service to the subscriber at all - the AON equipment was primarily intended for automatic billing of long-distance calls. Our AONs simply "deceived" the automatic telephone exchange, and she, "thinking" that the intercity station required the number from her, gave the number to the subscriber. But this trick does not work with foreign stations, it became possible to block the issuance of a number to a subscriber. But do not be very upset, because the old way of determining the number has its drawbacks. Only a number no longer than seven digits and the category of the subscriber are issued. To determine the number, a connection must be established between the subscriber and the PBX, which, with time-based billing, brings inconvenience to the calling subscriber. Each of us has repeatedly encountered a situation when you dial a number, the caller ID works on the other end, and there is no one to talk to. This is especially unpleasant for long-distance calls, where the rates are especially high. Now subscribers of modern digital stations can order the number identification service (CLIP), like other services, for money. But now this is a guaranteed service, pay money - you get a service. The CLIP service is free from the above described shortcomings and has wider possibilities. Naturally, in order to use this service, you must first order it from your telephone operator in the same way as other services. Secondly, you need to have a caller ID (commonly called Caller ID) that is compatible with the standard of your telephone operator's equipment. Since the beginning of the 90s of the last century, manufacturers of telecommunications equipment have provided for the possibility of issuing a caller's number as one of the services of digital stations. In parallel, two standards developed. The DTMF standard (Dual Tone Multi-Frequency - two-frequency coding) was first proposed by Bell Labs engineers for data transmission over radio channels, and then began to be used in other transmission systems. Here, each transmitted symbol is represented by the sum of two different frequencies out of eight possible. In total, we have sixteen characters at our disposal: ten digital from 0 to 9 and six service ones - "*'*, "#", "A", "B", "C", "D". The frequency layout is shown in the table.
Combining these characters, we get the required message. The advantages of this standard are the reliability and prevalence of DTMF and the simplicity of the number identification equipment. With regard to the CUP service, this standard has evolved in several stages, so not all stations support it in full. At the first stage, only the number of the calling subscriber or the last redirector was transmitted. In this case, it is not possible to determine if the call has been forwarded. Transmission format: D S1 S2 S3 ... Sn C. In the second stage, only the caller's number or the last forwarder was also transmitted, but in this case it is possible to determine which number was received: the caller's number or the forwarder's number. Format of transmission about the caller: A S1 S2 S3 ... Sn C. Format of transmission about the forwarding subscriber-DS1 S2 S3 ... SnC. At the third stage, both the caller's number and the number of the last redirector were transmitted: (A S1 S2 S3...Sn) (DS1 S2S3...Sn)C. In recent stages, the protocol has been extended to pass additional parameters. It is now possible to include up to five forwarding numbers in a message and additional information codes that indicate how to interpret the message. Transmission format: (A S1 S2S3...Sn)(DS1 S2 S3...Sn)......(D S1 S2 S3...Sn) (B S1 S2) C. The symbols A and D are the start symbols for the numbers of the calling and forwarding subscribers, respectively, B is the start symbol for transmitting parameters, Sn is the digit of the number, n is an integer from 1 to 15. The transmission of information always ends with the symbol C. The duration of the tone of each symbol and the pause between them 70 ms. At any of these stages, information is transmitted about the impossibility of providing the calling number, for example, if the number is protected (CLIR service). In this case, the sequence (B 1 0 C) is transmitted. The number of digits in transmitted numbers can be no more than fifteen. The first two digits are the zone number. In order for the called subscriber to receive the caller's number, it is necessary that the signaling system of the entire chain of stations supports the required data transfer protocol. The use of the DTMF standard for number identification has become widespread mainly in European countries. In the Americas and Asia, the FSK (Frequency Shift Keying) standard is mainly used. In my opinion, this standard is more mature than DTMF, at least at this stage. Initially, this method was developed specifically for data transmission over telephone networks between modems. Here, bit "0" is encoded at 2100 Hz, and bit "1" at 1300 Hz, the transmission rate is 1200 bps. The bits are assembled into eight-bit bytes, and the bytes are combined into messages. Thus, we have 256 characters at our disposal. It became possible to transfer not only numbers, but also alphabetic characters. Now a huge number of Caller IDs of the FSK standard are being produced, which allow you to provide the subscriber not only with the number, time and date of the call of the caller, but also with his name. As for the name of the caller, the possibility of its transfer depends, first of all, on the telephone service provider, the rest of the parameters are transmitted without fail. The telephone exchange, before issuing a number, must somehow inform the subscriber device about its "intentions". Here, too, there are several options: changing the polarity of the telephone line, turning off the line voltage for a normalized time interval, or reducing the line voltage to a certain level. The message may be transmitted before the first ringing signal or between the first and second. In this article, we will look at the DTMF standard Caller ID construction. The device works as a set-top box, connected in parallel to any telephone set on an analog telephone line with a linear battery voltage of 54 ... The set-top box does not interfere with the operation of faxes, answering machines and other devices operating in automatic mode and meets the requirements of standards for connecting subscriber devices. Structurally, it can be made in a separate housing or built into the telephone. The set-top box is powered by a battery of three galvanic cells or batteries of AA or AAA size. Provides constant recharging of batteries with low current from the telephone line. The current consumption from the telephone line with the handset in standby mode (at Upit = 4,5 V) is not more than 0,1 mA, and the power supply recharging current is not less than 0,01 mA. Current consumption from the power source: at the time of dialing or determining the number - no more than 5 mA, with the receiver off-hook or viewing the memory - no more than 0,3 mA. The console's memory is twenty-six incoming numbers, organized on a first-in, first-out basis. With two buttons, you can "flip through" the memory in the direction of earlier calls and in the direction of late calls. The zone, number, time and date of the call are recorded in the memory. Economical use of memory is provided, i.e. if the same subscriber calls you with a frequency of less than 10 minutes, then his number is recorded in memory once and the time of the last call is recorded. When the power is off, the information in the memory and the operation of the clock are retained for at least 3 minutes, which is enough to replace the batteries. The number of new calls recorded in the memory since the last viewing of it is displayed on the indicator. The counter of new calls is reset after viewing the memory. If your phone works in tone mode, the dialed number is duplicated on the indicator, so you can control the correct dialing. The attachment diagram is shown in fig. 1. The device is assembled on three microcircuits. As an indicator, a liquid crystal indicator from Chinese PANAPHONE phones or similar is used. This is a 10-digit indicator with a built-in Holtek controller. The main design element is the PIC16F84A (DD2) microcontroller. To decode DTMF signals, a DTMF decoder chip (DD1) is used in a typical inclusion. Hardware decoding provides higher noise immunity and reliability, in contrast to software decryption. In addition, the program is simplified and minimized.
The DD3 chip combines a clock, a timer, a calendar and a static RAM in which the defined numbers are stored. The l2C interface is emulated in software at the pins PB6 and PB7 of the DD2 controller. The larger the capacitance of the capacitor C7, the longer the memory of the numbers and the course of the clock are retained when the power is off. Trimmer capacitor C6 is needed to set the accuracy of the clock. The VT1 transistor cascade is the simplest comparator for analyzing the state of a telephone line. The RB0 pin of the DD2 controller is configured as an external edge interrupt source. Zener diode VD4 serves to protect the input from possible overvoltage. With a free line, transistor VT1 is open, and when the voltage in the telephone line drops below 50 V, it closes. The tuning of this cascade should be taken especially carefully, which will be discussed further. If your PBX signals the transfer of a number by changing the polarity of the line, then this node will need to be improved, since it is necessary to form a front when changing the polarity. For the sound accompaniment of pressing buttons and identifying the number, a sound emitter HA1 with a built-in self-oscillator for an operating voltage of 6 or 12 V is used. microcircuit This opens the transistor VT1, which turns on the sound signal and provides a low logic level at the output RA4 of the controller DD1. In the number identification mode, this output is configured as an input and the code is strobed through it at the inputs RA0-RA3 of the controller. In the absence of a DTMF signal at the input of the decoder DD1, a low level is present at its output DSO, the transistor VT2 is closed, and the input RA4 of the controller DD2 is connected to the power circuit through the internal circuits of the emitter HA1. In other modes, the DD1 decoder is disabled, the RA4 pin is configured as an open-drain output that controls the power supply to HA1. When the handset is on, the elements R10, VD5 provide the current flowing into the power circuit, sufficient to compensate for the current consumption in standby mode and recharge the batteries. The VD6 zener diode is used to protect power circuits from possible overvoltage. It is advisable to use a zener diode with a sharp drop in characteristic, the total consumption depends on this. Resistor R1,2 is used to power the indicator with a voltage of 1,7 ... 19 V. Selecting it within a small range, you can control the indicator contrast. The indicator is loaded from the outputs RB2 and RB3. Voltage dividers R13R14 and R15R18 are used to match the signal levels between the outputs RB2 and RB3 (DD2) and the inputs DI and CLK of the indicator. When the power is turned on, the registers of the controller DD2 and the clock DD3 are initialized. The power supply of the DD1 chip is turned off due to the low level at the output of RB1 DD2, the DD3 timer is set to an interval of 7 s. After that, the device goes into standby mode, the controller executes the SLEER command. It can be activated by one of the following events: a front at the RB0 input (incoming or outgoing call), a change in the state of the RB4, RB5 inputs (pressing buttons or a pulse at the INT DD3 output). Every 7 s, a pulse appears at the INT pin of the DD3 chip, by which the controller reads the minutes and hours registers from the DD3 chip and loads the HG1 indicator with these values. This prevents the indicator from automatically switching to stopwatch mode. In standby mode, the ratio of controller active time to SLEEP time is 1:7. With an incoming call, before issuing the first ringing signal, the PBX occupies the line and reduces the voltage to 43 ... 45 V. The transistor VT1 closes, the controller DD2 is activated, turns on the power of the DD1 chip and polls the outputs of the decoder D1 - D3 and DSO. The received code is written to the buffer memory, analyzed, and if the first character is A or D, it is decided that this is an incoming call with a number transmission. Information about the number, time and date of the call is packed, stored in memory and displayed on the indicator. Upon receipt of the stop character C, the power supply of the DD1 chip is turned off. If the first character is different from the above, it is considered to be an outgoing call. In this case, each received code extends the power-on time of DD1 by another 7 s. Thus, when making an outgoing call, the key codes of the parallel connected device are displayed on the indicator. Naturally, the device must work in tone (ie, DTMF) mode. In the mode of viewing the memory of incoming calls, pressing the buttons activates the controller, information about the number, time and date of the call is selected from the memory, unpacked and displayed on the indicator. The number is displayed for two seconds, the date and time of the call are displayed for the next two seconds. This cycle is repeated three times, then the device goes into standby mode. The incoming and outgoing call modes take precedence over the memory browsing mode. The device is assembled on a printed circuit board (Fig. 2). Before installing the components, you need to solder six jumpers. Resistors, diodes and the VD3 bridge are installed vertically. The distance between the centers of holes for resistors and diodes is 2,5 mm. The VD3 bridge can be replaced with an imported RB157, and the KP501 transistors with KR1014KT1. You can use SMD components that are soldered to pads. The DD1 chip can be replaced with KT3170, KT9170, KT9270, KT8870 (the first letters may be different) or the domestic KR1008VZh18.
To set up the device, you need a conventional multimeter (preferably digital), an oscilloscope with an input impedance of 10 MΩ, an adjustable DC voltage source up to 60 V, which will replace the telephone line, and a battery of cells or batteries with a voltage of 4,5 ... 4,8 V for power supply devices. You will also need a thin screwdriver with an insulated handle to adjust the trimmers. When properly assembled from serviceable components, the device starts working immediately, and you only need to set the clock frequency of the DD2 controller with resistor R5, adjust the input comparator with resistor R8 and check the current consumption. Before adjusting, you need to set the sliders of the tuned resistors to the middle position. DO NOT connect the set-top box to the telephone line without first installing the batteries! We turn on the power supply 4,5 ... 4,8 V through a milliammeter set to a measurement limit of 5 mA DC. After about 5 seconds, the set-top box will go into standby mode (the time and call counter will appear on the indicator), the current consumption should not exceed 30 μA. If the current is greater or the set-top box does not go into standby mode, it is necessary to check the VD6 zener diode, the quality of installation and the controller firmware. In standby mode, the controller regenerates the indicator every 7 seconds, so the current briefly increases to 100 µA. We turn on the power directly (without a milliammeter). We connect the oscilloscope probe to the output 15 of the DD2 controller and, holding one of the buttons, set the pulse period to 15 μs with a tuning resistor R5. We release the button. The clock frequency is not critical and can be set with an error determined by the sweep of the oscilloscope Without turning off the power, we connect the outputs of the diode bridge VD3 (intended for the telephone line) to an adjustable source of 60 V, and the oscilloscope probe to output 6 of the DD2 controller. At a voltage of 50 V, we set the voltage level at pin 8 to no more than 6 V with a trimmer resistor R0,3. We reduce the voltage to 46 V, while the level at pin 6 must be at least 3 V. Otherwise, you need to check the zener diode VD4 and transistor VT1. We set the voltage to 60 V, and turn on the milliammeter in the gap of one of the wires. The prefix must be in standby mode, while the current in the measured circuit must not exceed 100 μA. Now the set-top box can be connected to a real telephone line and check the operation of the DD1 decoder. Pick up the handset on the phone set to tone mode. The display will clear, you have 7 seconds to dial a random sequence of numbers. They should be displayed on the display, and each press should be accompanied by a sound signal. If there is no indication, it is necessary to check the correct installation, the serviceability of the decoder and the quartz resonator ZQ1. Note that the decoder remains powered on for a maximum of 7 seconds after the last received DTMF signal. Some numbers may not be displayed. This usually happens with Chinese-made telephones and other devices that heavily load the telephone line. In this case, measure the voltage on the telephone line while off-hook. If it is below 8 V, connect 100 ohm resistors with a power of at least 0,5 W in series with the terminals of the telephone set. This will not affect the quality of the connection, but it will help get rid of the problem. The correct setting of the comparator and the display of numbers when dialing from a parallel device is a guarantee of identifying the number during an incoming call. The last stage of adjustment is to adjust the accuracy of the clock with a tuning capacitor C6. Do this during operation. If the clock "runs", slightly turn the rotor C6. Repeat this operation until you achieve an accurate clock. Use a dielectric screwdriver, as adding capacitance to the oscillator circuit of the DD3 microcircuit can lead to its malfunction. The microcircuits used are sensitive to static electricity, so use a "grounded" soldering iron with a power of no more than 40 watts isolated from the mains. Perform all installation operations with the power off. A few words about how to manage the console. Everything is extremely simple. The SB1 "PREV" button scrolls through the memory towards earlier calls, and the SB2 "NEXT" button - towards later ones. To enter the memory view mode, the first press must be at least 0,5 s. The prefix will show the number, date and time of the call, and after that it will automatically switch to standby mode. To enter the clock setting mode, press both buttons simultaneously for at least 0,5 s. The display will show the date, month, hour and minute from left to right. To select a value, use the SB2 button, to set - SB1. To exit the setting mode, press the SB2 button and hold it for at least 0,5 s, and release it at the exact time signal. No other settings required On fig. 3 shows the assembled device.
Programming mode - with the WDT watchdog turned off, the PWRT timer turned on and the RC oscillator. Author: V.Bachul, Chisinau, Moldova
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