Reading Digital Logic Diagrams

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Last updated: Oct 25, 2021 by Mayurakhi Khan
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Table of Contents

Digital Logic Overview 

Digital, or boolean logic, represents signals in an ideal digital circuit, where the signals are typically in one of two states or levels [1]. Digital logic allows modern complex problems in electrical circuit designs and programming to be deconstructed into simple binary queries [2].


Digital Logic States (Logic Levels)

In digital circuits, input and output signals can only inhabit a single logic level at any time [3]. These logic levels, or states, are usually represented by the voltage difference.

Binary (Two-Level) Logic

In binary logic, multiple different sets of names are used for the "high" and "low" states depending on if the application is in a field of mathematics, computer science, electronics, engineering, etc [4].

Tri-State (Three-State) Logic

In tri-state logic circuits, input and output signals can be in a third state along with the binary “high” and “low” states [5]. This third state is the “high-impedance” state. An I/O pin/port/pad that is in this state is practically removed from the circuit as if it were not there in the first place. In this state, a digital signal on the pin/port/pad’s line would not affect the rest of the circuit. This allows multiple circuits' I/O pin/port/pads to share lines between them. This is done by having one circuit use the line at a time, while the other circuit’s pin/port/pads on the shared line are in a “high-impedance” state.

Digital Logic StatesAlternative Names
Logical High Logic '1'TRUEON
Logical Low Logic '0'FALSEOFF
High Impedance Hi-ZFloatingTri-Stated



Logic Gate Symbols

Logic gates can have different numbers of inputs, but the summary table will use 2-input logic gates, with the exception of the 1-input buffer and NOT logic gates. Logic gates can be used in different configurations to create complex digital logic circuits and some logic gates can even be used to create other logic gates.

XOR Gate Equivalent Circuit Constructed With NAND Gates [6]

Image of a XOR Gate Equivalent Circuit Constructed using only 5 NAND Gates.

Standards

There have been many different standards for logic gate symbols depending on the region and time period it was used in. However, there are two sets of symbols that are currently the most commonly used [7].

StandardSymbolsDescription

ANSI/IEEE Std 91a-1991


ANSI/IEEE Std 91-1984 


MIL-STD-806B

Distinctive Logic Gate Symbols [7]
Image of Distinctive Logic Gate Symbols.

This is the traditional and more widely used set of logic gate symbols, where each basic gate (Buffer, OR, AND, XOR) has a distinctive shape. Gates that are the result of combining a NOT gate and one of the basic gates have a circle in front of the distinctive basic gate (NOT, NOR, NAND, XNOR) to represent the complement logic.

IEC 60617-12:1997 


ANSI/IEEE Std 91/91a-1991


ANSI Y32.14

Rectangular Logic Gate Symbols [8]

Image of Rectangular Logic Gate Symbols.

These less common standards specify logic gate symbols to be in a rectangular shape with its corresponding function centered inside the rectangle. This design allows for a broader range of devices with more complex functions than with the basic distinctive shape symbols. In this design, inverse logic gates are identified with a line connecting the output signal line upwards to the vertical edge of the rectangle on the same side to create a right-angle triangle. Depending on the standard this “triangle” notation may also be interchangeable with a circle notation similar to the distinctive shape design.




Truth Tables

A truth table is used to present all the possible outputs corresponding to any combination of inputs for digital logic problems [1]. Truth tables are usually formated starting with all input values of Logic '0' and increasing the inputs by binary one for each row down the table until all inputs are Logic '1'. They can be used to represent individual gates or complex logic circuits with mutiple gates.

Half Adder Logic Circuit and Truth Table [9]

Image of a Half Adder Logic Circuit and its corresponding Truth Table.



Boolean Algebra 

Boolean algebra can be used to represent and analyze binary (0 and 1) logic circuits [10]. Boolean expressions or equations of logic circuits can be used to calculate the output signals of that circuit, given the input signals without needing to know the exact layout of the circuit. 

As boolean algebra is a different branch of algebra, it has its own set of operators, laws, and theorems [3]. These laws and properties create techniques that allow certain boolean expressions of logic circuits to be greatly simplified. As such, different digital logic circuits may have the same simplified boolean equation.

Half Adder Boolean Equation

Boolean Operator Logic Symbols

Similar to logic gates symbols and states, operator symbols can greatly vary depending on the region, time period, field of study, and application.

Basic Alternative Boolean Operator Logic Symbols [11] [12] [13] [14] [15]
Negation/Complement (NOT)Conjunction (AND)Disonjunction (OR)Exclusive Disonjunction (XOR)




Basic Logic Gates Summary Table

Name

Gate Symbol

Truth Table

Description

Example Boolean Equation 

Buffer Gate
1-input Buffer Gate Distinctive Symbol [16] 
Image of a Buffer gate with the input labeled A and the output labeled Y.

Input

Output

A

Y

0

0

1

1

A 1-input Buffer gate leaves the digital signal unchanged and outputs the same value as the input value.

NOT Gate
1-Input NOT Gate Distinctive Symbol [17]
Image of a NOT gate with the input labeled A and the output labeled Y.

Input

Output

A

Y

0

1

1

0

A 1-input NOT gate, also known as an Inverter, outputs the opposite value of the input.

OR Gate

2-Input OR Gate Distinctive Symbol [18]
Image of an OR gate with inputs labeled A and B and the output labeled Y.

Inputs

Output

A

B

Y

0

0

0

0

1

1

1

0

1

1

1

1

A 2-input OR gate outputs a Logic '1' if any input value is Logic '1'.

AND Gate

2-Input AND Gate Distinctive Symbol [19]
Image of an AND gate with inputs labeled A and B and the output labeled Y.

Inputs

Output

A

B

Y

0

0

0

0

1

0

1

0

0

1

1

1

A 2-input AND gate outputs a Logic '1' only if both input values are Logic '1'.

NOR (NOT-OR) Gate


2-Input NOR Gate Distinctive Symbol [20]
Image of a NOR gate with inputs labeled A and B and the output labeled Y.

Inputs

Output

A

B

Y

0

0

1

0

1

0

1

0

0

1

1

0

A 2-input NOR gate outputs a Logic '0' if any input value is is Logic '1'.

NAND (NOT-AND) Gate 

2-Input NAND Gate Distinctive Symbol [21]
Image of a NAND gate with inputs labeled A and B and the output labeled Y.

Inputs

Output

A

B

Y

0

0

1

0

1

1

1

0

1

1

1

0

A 2-input NAND gate outputs a Logic '0'  only if both input values are Logic '1'.

XOR (Exclusive-OR) Gate

2-Input XOR Gate Distinctive Symbol [22]
Image of an Exclusive OR gate with inputs labeled A and B and the output labeled Y.

Inputs

Output

A

B

Y

0

0

0

0

1

1

1

0

1

1

1

0

An 2-input XOR gate outputs a Logic '1' only if both input values are the opposite of each other.

XNOR (Exclusive-NOR) Gate
2-Input XNOR Gate Distinctive Symbol [23]
Image of an Exclusive NOR gate with inputs labeled A and B and the output labeled Y.
InputsOutput
ABY

0

0

1

0

1

0

1

0

0

1

1

1
An 2-input XNOR gate outputs a Logic '0' only if both input values are the opposite of each other.



Digital Output Configurations

Output ModesDiagramDescription
Push-Pull (Totem-Pole)
A Microcontroller's Push-Pull Configured Output Pin in Push Phase (Left) and Pull Phase (Right) [24]
Image of a Push-pull digital output schematic using two transistors (PMOS connected to VDD and NMOS connected to GND).

Push-pull is the most common output configuration where an output pin drives a logical "high" or "low" level [24].

In the push phase, the output pin "pushes" the power supply voltage so that the pin sources (supplies) current to the load connected to the output. In effect, this drives the signal on the output line to a "high" state.

In the pull phase, the output pin "pulls" to ground so that the pin sinks (absorbs) current from the load connected to the output. In effect, this drives the signal on the output line to a "low" state.

Open-Drain
A Microcontroller's Open-Drain Configured Output Pin
Image of an Open-Drain digital output schematic using a NMOS.

In an open-drain configuration, the output pin is typically connected to the drain terminal of a N-Channel MOSFET. As a result, the output pin can only drive a "low" level, but can also assume a "high-impedance" state [24].

When the drain terminal is closed (connected to ground), the output pin is then pulled to ground so the output pin sinks current from the load. In effect, this drives the output line signal to a "low" state.

When the drain terminal is open (not connected to ground), the output pin, and therefore the load, is left floating as the line is not connected to the "high" or "low" level voltages. Therefore, it is said the output pin is in a Hi-Z state. 

Open-Collector
A Microcontroller's Open-Collector Configured Output Pin
Image of an Open-Collector Digital Output schematic using a NPN BJT.
In an open-collector configuration, the output pin's functionality is the same as in open-drain. However, in an open-collector configuration, the output pin is typically connected to the collector terminal of a NPN BJT instead of the drain terminal of a NMOS.



References

[1]    J. Braza, “What is Digital Logic?,” Circuit Basics, 28-Jun-2020. [Online]. Available: https://www.circuitbasics.com/what-is-digital-logic/. [Accessed: 01-Apr-2021].

[2]    SFUptownMaker, “Digital Logic,” Sparkfun, 03-Oct-2013. [Online]. Available: https://learn.sparkfun.com/tutorials/digital-logic/all. [Accessed: 13-Apr-2021].

[3]    T. R. Kuphaldt, Lessons In Electric Circuits, Volume IV – Digital, Fourth., vol. IV, VI vols. 2007.

[4]    “Digital Logic Gates,” Electronics Tutorials. [Online]. Available: https://www.electronics-tutorials.ws/logic/logic_1.html. [Accessed: 01-Apr-2021].

[5]    Techopedia, “What is Three-State Logic? - Definition from Techopedia,” Techopedia, 07-Jul-2015. [Online]. Available: https://www.techopedia.com/definition/22641/three-state-logic. [Accessed: 13-Apr-2021].

[6]    Inductiveload, XNOR from NAND.svg. 2009. Available: https://commons.wikimedia.org/wiki/File:XNOR_from_NAND.svg.

[7]    W. McAllister, “Digital logic gates,” Spinning Numbers. [Online]. Available: https://spinningnumbers.org/a/logic-gates.html. [Accessed: 13-Apr-2021].

[8]    A. Saleh, “Electronics Done Quick 7 | Logic Gates,” RobotShop Community, 31-Aug-2019. [Online]. Available: https://www.robotshop.com/community/tutorials/show/electronics-done-quick-7-logic-gates. [Accessed: 13-Apr-2021].

[9]    E. Coates, “Building a Half Adder,” Raspberry Pi Learning, 23-Jan-2018. [Online]. Available: https://projects.raspberrypi.org/en/projects/halfadder. [Accessed: 13-Apr-2021].

[10]    “Boolean Algebra,” Tutorialspoint. [Online]. Available: https://www.tutorialspoint.com/computer_logical_organization/boolean_algebra.htm. [Accessed: 13-Apr-2021].

[11]    R. M. Meyer, Introduction to Computer Hardware and Organization . Buffalo, New York, 2002.

[12]    “Boolean Algebra (Boolean Expression, Rules and Examples),” BYJUS, 2020. [Online]. Available: https://byjus.com/maths/boolean-algebra/. [Accessed: 13-Apr-2021].

[13]    “Comprehensive List of Logic Symbols,” Math Vault, 20-Jul-2020. [Online]. Available: https://mathvault.ca/hub/higher-math/math-symbols/logic-symbols/. [Accessed: 13-Apr-2021].

[14]    “Logic Symbols,” RapidTables. [Online]. Available: https://www.rapidtables.com/math/symbols/Logic_Symbols.html. [Accessed: 13-Apr-2021].

[15]    H. Yan, “Appendix List of Basic Logic Symbols,” in Spatial similarity relations in multi-scale map spaces, Springer International Publishing, 2015, pp. 187–188.

[16]    Stefan506, Logic-gate-buf-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-buf-us.png.

[17]    Stefan506, Logic-gate-inv-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-inv-us.png.

[18]    Stefan506, Logic-gate-or-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-or-us.png.

[19]    Stefan506, Logic-gate-and-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-and-us.png.

[20]    Stefan506, Logic-gate-nor-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-nor-us.png.

[21]    Stefan506, Logic-gate-nand-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-nand-us.png.

[22]    Stefan506, Logic-gate-xor-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-xor-us.png.

[23]    Stefan506, Logic-gate-xnor-us.png. 2005. Available: https://commons.wikimedia.org/wiki/File:Logic-gate-xnor-us.png.

[24]    Y. Stoyanov, “Open Drain Output vs. Push-Pull Output,” Open4Tech, 18-May-2019. [Online]. Available: https://open4tech.com/open-drain-output-vs-push-pull-output/. [Accessed: 13-Apr-2021].

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Mayurakhi Khan 1123 days ago
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