# TTL circuits Help. (NOT, AND, OR, ETC.)

Discussion in 'General Electronics Chat' started by alejandrodj, Sep 29, 2012.

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1. ### alejandrodj Thread Starter New Member

Sep 10, 2012
13
0
Can someone explain to me step by step how TTL gates work? for the life of me i can't understand and my teacher as much as i hate to say it, he can't seem to teach it in a way i can understand it.

if someone can show me a step of step procedure of how a NOT gate works. NAND, and NOR. when is 0 and when is 1. thanks a lot guys in advance.

2. ### BSomer Member

Dec 28, 2011
433
106
When looking at the various logic gates you want to see what it will take to make the output High. So for a "AND" input A "AND" input B must be High for the output to be High. When you have a "NOT" gate, be it a NOR or NAND, everything is "inverted" or opposite.

Perhaps reading through the Logic Gates e-book on this site may help a little. Logic gates are fairly simple devices.

What exactly are you getting hung up on? Is it writing out the truth tables or just understanding what is a High (1) level vs. a Low (0) level?

3. ### ScottWang Moderator

Aug 23, 2012
4,933
777
NOT Gate - Input and Output like two sides of the Seesaw, never get the balance.

exp : Input = 0, Output = 1.
exp : Input = 1, Output = 0.

* Memory : The output is contrary forever with the input.

OR - This gate just like there are two(or three) doors of the room, you can get in or get out from any door.

exp : Input1 = 0, Input2 = 0, Output = 0, two input = 0, so Output = 0.
exp : Input1 = 1, Input2 = 0, Output = 1, any input = 1, so Output = 1
exp : Input1 = 0, Input2 = 1, Output = 1, any input = 1, so Output = 1
exp : Input1 = 1, Input2 = 1, Output = 1, any input = 1, so Output = 1

* Memory : any input = 1, then Output = 1.

NOR Gate - This gate is contrary with OR gate.

* Memory : any input = 1, then Output = 0.

AND Gate - This gate like two(or three) switches in one wire, Output = 1 when all condiction is completed.

* Memory : any input = 0, then Output = 0.

exp : Input1 = 0, Input2 = 0, Output = 0, two input = 0, so Output = 0.

NAND Gate - This gate is contrary with AND gate.

* Memory : any input = 0, then Output = 1.

When you do the labs more and more, then you will remeber the gates funtions.

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Are you specifically asking about the internal workings of a TTL gate?

If, what family? (i.e., 74xx, 74Sxx, 74LSxx, etc.)

I'd recommend first learning how the internals of CMOS gates work. They are extremely simple and you get to see the basic concepts at work. Then look at the 74xx TTL family.

In general, for the 74xx family, you have three stages of circuitry: A logic input stage, which produces a suitable single-ended signal at it's output for the logic being implemented; a single-ended to double-ended converter that creates two signals that are compatible with the output stage, and a totem-pole output stage in which one transistor pulls the output HI while another pulls it LO with the output from the second stage determining which transistor is turned on.

If this is what you are looking for, we can walk through a detailed analysis of the TTL internals.

5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The thing you have to understand is TTL is not really complex, but the reason it is so popular is in either state it draws about the same current. Other logic families, such as RTL, is much simpler, but when they are in one state the current draw is much different then if they are in the other. The result of this was massive current surges in large circuits, such as early computers. The power supplies had trouble keeping up. With TTL the current was relatively constant.

Our local ebook has a section on TTL...

As a logic family, I much prefer CMOS. It barely sucks current, works over a large range of voltages, has extremely high input impedance (which allows for large fan-outs), and low output impedance (which allows it to go rail to rail to the power supply on the output voltages). The downside is it can barely drive a LED, and sometimes not then.

Understanding all the logic families is important though. I frequently use Diode logic on the fly with other logic types, but you have to understand the requirement of each family to successfully do something like this.

6. ### alejandrodj Thread Starter New Member

Sep 10, 2012
13
0
thanks guys for your input. i do understand the truth table for each gate. however i wonder if its enough just to know how the TTL basic circuit looks and what it does. or is it necessary to know what happens internally when the signal is a 0 or a 1.

these links show how a TTL nand and and gates look like and i just wonder if i need to know what happens internally if 0 or 1. is that important or should i just know how to identify them and know what they do and not how they do it?

7. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
In general, the more you know how and why things work, the better. But, having said that, we all have to accept some things as being a "black box" and settle for knowing what they do and not how they do it.

Many, many people get along just fine working with digitial logic circuits without ever knowing or understanding what goes on inside. But they are still limited in what they can do -- it's just that what they can do is enough to get by. But those that know more can do more. They tend to make fewer mistakes and can troubleshoot malfunctioning circuits better and quicker (as well as recognize when a circuit is behaving as would be expected in the real world versus on paper) and, hence, are more valuable to employers and get paid more (at least ideally).

8. ### alejandrodj Thread Starter New Member

Sep 10, 2012
13
0
I get what you mean. i guess ill just make sure how to identify the different TTL gates and use them and not worry too much what happens inside. after all i think even when he is an a-hole about it he doesnt really make us explain it back to him or give us test about it. thanks a lot for the help guys.