Testing a TTL chip

Inputs can be put anywhere in 0-5V...

Tim

The standard advice used to be to connect unused inputs to the posituve rail via a 1k resistor.

TTL can survive brief excursions to 7V acroos the power pins. but the inputs are base-emitted junctions, and can suffer from destructive avalanching if the power supply gets over 5.6V.

Read a TTL data book if you can still find one.

-- Bill Sloman, Nijmegen

For TTL, I'd avoid any voltage between Vih min. and Vil max. so it doesn't go into its analoge range. At best, it will oscillate, and at worst, burn up. =:-O

There is absolutely nothing wrong with strapping inputs to VCC or GND other than if you strap, say, /reset to 0v, it will never set. ;-)

I'd recommend OP double and triple-check his wiring, and post further questions to sci.electronics.basics.

Cheers! Rich

t

I actually found the problem, but I can't seem to fix it. The reset button is supposed to be wired with one side to ground, the other side to a resistor. The other side of the resistor is connected to VCC. Output (wires to ~RESET) are from between the resistor and GND. I had the reset wires connected to ground (wrong side of the switch). I've fixed that, but now no voltage is showing up at the side of the resistor connected to the switch. And the resistor is good.

Why sci.electronics.basics? This isn't basic. It's a calculator, which I am *DESIGN*ING.

--
No, the wire should be connected from the junction of the switch and the
resistor to the \'74 \\RESET pins.

How a TTL chip works, and why it won't in a basic circuit, are certainly basic.

I noticed you've been multi-posting there - don't do that: Crosspost.

Hope This Helps! Rich

Uhh... give me an example of multi-posting that I've done. And, seeing as nobody else is complaining about my posts, I don't see why you alone will influence me.

7474 Reset pin must connect to where switch and resistor join. Your post implies your switch is NC type. Switch must be NO type - only grounds the wire when pressed.

Use the continuity tester and ohm meter of your meter to measure what you have actually connected.

When the switch is not pressed, then the 7474 reset pin should not be connected to ground.

r

Just so you know, the switch is definitely NO, however, I have hooked it up sideways! Here is a physical schematic...

1 2 |_____| | @ | \\ --- | @ | / |

----------- | |

3 4 I had it hooked up the wrong way (don't ask me why they make the switches like this), i.e. I was assuming that pin 1 and pin 2 were the ones getting switched, but it would actually be pins 1 and 3.

Thanks anyways!

Checking back, it was someone else, so please accept my apology; I spoke on impulse.

Again, I'm sorry for that.

But I will stand by my assertion that a basic TTL question should have been in the basics group in the first place.

Thanks, Rich

Which is why everyone should do these things. Example of learning why mistakes are made. You made a mistake by making an assumption. Then only discovered your mistake by discovering you made an assumption. Appreciate how significant that little lesson is.