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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Advanced TTL logic probe. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Many years of experience with digital devices allowed the author to improve the probe described in the Radio magazine in 1990. As a result of its modification, in particular, it became possible to count and indicate up to 20 pulses, use the probe for auditory frequency control and expand the operating frequency range of a simple frequency counter. This probe will be useful when setting up various electronic devices on TTL chips. In [1], a probe was described that determines the state of logical circuits and counts the number of pulses. It also provides for the possibility of auditory control of the frequency of oscillations entering its input in the range from audio frequencies to 10 MHz. When finalizing this device, some changes were made to it, which simplified the work with the probe. Firstly, the existing threshold values of TTL logic levels have been changed: 0,4 V - log. 0 and 2,4 V - log. 1. These voltage values correspond to the standard TTL output logic levels and allow us to judge the operation of the microcircuit as a signal source. It is often more important to know how some level in the logic circuit perceives the input of the next chip. Proceeding from this, the threshold voltage values are chosen respectively for the input: 0,8 V and 2 V [3]. The input switching voltage has a fixed value of 1,5 V, only for new series of TTL microcircuits, for example, K (R) 1533 and KR1531, and for old ones - K155, K555 and KR531 - it changes within certain limits. Thus, if we keep in mind only promising series of microcircuits, then the indication of an indefinite state is practically not needed - we can assume that the log. 0 is a voltage below 1,5 V, and log. 1 - respectively, above 1,5 V. But since the old series of microcircuits will work for many more years, an indication of an indeterminate state was left in this probe. Secondly, in the source device, inconveniently for perception, there is an indication of the number of logical pulses received at the input (in binary code). How many people can quickly convert the number of pulses, expressed in binary code, to decimal? The choice of the division factor of the input pulse frequency for listening to the head phone is also inconvenient. Taking into account these comments, the probe circuit had to be slightly changed. Now it contains five microcircuits and one seven-segment indicator (see figure).
The probe with three LEDs displays the logical states of the input: zero, indefinite indicator state and one. The display time of short pulses is lengthened in order to be able to evaluate them visually. If the pulse stretching is disabled, then the relative brightness of the LEDs can be used to judge the duty cycle and squareness of the input signal. To determine the number of pulses received at the input, the probe is equipped with a counter and a digital indicator that displays numbers from 0 to 9. The inclusion of a decimal point is used to indicate the unit of transfer to the most significant digit. Thus, a sequence of up to twenty pulses is fixed. If necessary, the counter can be reset to make it easier to keep counting. The probe also allows you to judge the frequency of the signal "by ear", by comparing the frequency according to the "higher - lower" principle, and after some training - to approximately determine the frequency of the signal entering the input. For this, a piezoceramic sound emitter HA1 is installed in it, connected to the output of the divider by 2 - pin. 12 DD3 (for frequencies 100 Hz ... 30 kHz). The control of pulse sequences with a frequency of up to 10 MHz is carried out through an additional divider, reducing it to sound. Now more about the probe circuit. Two repeaters are installed at its input (separately for log. 0 and 1) on transistors VT1 and VT2. Resistor R1 protects them from current overload when a voltage outside 0 ... 5 V is applied to the input. Resistors R2 and R3 create a load for the repeaters and a bias for the inputs of the microcircuit. Elements DD1.1 and DD2.2 form logical level thresholds for subsequent blocks, therefore, K1533 series microcircuits are used - they have a fixed input threshold. Element DD1.2 generates a signal of an indefinite state of the input. From the outputs of these three elements, the generated signals (active level - low) are fed to the inputs of three single vibrators on the elements DD2.1, DD2.3 and DD2.4, which control the LEDs for indicating logical states. The second inputs of the single vibrators are connected through resistors R14 - R16 to the microswitch SB1, which controls all the functions of this probe. In the position of the switch shown in the diagram, the single vibrators stretch the pulses arriving at them for their reliable detection. In the other position of SВ1, pulse lengthening does not occur, since the feedback signal at the upper inputs of the single vibrators does not reach the switching threshold. As a result, the duty cycle of the periodic sequence of the input signal can be estimated "by eye", comparing the brightness of the glow of the LEDs HL1 and HL3, and the squareness - by the brightness of the glow HL2. The brighter it is, the more shallow the fronts and decays of the pulses, but if they are almost rectangular, HL2 does not glow. Decimal counter DD3, the input C1 of which is connected to the output of the element DD1.1, counts the received positive changes in the input signal. (If this input is connected to the DD2.2 output, it will count negative drops). To the outputs DD3 connected to the converter code DD4 indicator HG1, displaying the number of received pulses in decimal form. The counter is reset during the switching of the contacts of the switch SВ1, since only at this time there is a log on both inputs R0 of the counter DD3. 1. Since the lower position of the switch SВ1 according to the scheme is used to analyze groups of high-frequency pulses, in this position a log is applied to the input DE of the code converter. 0 to turn off the indicator and reduce power consumption. To output 8 of the counter DD3 connected counter-divider by 64 (DD5). From output 1 of DD3 and from output 2 of the second counter of the DD5 microcircuit, the pulses are fed to the NAND elements DD1.4 and DD1.3, the other inputs of which are connected to the switch SВ1. In the position SB1 shown in the diagram, the element DD1.3 is turned off, and DD1.4 is turned on - a signal passes to HA1 with a frequency that is 2 times less than at the probe input. When you press the button SB1 through the element DD1.3 to HA1, the input signal passes after the frequency is reduced by 640 times. From output 8 of the DD3 microcircuit, an output was also made to an external connector for connecting to a frequency meter probe, so the probe can also be used as an active input probe for measuring the frequency of digital signals (in this case, the frequency meter readings are multiplied by 10). Dividing by 10 is necessary here so that when pulses with a frequency of up to 10 MHz are applied to the input, a signal with a frequency of no higher than 1 MHz arrives at the external connector for the frequency meter. This allows the use of a relatively cheap frequency counter. The counter DD5 from output 1 through the transistor VT3 controls the glow of the decimal point on the indicator, which displays the unit of transfer to the most significant digit (a luminous dot indicates that 10 should be added to the indicator reading). A little about the design of the probe. Its body is a plastic case from a ballpoint pen measuring 149x21x15 mm. At the end of the case, a steel needle is installed as a probe (it is convenient to pierce the protective varnish on the terminals of radio components and printed tracks of boards), and on the opposite side there is a female part of a small-sized three-pin connector (for head stereo phones). Wires are soldered to the pin part of the connector (pin diameter 3,5 mm), through which power is supplied, as a rule, from the device under test and the output signal is transmitted. The ends of the wires are equipped with crocodile clips. The probe can also be powered from an independent power supply, but in this case, the common wire of the probe and the microcircuit under test should be connected together. Holes are cut out on the side of the case for LEDs placed on the board, displaying logical levels, and a seven-segment indicator of the pulse counter. In addition, the head of the microswitch button is located in a place convenient for pressing with the index or thumb. All parts of the probe are mounted on a single-sided printed circuit board; most of the connections are made with printed conductors, the rest - with a thin wire insulation. The pins of the microcircuits not indicated in the diagram are not connected to anything. Capacitors C1-C3 are placed above the microcircuits, the piezoelectric element of the signaling device HA1 is also placed, opposite which several small holes are made in the case for the passage of sound. Microcircuits DD1 - DD3 in the probe can be replaced with similar ones from the K (KM) 555, K155, KR1531 and even KR531 series, but this will lead to an increase in current consumption and a decrease in operation stability (it would be much better to use DD3 from the KR1533 series). The K561IE10 chip can be replaced with the same one from the 564 series, and instead of DD4, you can use, for example, K (P) 514ID1 along with replacing DD6 with an indicator with a common cathode and the corresponding operating current (in this case, resistors R6 - R12 are not needed). When using other decoders and indicators, they can be matched as described in [2]. The indicator should be selected based on the appropriate dimensions, the size of the familiarity and the brightness of the glow (preferably red). LEDs HL1, HL3 - any low-power suitable size. They should be taken of the same color, otherwise it is difficult to determine the duty cycle of the pulses by brightness. Any high-frequency low-power silicon transistors of the appropriate structure with a base current transfer coefficient of at least 100 are applicable in the device. Resistors - MLT 0,125 (R1 - 0,25 W), capacitors C5 - C7 - K50-16, K50-35 or similar. Pushbutton switch SВ1 - any small-sized with one switching contact without fixing. To preserve the small dimensions of the probe, the HA1 piezoelectric element placed in it was removed from the body of the ZP-3 sound emitter, but it is better to use some small-sized one, used, for example, in electronic watches. To protect against incorrect power connection, it is easiest to install a D310 type germanium diode (with a minimum forward voltage drop) in the gap of the positive supply wire in the same way as done in [1], but in this case the supply voltage will drop by about 0,2 V. The best for the probe, an option would be to connect a zener diode for a voltage of approximately 5,5 ... 6 V between the power buses of the probe, and instead of a germanium diode, a 250 mA fuse that will withstand the normal supply current of the probe, but if the supply voltage is exceeded or its polarity is changed, it will be burned increased current. The disadvantage of such protection is the need to replace the fuse (however, if the power supply of the tested design can withstand the increased current). Other protection devices are also possible. The maximum current consumption of the probe is about 200 mA, and the microcircuits consume only about 40 mA, and the rest is the indicator circuits. You can reduce the power (and brightness) consumed by the indicators by doubling the resistance of resistors R6 - R13 and R20 - R22. In conclusion, it should be said about the adjustment of the probe response thresholds. If desired, they can be changed, including low-power germanium diodes in the discontinuities of points A - E. The introduction of diodes at points A and B increases the threshold between an indeterminate state and a log. 1 (but by a different amount), and at point G - slightly lower. Diodes at points B, D and E lower the threshold between an indefinite state and a log. 0. If it is necessary to achieve logical thresholds similar to those indicated in [1], one low-power silicon diode should be included in the gaps at points B and D. The ability to control a level exceeding 2,5 V, which corresponds to the threshold for CMOS microcircuits, and the small input current of the probe make it possible to use it to control devices on microcircuits of the K561, K176 series with a supply voltage of 5 V. Literature
Author: V. Kirichenko, Shakhty, Rostov region
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