# Digital logic gates

There are two common sets of symbols for logic gates,

- The older
*distinctive shapes*of MIL-STD-806B - The more modern and uniform boxy shapes of IEC 60617

We describe what these gates do in the article on Logic.

Written by Willy McAllister.

### Contents

- Symbols for AND OR NOT and XOR
- Symbols for NAND and NOR
- Symbols for complex logic functions
- Standards are for sale
- Summary
- Appendix - How to draw the OR symbol

Logical thoughts can be represented in text form as Boolean equations, or as symbols in a logic schematic. I often find it is easier to understand the meaning and *intent* of a logic design when it’s drawn with symbols rather than written out as equations.

## Symbols for AND OR NOT and XOR

There are two common sets of symbols,

**MIL-STD-806B**defines the*distinctive shapes*for the simple gates. I prefer these symbols for simple gates because they are easier to identify at a glance. These are the symbols I use at Spinning Numbers. Notice how NOT is indicated with a bubble.

**IEC 60617, ANSI/IEEE Std 91-1984**defines rectangular shapes for both simple gates. Notice how the NOT symbol has a*flag*instead of a bubble.

**DIN 40700**There’s a chance you may come across some antique symbols in academic papers—the third column of symbols is an older set once used in Europe.

## Symbols for NAND and NOR

The distinctive and IEC/ISO symbols for NAND and NOR,

The bubble or flag indicates a logic inversion.

## Symbols for complex logic functions

IEC 60617 also defines symbols for more complicated digital functions like multi-bit registers. Here is an example, an 8-bit D flip flop with edge-triggered clock and enable,

If you use a CAD program to draw logic, it’s symbol library may follow the IEC convention or they may just wing it and make up complex symbols in their own style.

## Standards are for sale

If you search for these standards on the web you won’t find the standards themselves—they are not available for free on the web. Instead you will come across offers to purchase a copy. This is a fact of life in engineering. Standards organizations spend resources to create and maintain these standards. To recover that expense they provide them for sale, not for free. Your company or your school’s engineering library might have a copy if you are interested in the tiny details.

## Summary

Which symbols should you use? That is up to your preference or your school’s or company’s policy. The IEC standard is more modern than the older MIL-STD-806B, but it hasn’t been universally adopted. IEC allows you to use the older distinctive shapes for the basic gates and still be standard-conforming.

I recommend you pick one set or the other—don’t mix them together in the same schematic. I like the distinctive shapes because they are quicker to recognize and are truly beautiful.

For complex symbols I use the IEC style whenever possible. I like how data and control are clearly separated.

## Appendix - How to draw the OR symbol

The OR symbol is beautiful—it is constructed from three circles and two short lines,

Click here for the .svg source image, drawn on a 1 mm grid with 5 mm major grid lines.