So is it possible to protect AON from failures? Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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We have repeatedly talked about methods for improving the reliability of telephones with automatic identification of the caller's number (CALLER), powered by AC power. Some of the restart devices may not work with all software versions. There were other restrictions as well. The proposed article discusses options for failure protection devices that can be used not only in phones on the Z80 processor, but also in other caller IDs.

Consider the main reasons for the unstable operation of AONs.

1. Failures due to impulse noise in the mains. Strong interference is caused by household appliances containing power transformers or electric motors, especially a refrigerator. According to the author's experience, the best protection measure is to allocate a separate outlet for powering the AON, connected to the electrical wiring as far as possible from such devices.

2. The build quality of the device itself. I emphasize that I tested only on AONs assembled on high-quality printed circuit boards with good soldering (otherwise, is it worth spending time and effort on modernization?). The panels in which the microcircuits are installed must provide reliable contact. At the slightest suspicion of poor quality panels, they must be replaced.

3. Power outage transients are the cause of the vast majority of failures. The most characteristic consequences of this for AONs on the Z80 are as follows:

Failures of the last two categories and similar ones have very unfortunate consequences, as they are associated with the distortion of system variables of the GA program that are inaccessible to the user. This entails a "freeze" of the processor and a subsequent restart with a complete loss of information stored in RAM. Often, such a failure does not immediately cause "freezes", but remains in memory and later manifests itself like a computer virus, creating the illusion that the device is working. For this reason, security devices that track indicator scanning are not always effective. Disabling the processor buses with the BUSRQ signal (for the Z80) does not solve the problem either.

Unfortunately, similar shortcomings are also inherent in AONs made on a different element base, in particular on 80s31 microcomputers. Devices that use FLASH memory are better protected from failures.

An analysis of the work of the AON shows that the reason for these phenomena is the insufficient study of the digital part of the apparatus. In particular, when the supply voltage changes from +5 V to zero ( power outage), the WR and RD signals at the inputs of the RAM chip have uncertain values ​​for some time, since the voltage on these lines drops synchronously with the supply. The prohibitive level of such signals for RAM is high. In addition, the possibility of a false selection of the RAM by the CS signal is not excluded. The combination of these two factors can lead to a parasitic operation of the RAM, writing to which information not intended for it creates the effects described above. False selection of RAM in read mode is also harmful: in this case, the data bus begins to be powered by the RAM support capacitor. As a result, in 2 ... 3 s it is discharged by more than half. Naturally, there is no need to talk about long-term storage of data in RAM.

The most effective way to protect against such failures involves monitoring the supply voltage and blocking the RAM at the moment when the voltage drops below a certain level. In this case, the inhibitory signal generated at the CS input of the RAM chip disables it for the entire duration of the transient process. This eliminates both the distortion of information in the memory and the rapid discharge of the support capacitor.

The proposed method has a very high efficiency (more than 99%), since not only the consequences, but also the cause of failures are eliminated. Such protection is applicable in devices with any version of the ROM program, with different types of processors (both Z80 and single-chip microcomputers) and RAM (both two- and eight-kilobyte), i.e., in almost all caller IDs using power from electrical networks. The disadvantage is the lack of protection against impulse noise. If this problem still occurs, you can additionally use a restart device for the Z80, for example, as indicated in [1]. In AONs based on microcomputers, an automatic restart device is usually included in the apparatus.

On fig. Figure 1 shows the basic version of the protection device and its connection to a typical Z80 caller ID scheme using 2 KB RAM. The designation of elements on the AON board corresponds to [2]. The comparator DA1 is used as a Schmitt trigger, the response levels of which depend on the ratio of the values ​​of the resistors R3 - R5 (practically only the value of the lower threshold is important).

When the supply voltage drops (and hence the voltage at pin 4 of DA1) to a certain value, a high level occurs at pin 9 of DA1. Transistors VT1 and VT2 open, while the transistor that controls the AON RAM selection closes. The capacitor in the processor reset circuit quickly discharges through the open transistor VT1, which protects the processor from freezing during short (less than 2 s) interruptions in power supply. The power supply of the comparator itself during the transient process is provided by the capacitor C1.

The device used MLT resistors, capacitor C1 - K50-35. The PCB drawing is shown in fig. 2.

To set up the device, you need a digital voltmeter with an input resistance of at least 1 MΩ and a resolution of at least 0,01 V. First, the resistor R4 must be replaced by a circuit of series-connected constant resistors with a resistance of 2 kΩ and a variable 4,7 kΩ, with the latter set to the minimum position resistance. Then they measure the voltage at pin 4 of the DA1 microcircuit and, slowly rotating the variable resistor slider, set the voltage at pin 3 of DA1 by 0,04 ... 0,08 V below the measured one.

It should be borne in mind that a potential difference of more than 0,1 V can reduce the effectiveness of protection, if the difference is too small, false alarms may appear, for example, due to the temperature instability of the elements. When measuring, care must be taken to ensure that the comparator does not switch to a high level state at pin 9.

After that, the resistance of the circuit of two resistors is measured and replaced with one constant resistor, selected as accurately as possible. The configured board is placed in the AON case, while the connecting wires should be made as short as possible.

To check the protective properties, you need to turn on the caller ID in the network and restart the program (in particular, for the Rus versions, press the keys: "&№42;", "&№42;", "3", "5", "1") . Then repeatedly (30...40 times) cycle the power off and on using an electric extension cord with a built-in switch. After that, you need to view the contents of the AON memory areas available to the user: archives of incoming and outgoing calls, a notebook, alarm clocks. The lack of information in them indicates the reliability of the protection. It is also useful to look at the user constants and compare them with the values ​​that were in memory after the restart. If memory failures are still detected, the setting should be repeated (see above), setting the resistor R4 with a slightly larger resistance.

Now a few words about the RAM recharge capacitor in AON. The optimal capacity can be considered 220 ... 470 microfarads. The main role is played not by the capacitance value, but by the quality of the insulation, i.e., the leakage current. The type of capacitor is selected experimentally. So, cheap Chinese-made capacitors and domestic K50-35 are usually able to maintain RAM power for 3 ... . The best option is to use an ionistor or a battery of 4-2 "finger" cells connected through a diode, this makes the memory of the device practically non-volatile. To place the elements, it is convenient to use the battery compartment available in many devices, in particular "Technica".

One more remark concerns the power supply unit (PSU) of the AON: due to the high sensitivity, the protection device imposes increased requirements on it. The presence of noticeable ripples is highly undesirable, and in some cases generally unacceptable (especially if a very small potential difference is set between the comparator inputs, see above). Therefore, you should check the operation of the PSU under load: the minimum instantaneous voltage at the input of the KR142EN5A stabilizer should not be lower than 8,5 V. It is useful to test the source even at low voltage in the network, using LATR for this. If ripples appear at the output, the PSU should be replaced or measures should be taken to refine it: increase the number of turns of the secondary winding, replace the midpoint rectifier with a bridge rectifier powered by the entire winding, etc.

The second version of the protection device is shown in Fig.3. It is based on the integral timer DA1, which is turned on atypically: the UR input (pin 5) is used to supply the working voltage, and the R input (pin 6) is the reference voltage. The divider R1 R2 allows you to set a voltage of several hundredths of a volt between pins 5 and 6 of DA1, which determines the sensitivity of the device.

The principle of operation is the same as in the first version: during a power outage, the voltage at pin 5 of DA1 drops much faster than at pin 6, as a result, the upper level comparator, which is part of the DA1 timer, is triggered, and a low level occurs at the DA1 outputs. When the power is subsequently turned on, the outputs of the DA1 chip are held high due to the action of the low-level comparator, the input of which (pin 2 of DA1) is connected to the common wire [3].

The DA1 output, which has a push-pull output stage (pin 3), is used to block the machine's RAM. Depending on the processor and RAM used in AON, one of three options for switching on is possible.

1. The device uses RAM KR537RU17 or similar, regardless of the type of processor. In this case, we use the non-inverting CS input (pin 26) of the RAM chip, which is usually not used and is connected to the positive output of the power source. It is necessary to disconnect the indicated output from the power circuit and apply a signal to it directly from the output 3 of the DA1 chip. Resistor R', which maintains an inactive level at the CS input in storage mode, must be mounted on the AON board (Fig. 3).

2. RAM KR537RU10 (RU8) was used, the sampling circuit of which contains a transistor [4]. This construction of the node is used in almost all devices on the Z80 and quite rarely - in other callers. In this case, it is necessary to install the VD3 diode and connect its anode with a conductor to the base of the above transistor, as shown in Fig. 4.

3. RAM KR537RU10 (RU8) was used, the sampling circuit of which does not have a transistor. Such a connection is typical for most AONs based on microcomputers (for example, 80s31) and is extremely rare in devices based on the Z80. Blocking is carried out at the CS input (pin 18) of the RAM microcircuit, for which a VT transistor and a resistor R & No. 39 are installed on the AON board; (Fig.5). It is necessary to cut in a convenient place the printed conductor going to the specified output of the microcircuit, and carefully solder the transistor to it with the outputs of the emitter and collector. To the conclusion of the base VT' connect the conductor from the protection device, while instead of the VD3 diode, a resistor R3 is installed. Resistor R' installed on the AON board between pins 18 and 24 of the RAM chip.

It should be noted that the whole variety of types of foreign RAM chips used in AONs, in practice, comes down to only two types of chips, different in capacity: 2 kB and 8 kB. In particular, microcircuits with 24 outputs are analogues of domestic devices KR537RU10 (RU8) both in terms of functionality and pin arrangement. Similarly, foreign microcircuits made in 28-pin packages are interchangeable with domestic KR537RU17. Separately, we can mention FLASH memory chips (usually manufactured in 8-pin packages); they are used relatively rarely in caller IDs and do not require any protection against interference due to a different physical principle of operation.

The open collector timer output DA1 (pin 7) is used to restart the processor. In the case of the Z80, it is enough to find the initial start-up circuit capacitor on the AON board, to the positive terminal of which the conductor is connected from the specified output DA1.

In caller IDs made on single-chip microcomputers, the protection device supplements the standard automatic restart system, making its operation more correct. To implement protection, you first need to find a conductor going to the reset input of the microcomputer (for example, for 80s31 in the DIP package, this is pin 9 [4]). Then, the logical elements involved in the operation of the restart system are identified (usually it is performed on K561LN2 or K561LE5 microcircuits), and, finally, the initial start capacitor. The negative terminal of this capacitor, as a rule, is connected to a common wire, while a conductor from pin 7 of the DA1 microcircuit must be connected to the positive one.

To establish the device, the resistor R2 (Fig. 3) must be temporarily replaced by a circuit of a series-connected constant resistor with a resistance of 10 kOhm and a variable 47 kOhm. Then they turn on the AON in the mains and, slowly increasing the resistance of the variable resistor from zero, they achieve a malfunction in the operation of the device (disappearance of the readings on the scoreboard). After that, the resistance of the circuit of two resistors is measured and replaced with one constant resistor having a resistance of 4 ... 5 kOhm less than the measured one.

You can check the operation of the protection in the same way as in the first version of the device, and if necessary, repeat the setting. The use of resistor R2 of lower resistance entails a decrease in the effectiveness of protection, and too much resistance can cause malfunctions of the device.

The requirements for the quality of the AON power supply and recommendations for choosing a RAM support capacitor remain the same as for the first option. I will only add that RAM chips with a capacity of 8 KB (KR537RU17 or similar) have a significantly higher current consumption in static mode than two-kilobyte ones. For this reason, even with a high-quality capacitor, it is rarely possible to achieve a storage time of more than one hour; it is advisable to use an ionistor or a battery of galvanic cells for feeding.

On fig. 6 shows a drawing of a printed circuit board.

Literature

1. Turchinsky D. Completion of AON on Z80. - Radio, 1998, No. 8, p.50.
2. Schematic diagram of AON. - M.: Center SCS, 1993.
3. Pukhalsky G., Novoseltseva T. Design of discrete devices based on integrated circuits. Directory. - M.: Radio and communication, 1990.
4. Multifunctional telephone set "Telink". - Radio, 1996, No. 6, pp. 43-46.

Author: D. Nikishin, Kaluga

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