Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [7400]

48 questions
22
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3 answers

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 series chips in his 8-bit computer. Why would he do…
rjm27trekkie
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3 answers

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. Unfortunately there's no datasheet for them so I've…
Anthony
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8
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1 answer

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 gradual transition in VOUT as VIN goes from 1.2 to…
Eeyn
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8
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4 answers

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've found that the pins seem to float a little…
rjm27trekkie
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4
votes
1 answer

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’s no voltage coming to the output. I’ve ensured…
George
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3
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3 answers

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 currently analyzing power supply requirements, and…
mmap32
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3
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1 answer

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 I like to implement "quick and dirty" 5V to 3V3…
Kevin H. Patterson
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2
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2 answers

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 considering using a "good" (thermocompensated) VCO…
valioiv
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2
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1 answer

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 blanking interval for controlling the brightness.…
Matthias
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2
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2 answers

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 an output. If neither of those outputs are enabled…
Richard
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2
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2 answers

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 both and, though the DIP package pinouts are labeled…
cjs
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2
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2 answers

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 designs? Is one better than the other? Or both will…
PotatoSalad
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1
vote
1 answer

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 that would be appropriate for driving some VMEbus…
Tom S
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1
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4 answers

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 BCD and a chip will then display the number on a…
Richard
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1
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1 answer

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 2.0 boards from Aliexpress, a few years ago and…
maxschlepzig
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