Watchdog for the computer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers
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Often, personal computers are used in continuously operating systems for managing, collecting and processing information. Unfortunately, computers are prone to crashes that cause the program being run to hang. In well-established systems, such events are rare, but they cannot be completely excluded. You have to keep the operator on duty, whose only task is to find a "freeze", press the "Reset" button and restart the computer.
In specialized computers and controllers for industrial use, the problem is solved by using the so-called watchdog timer (translated as "watchdog"). A normally running program periodically writes to its register a number that specifies the delay, after which the timer generates an interrupt request signal or processor reset, if no new writing to the register has occurred during this time. A "hung" program does not do this in a timely manner, as a result, the computer restarts.
General purpose computers are not equipped with such a timer. The device whose circuit is shown in the figure will successfully replace it. A sign of normal computer operation in this case is the presence of a signal at the TXD output of one of its communication ports. The program to be protected from "hangup" must constantly output data to it. Which one doesn't matter. This can be useful information intended for a peripheral device connected to the port, or any other, so long as the signal at the TXD output is not interrupted for a long time.
The values of the voltage rectified by the diodes VD1 and VD2 on the capacitors C2 and C3 are equal in absolute value (about 10 V), but opposite in sign. As a result, the voltage at the base of transistor VT1 is close to zero and it is closed. After the termination of the TXD signal, the capacitor C3 discharges faster than C2, the capacitance of which is several times larger. The voltage at the base of the transistor VT1 becomes positive and opens it. The "Reset" circuit of the computer is connected to a common wire until the capacitor C2 is completely discharged.
As a result, the computer will start by loading the operating system and executing commands from the AUTOEXEC.BAT file. Among them, there should not be any requiring operator intervention, for example, entering a password, selecting a mode. Only loading the necessary resident drivers and calling the working program. During the entire restart process, the watchman does not affect the operation of the computer, since there is no TXD signal, the capacitors are discharged, and the transistor is closed. With the appearance of the signal, the capacitor C3 is charged first, then C2, and the device is again ready for operation.
Author: A. Dolgiy, Moscow
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