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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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From what I read, if two input are high on a buffer, then the output will be high. This is clearly what I am expecting.

74LS241 Datasheet PDF

Therefore any results you get using a power supply of 3. Unless you know exactly what you are doing and its consequences, do not apply voltages to the other pins of an IC, without the correct power supply voltage s also being supplied to its specified power pins i.

Then [you connected 3. 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. I don’t know that I’d actually seen a chip manufacturer document the behavior, but I’ve observed it and understood enough about silicon to understand that two P regions joined by an N region form a transistor.

Why is my microcontroller passing a current through it when not connected? Please show the schematic diagram. Understanding how a tristate buffer works 74LS Ask Question. MrBonjour Try another gate then. So your first tests were done without any power connected to the Vcc pin! I have the feeling that I have understood the schematic, but the fact is this is not working.

For some reason, this isn’t working on my circuit. Which I have interpreted as: And when I connect the Vcc pin, without 19 and 17, my led is on, where I expected this to be off. I didn’t put any resistors on the input. This is not unexpected, since you didn’t power the IC via its power pins for dstasheet test.


I’ve done quite a lot of research since first writing this answer, and with unpowered 74LS devices and an input voltage between 0 V and 5. But I thought gate like those didn’t need to be protected. 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.

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I was expecting that my led will 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. The LED being lit is not a surprise. And when I connect the Vcc pin [ Do you have 5 volts Vcc connected to pin 20, and have pin 10 grounded? This circuit will help: What does “put a led on the 3 output pin” mean?

I have put a led on the 3 output pin, in order to test the output. JvO 2 Note that you may not enable both G inputs e. Your explanation “two input are high” sounds more like the definition of an AND gate. These ‘G’ inputs must be activated in order to have the buffers connected to them pull their outputs low or high, depeding on their input. So I clearly don’t understand at all. I don’t use the two G inputs at the same time.

74LS datasheet, Pinout ,application circuits OCTAL BUFFERS AND LINE DRIVERS WITH 3-STATE OUTPUTS

Email Required, but never shown. But my led at pin 18 just glow dimly and never change state, no matter if pin 2 adtasheet high or not connected. And we still don’t know how you have your LED connected???? I’m trying to understand how the 74LS works. By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand that your continued use of the website is subject to these policies. Sign up using Facebook.


When you connected [3.

Sign up or log in Sign up using Google. Unfortunately your test is invalid because according to your recent schematic your Vcc supply to the 74LS is only 3. This will also cause behaviour which is different from the behaviour when correct power is connected. I have tried other gate 19 with 15, 13, 11 and I have the same behavior. It is not guaranteed to operate at 3. Texas Instruments discusses some of these results of reduced voltage power supply voltages to ICs, in this short ” Designing With Logic ” document.

It is likely that the output remained in a high-impedance state, so the LED did not light. Which I have interpreted as being: Although it is not “best practice” to do this, unconnected LS TTL inputs will usually float high, as PeterBennet has already kindly explained.

SE topics where powering devices via other pins, instead of the power pins, caused unexpected behaviour:. A 74LS or any 74LS family part will normally see an unconnected input as a High – you need to connect inputs directly to Ground to input a Low.