Traditional symbols

  IEC symbols

  Truth tables

AND gate.

OR gate.

EXCLUSIVE OR gate.

AND gate.

On a basic electrical circuit making a AND gate using just switches, a lamp and power supply, it has just switches in series with a lamp. All the switches must be on for the lamp to go.

All right we all might have a list of Integrated Circuits (IC) with AND gate functions in them we like using, but some times you just can not beat old technology and use components that were used before IC were invented. with a couple of signal diodes (like 1N1416) and a pull up resistor you can not only save on component cost but can save space on your Printed Circuit Board (PCB).
When the Anode of one diode goes high, the low on the Anode of the other diode will keep the output low until both Anodes go high.

If you are not so sure on those "truth tables" and what they are trying to tell you , try my little web site toy below, each time you click the "Next State" button the Inputs and Outputs change, with that line been highlighted on the table. Don´t spend two long clicking onto it or you might need to see a doctor, I wonder if I should go and see a .............. oh never mind.

Sometimes the truth table seen to the right might be shown in a format the same as what is on the left

Now we have a different type of truth table chart, for a two input AND gate.

The last word on the AND gate is the Equation, I must say as a service-person I have never found this very useful, I have done many other useful things both at work and at home that I have learnt from that digital electronics course, but using the gate equations ranks very low. By the way that character in-between the A and B is a dot.

Now you know what the AND gate does (I hope) if you would like to see how the basics of a computer program works have a look at the code below written in C++ for the two lines

 if (input_A==1 && input_B==1)
 std::cout << std::endl << "Output is logic one" << std::endl;

Both input_A and input_B have to "1" (or true is a better word to use in programming) (same as a AND gate function) for the program to print "Output is logic one"
You have to put in the double equals sign otherwise the code with one equals sign will make input_A and or input_B equal to 1 even if it was or should be some other number.

Whether checking a electronic circuit or a computer program a very good short test is as soon as you see one condition low or false then the result will be low or false. Some AND gates can have more than two inputs.

#include <iostream>
using namespace std;

    int input_A;
    int input_B;

int and_gate(void);

int main (void)
{
   int choice;
   std::cout << "Enter 1 to try the AND gate functions " << std::endl ;
   std::cin >> choice;

 if (choice==1)
 {
     and_gate();
 }
      return 0;

}


int and_gate(void)
{

    std::cout << "input Gate A, enter 1 or 0, enter 2 to end program" << "  " ;
    cin >> input_A;
    std::cout << "input Gate B, enter 1 or 0, enter 2 to end program" << "  " ;
    cin >> input_B;
    if (input_A==2 || input==2)
    {
        return 0;
    }
    if (input_A==1&& input_B==1)
         {
         std::cout << std::endl << "Output is logic one" << std::endl;
         }
         else
         {
         std::cout << std::endl <<  "Output is logic zero" << std::endl;
         }
    system("PAUSE");
    and_gate();
}

So why the C++ bit?

You might ask why the C++ when the page starts off with electronic logic gates, I suppose in all honesty I should of used just C since it does not use any of the C++ features, (in that program listed) and the recursion as well, were a function calls a function, well I just don´t think recursion are used enough and this is a useless example to describe the benefit of recursion, so if you are studying C/C++ don´t forget to have a quick look at recursion, who knows one day it might take off, have a nice day ~ someone has too.
Since there are basically three types of gates in electronics and three Operators, not only in programming but also the microprocessor and they work the same way, I thought why not tie them together on this page. In programming they are,

Table of Logical and Bitwise Operators.
	AND	OR	EXCLUSIVE OR
Bitwise Operators.	&	|	^
Logical Operators.	&&	||
Boolean Operators are not used in programming.
Boolean Operators.		+ orV

By the way,
Anybody that has learnt C and / or C++ can easy go on to learn Javascript, Java, PHP just to mention three.

OK then lets have the same code written in C.

... do you get the feeling I am making this up as I go along?

#include <iostream>
#include <stdio.h>

    int in_A;
    int in_B;
int and_gate(void);


int and_gate(void)
{
    printf("input Gate A, enter 1 or 0, enter 2 to end program ");
    scanf("%d",&in_A);
    printf("input Gate B, enter 1 or 0, enter 2 to end program ");

    scanf("%d",&in_B);
    if (in_A==2 || in_B==2)
    {
                   return 0;
    }


    if (in_A==1 && in_B==1)
         {
          printf("Output is logic one \n");
         }
         else
         {
          printf("Output is logic zerro \n");
         }
    system("PAUSE");
    and_gate();
}

int main (void)
{
   int choice;
  printf("Enter 1 to try the AND gate functions " );
 scanf("%d",&choice);

 if (choice ==1)
 {
     and_gate();
 }
      return 0;

}

I shouldn´t be so horrible picking on those recursion functions all the time.
Note the the main function is made the last function in C but the first in C++
We still need #include <iostream> for the line system("PAUSE"); to work,
I could of replaced

#include <iostream>
system("PAUSE");

with

char pause;
scanf("%c",&pause);
So the program stops and waits for you to press any key.

Ok so I never realy answered the question "why the C++" or the "C" for that matter, the point is anybody that has studyed logic gate could very well go on to study microprocessors, (like I did) and that is were the "C" comes into it, programing your micro in "C" , you can also program in assembly code, and that suits shorter coding, programing in assembly code also seems to make more efficent use of the program space, I have read programing in assembly code take up about a third less space compared to "C"

OR gate.

Getting back to our basic electrical circuit using just switches, a lamp and power supply. For a example of a OR gate, this has all the switches in parallel, and in series with a lamp. Any switches can turn the lamp on.

Once again as with the AND gate example above, the OR gate can be made with signal diodes.
When the Cathode on one of the diodes goes high this will make the output go high.

Notice there is a plus sign for the OR gate and a dot for the AND gate, brilliant! so easy and considerate to remember, give us a break. Just to confuse the issue, sometimes the letter V is used instead.

EXOR (EXclusive OR) gate.

Our basic electrical EXOR circuit using switches, lamp and power supply, this has two single pole, double throw switches in series but the Normally Closed (N.C.), Normally Open (N.O.) contacts. Either switches can turn the lamp on or off, great for having the two switches at opposite ends of the house.

You can make a Exclusive OR gate with a OR, NAND and AND gate but I don't have time to tell you more right now. In the mean time if you look at the equation you could make up a different combination.

The microprocessor.

As you might expect by now the microprocessor would also have the three logical instructions, one each for the AND, OR, Exclusive OR function they are AND, OR and EOR, (not to mention ANDI and ORI, for Load Immediately), they work the same logic as always, so a AND 0b10100101, 0b11000011 doing a bit by bit AND would give a result of 0b10000001.

The thing to note here, when ever the result is zero or 0b00000000 then the "Z" (Zero) flag will the set in the status register.
Also because the most significant bit sets the sign of a binary word (on a signed word), on a eight bit word, when ever the result on bit eight is a one (0b1xxxxxxx) then the "S" (Sign) and "N" (Negative) flags will be set in the status register.

The NOT device.

Some people claim this NOT device is a gate, I don´t think so, it just inverts what goes into it. It is not controlled by any other input. On the IEC symbol the square box has a 1 on it, the backward slash on the output tells us the output is inverted.

We can not make a diode (or diodes) do a NOT gate function , we need a transistor and a few resistors, but guess what? there are called Digital transistors out there been used today, I will not tell you what I first thought of when I first saw them named as digital transistors in the data books, who said Integrated Circuits (IC´s) were taking over the world? They are also called Resistor-Equipped Transistors (RET). Whatever you would like to call it, they normally only have either one or two internal resistors connected to the base (input).
Don´t you just love my resistors? I used a W to make them, I suppose you could say they are dubdubdub devices, but then I turned the middle dub upside down to get a more even flow, so I guess it must be a dubmdub device.

NAND gate.

This NAND gate is just the same a AND gate but the output is inverted, and the drawing shows a circle on the output and forward slash on the IEC symbols.

NOR gate.

This NOR gate is just the same a OR gate but the output is inverted, and the drawing shows a circle on the output and forward slash on the IEC symbols.

XNOR gate.

This XNOR gate is just the same a EXOR gate but the output is inverted, and the drawing shows a circle on the output and forward slash on the IEC symbols.

TRI-STATE.

So what is a “Low” or a “High”?

So what is a low or high for that matter?
I suppose the best place to get the answer is look at the data sheet, but since this is a tutorial at a web site, I will use a Transistor Transistor Logic (TTL) gate for the example.

The reason why I used TTL in this example over say CMOS is because the power supply to CMOS can very over a wider supply range, But TTL uses more power, and you need lots more bypass capacitors. Notice the big gaps between input and output, high and low this is done to help weed out any noise that might get into the system.

Component handling warning.

Don´t forget when you see this Anti-static sign (or if you see any other warning that the components you are using could be damage by static electricity) to use Anti-static precautions. While you might get away with out using anti-static precautions, sometimes you can partly damage the device, it will still work, but then it could very well fail premature, so the equipment will not last as long as it could do.

more to come, and tidy up.