SN54/74LS • SN54/74LS • SN54/74LS DC CHARACTERISTICS OVER OPERATING TEMPERATURE RANGE (unless otherwise specified). S. b l. P. 74LS Octal Buffers AND LINE Drivers WITH 3-state Outputs SN74S, SN74S OCTAL BUFFERS Details, datasheet, quote on part number: 74LS Texas Instruments 74LS Buffers & Line Drivers are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for Texas Instruments .

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I have tried other gate 19 with 15, 13, 11 and I have the same behavior. Another issue I’ve observed is parts that behave as though pairs of pins are connected via PNP transistor, such that excessive voltage on one will cause the device to source current into the adjacent pin. But I thought gate like those didn’t need to be protected. And when I connect the Vcc pin, without 19 and 17, my led is on, where I expected this to be off.

I have the feeling that I have understood the schematic, but the fact is this is not working.

74LS Datasheet(PDF) – National Semiconductor (TI)

Unfortunately your test is invalid because according to your recent schematic your Vcc supply to the 74LS is only 3. If you pull pin 19 down, the output will float and both LEDs will glow dimly because current is passing through both of them. When you connected [3.

And when I connect the Vcc pin [ Then [you connected 3. Pins 1 and 19 of the LS control this state for the buffers inside the chip. Note the invertor symbol tiny o ring at the control gate connected to pin 74ls24 you must pull pin 19 high to activate, and pin 1 low to activate. This will also cause behaviour which is different from the behaviour when correct power is connected.


A 74LS or any 74LS family part will datashdet see an unconnected input as a High – you need to connect inputs directly to Ground to input a Low. Your explanation “two input are high” sounds more like the definition of an 74lw241 gate. SE topics where powering devices via other pins, instead of the power pins, caused unexpected behaviour:.

MrBonjour Try another gate then. Sign up using Facebook.

Sign up or log in Sign up using Google. If you use a 5V power supply, datsheet make sure this is always connected to the device Vcc and Gnd pins see belowthen you should see expected behaviour. I was wondering if I didn’t have broke my chip because I don’t have protected the input gate, but it wasn’t asked in the schema. Do you understand WHY we have tri-state outputs? So here is what I did: Then i wired the 19 and 17 pin with 3. I was expecting that my led vatasheet be off with pin 17 not connected but pin 19 still high, and the led on with 17 and 19 high, but this is not happening.

74LS241 Datasheet

Which I have interpreted as: A tristrate output can have 3 different states: I didn’t put any resistors on the input. Do you have 5 volts Vcc connected to pin 20, and have datashewt 10 grounded? Texas Instruments discusses some of these results of reduced voltage power supply voltages to ICs, in this short ” Designing With Logic ” document.

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But my led at pin 18 just glow dimly and never change state, no matter if pin 2 is high or not connected. Why is my microcontroller passing a current through it when not connected? The LED being lit is not a surprise. Do you understand what a buffer is and why we use it?


Although it is not “best practice” to do this, unconnected LS TTL inputs will usually float high, as PeterBennet has already kindly explained. I’m trying to understand how the 74LS works. Mr Bonjour 1 2 SE topics where powering devices via other pins, instead of the power pins, caused unexpected behaviour: Please show the schematic diagram. So your first tests were done without any power connected to the Vcc pin!

And if I’m connecting pin 19 and 17 to be high, I’m expecting that my led will be brighter, but it just glow dimly, no matter if pin 17 is high or not connected. I have put a led on the 3 output pin, in order to test the output.

And we still don’t know how you have your LED connected???? 74l241

74LS 데이터시트(PDF) – Motorola, Inc

What does “put a led on the 3 output pin” mean? Email Required, but never shown. Pull pin 19 high, and the output will follow the input 17 and either the green or the red LED will glow brightly; the other one will be off. Sign up using Email dwtasheet Password.

Without it, the outputs float. Which I have interpreted as being: From what I read, if two input are high on a buffer, then the output will be high.

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