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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING On the repair of microcomputers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Microcontrollers The time of microprocessors of the 8080, 8085, Z-80 series, their domestic counterparts of the KR580, KM 1821, K1858 and other similar series has gone forever. However, many radio amateurs still use homemade or industrially manufactured microcomputers based on first generation microprocessors. The enterprises continue to operate CNC machines and other technological installations with controllers based on such microprocessors. All this equipment breaks down from time to time. The author of the article shares his experience in repairing microprocessor devices. The most effective way to troubleshoot a microcomputer or microprocessor controller is to alternately replace the main LSIs (including the microprocessor and ROM) with known good ones. But if the LSI is soldered to the board, and not installed in the panel, the efforts expended on its dismantling with almost inevitable damage to the printed conductors often turn out to be useless if the culprit of the malfunction is a completely different microcircuit. You can temporarily replace a "suspicious" microprocessor with a known good one without physically removing the first one from the device. It is enough to transfer its outputs to a passive high-impedance state, in which they are actually disconnected from the internal nodes of the microcircuit. This mode is provided in all microprocessors, it is intended primarily for organizing direct, without the participation of a microprocessor, data exchange between memory and input / output devices. For this reason, it is called DMA - direct memory access. In most microcomputers, the DMA mode remains unused, and the signal input of the microprocessor bus transfer to the passive state is simply connected through a resistor to the power circuit. For K1858VM1, T34VM1, Z-80 microcircuits, this input is pin 25 (BUSRQ), for KR580VM80 it is pin 13 (HOLD). It is enough to connect it with a jumper to a common wire and the microprocessor will be turned off. In parallel with the passive standard microprocessor, you must first connect the same known-good microprocessor by combining their outputs of the same name, with the exception, of course, of the transfer to the PMA mode input and several outputs that do not have a third state. For connections, you can use flexible insulated wires (for example, MGTF-0,14) with a length of no more than 50 mm, and in order to protect the new microcircuit from damage during soldering, provide a panel for it. For the Z-80CPU microprocessor and its copies, only the outputs M1 (pin 27) and BUSAK (pin 23) do not have a high-impedance state. The latter is usually free - check this on the microcomputer circuit or printed conductors on its board. The conductor going on the board to pin 27 must be cut and connected to the same pin of the "hinged" microcontroller. Similarly, they deal with similar conclusions of microprocessors of other types. Now all the functions of the standard microprocessor are taken over by the additional one. If, as a result, the microcomputer started working, the culprit of the failure was found. Since operability has been restored, think about whether it makes sense to leave the microprocessor "attached"? Strengthen temporary solderings, and to prevent accidental shorts, strengthen and insulate the panel. Having decided to completely replace a faulty microcircuit, it is best to bite each of its outputs with specially sharpened wire cutters (their sponges must pass between the outputs), and then, after removing the microcircuit, clean the mounting holes one by one from the remnants of its outputs. If the defect persists, proceed to check the other microcircuits as described above. Chips of RAM, ROM, and many interface LSIs can be switched to a passive state by setting a high logic level at their CS (Chip Select) input. To turn off the LSI of a single-bit dynamic RAM, it is enough to cut the printed conductor going to its only output. Of course, the task must be approached creatively, taking into account the features of specific microcircuits. For example, some of them (K588 series) have a different function of the CS input. For others, the required input is named differently (CE, OE). A temporary ROM replacement can be used to force the microprocessor of the microcomputer under test to execute a specially designed test program that helps to identify and localize faults. Having transferred the microcontroller or other LSI to a passive state, it can be useful to measure the voltage levels at the released outputs with a voltmeter or using an oscilloscope even before connecting the "understudy". If such an output is connected only to the high-resistance inputs of CMOS microcircuits, the voltmeter readings can turn out to be anything, it all depends on the input resistance of the latter and current leakage both inside the microcircuits and between the printed conductors on the board. If one or more inputs of TTL microcircuits are connected to the passive output, the voltage must be within 1 ... 2 V. The presence of resistors in the circuit connected to the power bus or a common wire leads to the setting of the corresponding potential. In any case, it is useful to make sure that the voltage levels on all pins, for example, the data bus of a disconnected LSI, are approximately equal. A significant difference is a reason to think and carefully check the corresponding circuits. The most difficult case is when several outputs of different microcircuits are connected to the same circuit. In a normally working device, they are never active at the same time. Violation of this condition as a result of a malfunction of the control circuits or decryption of the address of one or more microcircuits often causes the microcomputer to fail as a whole. The description of the Z-80CPU microprocessor says that all of its outputs are in a high-impedance state during the operation of the RESET signal. In fact (at least for its domestic counterparts) this is not so: the mentioned signal sets low logic levels at the outputs. Transferring the microprocessor to the DMA mode can be used to connect a test console to its buses with switches that set the address and control signals, and an LED indicator of the address bus status. With the help of such a remote control, you can quickly check the memory and many input / output devices of the microcomputer. The information stored in static RAM (on chips of the K537, K541, K132 series), the remote control will allow you to read and write without time limits. However, please note that a stopped Z-80CPU microprocessor stops generating signals for regenerating the contents of dynamic RAM (usually on K565 series microcircuits) and the data stored there will be lost. Although in microcomputers on microprocessors of other series, dynamic memory regeneration is usually handled by the video adapter or the LIS of the PDP controller (KR580VT57), microprocessor commands may be required for the correct operation of the latter. And in conclusion, I will tell you about the repair of the Delta-S-02 microcomputer with the microprocessor and ROM soldered into the LSI board using the PDP mode. Externally, the defect manifested itself in the fact that when the microcomputer was turned on, only a black frame with a white border appeared on the screen of the TV connected to it. Before displaying the usual initial messages and splash screens, the microcomputer "hung up". Replacing the K1858BM1 microprocessor with a "mounted" one according to the above method did not give any result. However, the performance of the microprocessor even before this check followed from the partial execution of the initialization procedure - after pressing the "Reset" button for some time, stripes running down on a black background were visible on the screen. The renovation was continued in a somewhat exotic fashion. Another, serviceable Spectrum was connected to the Delta through the system connector, the microprocessor of which was stopped by connecting the BUSRQ input to a common wire. The M1 signal missing in the connector was fed from one microcomputer to another by a separate wire. The Delta ROM was disabled via the CS circuit, and the RAM was disabled by a read buffer lock, so that writing to it remained possible in parallel with writing to the RAM of a working microcomputer, but the microprocessor could only read data from the latter. The BASIC program for testing the screen area of the RAM was loaded into this system. The result of its work could be observed on the screen connected to the output of a faulty microcomputer TV. This made it possible to identify the malfunction: when writing the log. 1 in the DD27 chip of the Delta RAM, it simultaneously appeared in a similar DD31 chip. Although it was not possible to find the root cause of the phenomenon, the malfunction was fixed without replacing the microcircuit. It turned out to be sufficient to reduce the signal amplitude by 30%, and with it the interference at pin 2 (information input) of the DD31 microcircuit. This was done using a voltage divider of 2 and 5,6 kΩ resistors. A small addition for the repair services of enterprises. The most common racks in CNC systems 2R22,2U22,2S42 based on the main domestic general-purpose microcomputer "Electronics-60" have a machine line of exactly the same organization as CNC lathes based on the "Electronics NTs-31" microcomputer. Therefore, repair stands, designed for one of these systems, will fit others. It is only necessary to make adapters with the appropriate connectors, while taking into account the differing names of a number of control circuits similar in function. Author: V.Smirnov, Nizhny Novgorod
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