Questions tagged [7400]
The 7400 tag has no usage guidance.
50 questions
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Why is the 74L86 pinout different from the 7486 and 74LS86?
Someone mentioned in a forum that the 74L86 pinout is different from the 74LS86. A quick look at a TI datasheet shows that it covers the the 54 and 74 versions of the '86, 'LS86A and 'S86, and all DIP ...
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3
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Simple TTL: Is there a way to "detect" a high-z state and easily use that to drive other logic?
I have two TTL style logic chips, call them A and B (*), where the outputs of A are the inputs to B (think: 8 data lines). A has tri-state outputs, so when they are turned off they float. B is an ...
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1
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1
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Buffer for driving VMEbus
I am working on a project where I am reusing a VMEbus backplane to create a bus between some cards of my own design. This backplane has passive termination on both ends.
I am trying to find a buffer ...
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2
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2
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278
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Differential capacitance change measurement (sensing) with 74LS628
I'm researching the possibilities to accurately register the capacitance changes of a big metal plate (area 10 m2) to the ground (GND, yes - earth, soil). It varies around 100-200 pF.
For now I'm ...
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8
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1
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74HC being labelled as 74LS?
Bought several tubes of 74LS00. We test the datasheet spec for VIH by sweeping one input from 0 V to 5 V while holding the other input at 5 V, and checking the output. Usually we expect to see a ...
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3
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3
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370
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Power supply for medium-scale 74HC TTL circuit
I've developed an 8-bit computer, in the same vein as Ben Eater's wonderful 8-bit computer series. It's based on a collection of 74HCxx-series ICs, and fits (in theory!) on a single PCB board. I'm ...
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2
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4
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245
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74xxx logic. Cheapest, most space saving way to invert 10 logic outputs
Please see this circuit.
I'm trying to design a circuit that allows you to use a single button to increment a counter from 0 to 9 (then it will loop). Diode logic will convert the 10 logic outputs to ...
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1
answer
360
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74HC125/74AC125 vs. 74AHC125N/74AHC125 for overvoltage tolerant USBtinyISP 2.0
The schematic of the USBtinyISP 2.0 makes a point of using a non-inverting buffer IC with overvoltage tolerant inputs:
Only 74AHC125N is 5V compliant at lower VCC!
I bought a couple of USBtinyISP 2....
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0
votes
1
answer
248
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DIY Multiplexing circuit to drive 7-Segment displays like they did in the 1970's
My old CMOS cook-book shows a generic block diagram for a MUX circuit to drive many 7-segment LEDs.
It's assumed the reader will use an off-the-shelf MUX chip to drive the displays, but I don't want ...
- 123
0
votes
2
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211
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Trying to drive the clock pulse of a counter by a NAND gate output but the counter misbehaves
I have connected the output (2Y) of the NAND gate to the clock pulse (CP0) input of the counter, but as soon as I send an input to the NAND gate the counter misbehaves; it does not count in sequence ...
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0
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1
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226
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Max clock frequency when cascading several 74HC for Address Decoding
I'm building a 6502 (WDC 65C02S) computer, for now on a breadboard (maybe later will I hopefully learn and move to a PCB).
In my Address Decoding Logic I'm using several NAND and 4-input NAND gates (...
0
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1
answer
108
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What's the difference between Fmax and Fclock for 74HCxx series chips?
I'm trying to figure out what's the difference between Fmax and Fclock on an example of 74HC74 D flip-flops. I have two datasheets, one from NXP / Philips and another from TI.
The NXP one is pretty ...
0
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1
answer
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74LS14 datasheet question (Short-circuit output current)
the 6th row in the table (Short-circuit output current).
When exactly this value is to be considered? (When output shorted to ground? or Vcc?)
And does it mean I have to consider this value (i.e using ...
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2
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378
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feeding OR gate output to its own input
I'm %100 sure it's been asked before many times, but seriously can't find an answered question here.
let's say I have 5 logic outputs from a chip.
in order to OR them into a single Hi/Low signal, can ...
- 318
1
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1
answer
97
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7408 quad-AND with diode latching: sporadic (and undesired) self-latching at power up
As an introduction to electronics, I am still working my way through Charles Platt's Make: Electronics (2nd edition).
This issue is about Experiment 21: A Powerful Combination
In circuit on ...
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2
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1
answer
503
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74LS47/74LS247 BI/RBO-pin usage
I am currently playing around with a 74LS247 BCD to 7 segment decoder. The basic usage for decoding works perfectly fine.
But I would like to test out everything this IC has to offer including the ...
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0
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40
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74LS pull line low in-circuit
I am working on repairing a TRS-80 Model 4P and would like to test to see if a buffer is working. The problem is the line keeps being held high and I would like to pull it low to see if it is working. ...
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2
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1k
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Hardware debouncing using 74LS14 - inverter input does not drop below 800 mV
I just recently started dabbling with electronics, so probably I just made some beginner's mistake. Here's my problem: I want to debounce a switch in hardware. During my internet research, I found ...
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0
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1
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174
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Automatic switching to the changed set of TTL inputs
I have 2 sets of 4 logical inputs.
All inputs are active-low 5V TTL level lines with 4k7 pull-up resistors, as they may be pulled to ground by a a user pressing a switch or flipping a transistor with ...
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2
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2
answers
644
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Mutually exclusive latching momentary pushbuttons
Here's a circuit which implements a radio button arrangement.
All switches activate the clock.
One of the switches enables a latch which has no output connection.
The other two switches each enable ...
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0
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1
answer
232
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Reducing the number of expensive capacitors in an audio opamp circuit
I'm designing a guitar effect (simple booster) which uses a CD4053 cmos switch as outlined here (http://www.geofex.com/Article_Folders/cd4053/cd4053.htm)
As you can see from the second diagram, RG ...
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Crystal oscillator using 7404 is unstable
I am currently building a small Z80 microprocessor system and require some kind of clock generation. It is not my first design: Earlier I used an Arduino pin that generated a slow clock or a 555 timer....
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2
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105
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What is the output type of the 74AHC594?
What is the output type of the 74AHC594 IC?
I didn't find the answer in this datasheet. There is stated in some datasheets for the 74594 IC from other manufacturers that the output type is buffered. ...
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1
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317
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What are the additional transistors used for in this hex inverter?
I'm learning about electronics and was studying the SN7404 hex inverter schematic when I saw this:
I naively expected just a single transistor (the one in the bottom right) and the input at the base ...
2
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2
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What's the difference between 74LS00 "gates" and 74LS37 "buffers"?
The TI 74LS00 is "quadruple 2-input positive-NAND gates" and the 74LS37 is "quadruple 2-input positive-NAND buffers."
The pinouts are identical. The LCCC package is documented exactly the same for ...
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1
answer
105
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Unpowered 74ls chips vs Powered Nano?
I have a power-hungry 74ls digital logic project that I want to connect a USB-connected Arduino Nano to, but I'm not sure how design the interface between them safely.
The project has an external 5V ...
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1
answer
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STM32F407 CMOS or TTL or HC
I wonder which logic technology is STM32F407 MCU based on.. CMOS or TTL or HC or LVT etc? Although its mentioned in the datasheet that the GPIO's are CMOS and TTL compliant. I am concerned about this ...
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1
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HC vs LVC for 3.3V Logic
I am designing a circuit in which Vdd=3.3V. I have checked datasheets of hex inverters 74HC04 and 74LVC04 and both of these can work good at 3.3V logic circuits. If I have to select one of them then ...
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1
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Clock signal generator using 7400
I have an old PCB that uses this clock generator (means I'm absolutely sticking to the schema). The IC is standard 7400, resistors are the same, but the capacitors I'm using have 470 uF (500's are ...
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2
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2
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516
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Using 74HC154 with LEDS current resistor
I am using a 74HC154 to control 10 LEDs in a design.
Since only one LED will be on at a time I wanted to use one resistor rather than a resistor per LED. Is there any difference between the two ...
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-1
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1
answer
206
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Atari Punk Console with 7400 chips?
How would I create a stepped tone generator using 7400 logic?
For the first part, it's easy to have same range generator, with an additional transistor.
Is there any way to make the same range a ...
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8
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4
answers
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10k Resistor Pull Up/Down Standard for 74 series chips
I've been having some issues lately with the usage of 10k ohm resistors as pull down resistors for 74ls series chips. Most people seem to choose 10k resistors as their default pull up/down value but I'...
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3
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74LS vs. 74HC series ICs for Ben Eater's 8-bit Computer
What are the primary differences between 74LS series and 74HC series ICs? I know that LS series are TTL and HC are CMOS, but beyond that what are the key differences?
For example, Ben Eater uses LS ...
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3
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How can I implement a very simple asynchronous DRAM controller?
I'd like to know how to build a bare bones asynchronous DRAM controller.
I have some 30-pin 1MB SIMM 70ns DRAM (1Mx9 with parity) modules that I'd like to use in a homebrew retro computer project. ...
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0
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0
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Controlling two things with a tri-state output using discrete logic [duplicate]
I'm trying to control a bunch of MOSFETs with an Arduino microcontroller, and was wondering if it would be possible to control two FETs with one tri-state Arduino output. There are three states that ...
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2
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648
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Why does the potentiometer have no effect on the output voltage here?
I have the circuit below, where D and Q are input and output pins (respectively) of a 74373 octal D-type transparent-latch IC. As I vary the pot from 0-10k when D is high, the voltage goes high and ...
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1
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7405 TTL Chip outputting nothing when given low input
I’m struggling learning how to use some of these components for a lab. One thing I noticed is that when giving the 7405 hex inverter a low input, the output when checked by my logic probe shows there’...
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2
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7400 chip register with common in/out pins
First of all, I don't know if this kind of question is allowed here, so sorry if it isn't.
I am looking for a 7400 series register which has common in and out pins (like for the RAM ou EEPROM: ) with ...
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LTSpice: 74HCT Library - Setting 74HCT74 D F/F Initial State
I am using the 74HCT library from the Yahoo user group. I currently have two 74HCT74 D flip-flops in my circuit but I do not know how to set their initial state (Q = LOW for both). Is this done in a ...
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0
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2
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7408 AND Gate, Odd Behaviour
I'm putting together a simple DTACK circuit for a 68k homebrew machine, and part of that is requires me to know when either the Upper Data Strobe (UDS) or Lower Data Strobe (LDS) is asserted LOW. I've ...
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How does an output switching from HI or LO to HI-Z affect an input in CMOS?
Let's say we have a tri-state buffer output connected to an inverter input, implemented in 7400 series CMOS chips.
If the buffer output is HI, the inverter output is LO. If the buffer output is LO, ...
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6
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What are the available solutions for a 8-input OR function that is 74xx compatible?
I need to implement an 8-input OR function that is compatible to IC chips of the 7400 series.
I already found a few solutions, but none of them is perfect. A perfect solution would be an OR-gate chip ...
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1
answer
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Why is there no open-collector OR-gate in the 7400 series? [closed]
I searched in the "List of 7400 series integrated circuits" on Wikipedia for a open-collector OR-gate, but couldn't find any.
The list looks complete, so I assume there is no such chip in the 7400 ...
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1
answer
705
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Why do some logic gate chips have a reversed pin layout?
I am using some logic chips by Texas Instruments, namely SN74HC02N, SN74HC04N, SN74HC32N and others.
When I connected them in my circuit, the SN74HC02N NOR-gate didn't work as expected, so I searched ...
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2
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Flip-Flop Made with 74xx Chips not Latching Consistently
I'm trying to connect two latches made by 74xx chips (normal output: 7404, 7408, 7432...), so that the inverted main output signal is latched into the first circuit on the falling edge and into the ...
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2
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Having issues with clocking a 74S374 (D Flip Flop)
I'm having issues setting the state of any of the D Flip Flops, I have jumpers and a multimeter to work with. does the clock pulse have to more precise than touching the jumper to +5v?
74s374J
data ...
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2
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What is the difference between a 74*374 and a 74*377
What is the exact difference between a 74*374 and a 74*377? The first is classified as an octal edge triggered d-type flipflop while the latter is an 8-bit register. I don't understand the difference ...
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1
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61
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How to produce five outputs, one low, shifted on clock edge. 74HC logic
I want to produce five outputs, one of which should be low, unless the circuit is in reset, when all should be high.
The repeating sequence, on a rising clock edge, should be:
...
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3
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1
answer
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What 7400-series logic variants include input clamp diodes?
When prototyping, I often use spare 7400-series chips I have lying around. I have a good variety of chips, including many 74HC and 74AHC parts, as well as older varieties like 74LS.
From time to time ...
-1
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1
answer
819
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Advantages of 7400 series ICs over diode logic?
I can use diode logic (DRL) or 7400-series logic ICs when I need OR gate or AND gate.
Say 74HC08, 74HC11, 74HC21 (AND gates), or 74HC32 (OR gates).
Diode logic seems to me simpler, cheaper and thus ...
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