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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Probe-indicator for logical signals. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Readers are offered a relatively simple probe to check the performance of logic circuits, the presence and evaluation of the duration of pulse sequences. This, of course, is not an oscilloscope, but such a simplified visual representation of logical signals over time is often very useful when working with digital devices. Anyone who works with CMOS or TTL chips needs a reliable, cheap, and easy-to-use device for testing and tuning logic devices. The purpose of creating such a device was pursued by the author when developing his logic probe. So, in a pulse matrix oscilloscope [1], amplitude measurement is provided. In reality, this property is not required for detecting and indicating pulses in common TTL and CMOS microcircuits, and. excluding her. can significantly simplify the device, reduce its dimensions. The device, named by the author as a logical probe-indicator (hereinafter, for brevity - the probe), allows you to observe the logical signals deployed in time and has the following specifications:
It is possible to use the device as a stable frequency source. The principle of operation of the probe is that the logical levels of the input signal are stored sequentially in time in the shift register and displayed on the indicator. The probe, the schematic diagram of which is shown in fig. 1 consists of a number of the following functional units. The master crystal oscillator for a frequency of 1 MHz is made on the elements DD2.1, DD2.2. frequency divider - on microcircuits DD4 and DD6. The control device, consisting of a start trigger and a key, is assembled on elements DD1.3, DD1.4. The short pulse shaper is made on DD2.4-DD2.6 and C4, R4, the input shaper is on DD1.1. Serial sweep registers are assembled on DD3, DD5, DD7 microcircuits. The indicator is a line of LEDs HL1 - HL24.
Shown in fig. 1, the device circuit corresponds to the 24-count variant, although the author made a 48-count indicator probe and some of the information given above refers to the latter variant. An increase in the number of readings is achieved by introducing additional registers and LEDs. The quartz oscillator is assembled according to a well-known scheme. Pulses with a frequency of 1 MHz from pin 10 DD2.3 are fed to the input of the SR (pin 2) of a five-bit BCD counter DD4. It is enabled in decimal mode using the fifth digit to increase the sweep range. Thus, the counter divides the initial frequency by 10 and 20. Turning on the counter according to the standard scheme did not ensure its stable operation. Therefore, the control input CN (pin 3) of the counter is connected to the third digit output (pin 12), as recommended in [2]. to the input of the logic element DD1. Its other input is connected to an RS flip-flop controlled by the Start button SB10. When the button is pressed, the passage of clock pulses through DD20 is allowed. Then these pulses are shortened by the differentiating chain C100R200, formed by inverters DD1.4-DD1 and fed to the synchronization inputs of registers DD1.4, DD4, DD4. The investigated logical signals are fed to the inverter DD1.1 and, depending on the position of the switch SA1. pass to the input of the register information in direct or inverted form. When a synchronization pulse appears on the registers, the logic level that is currently in effect at its input is written to the first cell (bit) of the register. During the recording of the next reading, information about the previous ones is transferred to subsequent cells. Each shift register chip consists of two four-bit sections. Therefore, the information input D (pin 15) of the next section is connected to the output (pin 10) of the fourth bit of the previous section. Thus, three register chips make it possible to store 24 readings of the logic signal level. Since CMOS chips have a larger output current in the log state. 0, LEDs are connected between the outputs of the microcircuits and the power supply plus. Since it is more common to see a high level in a luminous indicator, in direct indication mode (switch SA1 in position "D") the input signal is inverted by element DD1.1. When the button SB1 ("Start") is pressed, the information is written to the registers, after releasing it, the recording ends only after the first of the recorded pulses reaches the last bit of the register DD7 and blocks the passage of clock pulses by switching the start trigger DD3, DD1.3 through the capacitor C1.2 .XNUMX to original condition. When evaluating the indicator readings, it must be taken into account that the states of the LEDs correspond to the logical levels at the input of the probe at the moments of the arrival of the next clock pulses. If the SA3 switch is set to the "1 µs" position and five LEDs are lit in a row, then the pulse duration is about 5 µs. If all LEDs are lit, go to a longer sweep period. An additional switch SA2 ("Control 0.1 ms") was introduced to control the device's operability. In this case, pulses are fed to the input of the probe from output 11 of the counter DD6. They have a duty cycle of 5, i.e., a log is active for 20 ms. 1 and further 80 ms - log. 0. Socket XS1 in the described version of the 24-count probe is used to issue control pulses to the microcircuits under test when the "Start" button is pressed. An increase in the number of LEDs makes it possible to achieve an increase in the accuracy of measuring the pulse duration. A 48-count device requires the addition of three 564IR2 microcircuits connected similarly to registers DD3, DD5, DD7 without an input inverter. A variant of the probe with an indicator for 48 diodes arranged in two identical lines can be used as a double-beam 24-count and as a single-beam 48-count. When the main and additional (without inverter) inputs are connected to view one signal and when one line is turned on to view the direct signal, and the second - the inverse signal, a pulse is displayed on the indicator, as on the oscilloscope screen. When connecting the input of an additional block of registers to the output of the 24th bit of the register, we get an indicator for 48 counts, and the pulse is observed in the polarity determined by the SA1 switch. To work with TTL microcircuits, a stabilized supply voltage of 5 V is required. About construction details. The probe uses AL102BM LEDs (in a metal case) and MLT 0,125 resistors. capacitors C2 - KM-6, C3 - KM-5b, C1 - K50-35 or other small-sized. Quartz resonator - RG-06 at a frequency of 1000 kHz. Buttons SA1, SA2 and SВ1 - MP7. Switch SAZ - MPN-1 for ten positions or similar. XS1 socket - small-sized for a pin with a diameter of 1 mm. Replacement parts with suitable specifications are possible, which will likely affect the dimensions of the PCB and package. Small-sized microcircuits of the 564 series have planar leads. When replacing microcircuits, it is advisable to choose the 164 series. There are no IE561 counters in the K2 series, they are replaced by an analog from the K176 series. Although many microcircuits from this series work at a voltage of 5 V, a preliminary check of their performance at a reduced power supply is necessary. The master oscillator frequency should not exceed 5 MHz, this limitation is related to the maximum switching frequency for CMOS chips. However, one should be aware of the possible inconvenience of counting the pulse duration at a non-multiple value of the resonator frequency and focus more on measurement practice. For example, if you often have to measure pulses of long duration, then the generator frequency can be chosen below the specified one, and vice versa. The printed circuit board for the probe with 24 LEDs is shown in fig. 2. The board is made of double-sided foil fiberglass 1 mm thick. Transition holes are drilled with a drill with a diameter of 0.6 mm. The board has two holes with a diameter of 3 mm. One - fixing, the second - for the withdrawal of the nest; it is attached to the top cover of the case. Four holes with a diameter of 1 mm are designed for fixing MP7 buttons with copper wire rivets.
The SA1 switch is installed on the reverse side of the board opposite the SA2 switch. Two sliders for fixing the microswitches are machined with a plastic needle file. The spring for the SВ1 button is made from the contact plate of the RPU-type relay; the start button is made of textolite. On fig. 3 shows the printed circuit board of the indicator (for 24 LEDs) with the arrangement of elements on it. When installing, first install the LEDs like this. so that their cases do not touch, then resistors are soldered from the side of the printed conductors.
The body is glued with fiberglass epoxy resin. Holes are made in the case for fastening the probe, sliders, switch and three holes for fastening screws. They are installed as follows: one is in the center, and a board with elements is fixed on it, the other two are at the edges. At the place where the board is attached, there is a contact pad through which the screw is connected to a common power bus. Under the nut of this screw, a wire with an alligator clip is attached to connect to the common wire of the device under test. The device was mounted with MGTF-0,07 wire. The board is installed in the case with the elements down, the indication board is laid on top without fastening and pressed with its top cover, which has holes for the LEDs. The probe is connected to the power supply unit with a MGTF-0,07 wire. Literature
Author: N.Zaets, Belgorod region
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