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Byte Adder Subtractor: an 8-bit Full Adder

With an accumulator to store one operand. Do binary addition (and subtraction) with the 8-bit Workbench™ and Binary (or Hex) Keyboard.

nijssenNijssen
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With this board, the idea is to gain understanding of the inner workings of an Arithmetic Logic Unit (ALU), one of the building blocks of processors. It is an annotated full byte adder that enables addition and subtraction of two bytes (operand A and B). It explains by doing. A byte is put in as input at the bottom edge connector. This byte can than be loaded into the accumulator, so is is being used as operand A. Then a second byte can be entered, used as operand B. The Sum-output (A + B) is available at the top edge connector. Three carry-bits are shown by the small LEDs: the carry-in, intermediate carry and the carry-out. The carry-in can be set by the small slide switch. Subtracting can be done using the method of complements: first you need to 'flip the bits', then add 1 using the carry-in. This way you transform a value '6' to a '-6' for example. It shows how a computer does things 'under the hood'. Add a hexadecimal and octal view with the 8-bit Workbench™.

The project is compatible with the 8-bit Workbench™ extension bus and can connect to other projects of mine:

  • 8-bit Workbench™; an annotated 8-bit display that talks hexadecimal and octal. An Action-button provides a properly de-bounced signal to use with microprocessors or TTL logic.
  • Super Breadboard; enjoy short circuit protection. Have both +3.3V and +5V at hand. Clearly separates data input and output of your prototype. Can work standalone as well.
  • Terrific Timer; a versatile clock board that provides an adjustable astable clock (0.7 - 480 Hz) and a monostable pulse (110 ms). Lets you insert custom 555-timer parts.
  • Binary Keyboard; adds a retro 8-bit toggling experience to your experiments.
  • Hex Keyboard; turn-in a hexadecimal code (0-F) for the high- and the low-nibble. The output, a full byte (8-bits), can then be used with attached experiments. Supports a Logic Probe as well.

These are all available as kits. 

More information is on my website.

ByteAdderSubtractorQuickStart.pdf

Quick Start

Adobe Portable Document Format - 1.06 MB - 05/08/2026 at 09:12

Preview

ByteAdderSubtractorBoM.pdf

Bill of Materials

Adobe Portable Document Format - 155.22 kB - 05/08/2026 at 09:12

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ByteAdderSubtractorSchematics.pdf

Schematics

Adobe Portable Document Format - 328.35 kB - 03/04/2026 at 11:18

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  • Quick Start documentation added

    Nijssen • 05/08/2026 at 09:14 • 0 comments

    Added a Quick Start that shows how addition and subtraction is done. 

    Attach the 8-bit Workbench™ at the top edge of the Byte Adder Subtractor to display the output of the adder. For input, use the Binary Keyboard or Hex keyboard that can attach at the bottom edge.

    Instead, use bread boards to create your own LED output display and input mechanism, e.g. using DIP switches with pull-down resistors. See the schematics of our other products for inspiration how to wire this up using your own parts.

    Kits are available now!




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Nijssen wrote 03/26/2026 at 08:51 • point

Kits coming soon...

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