What Is an Expansion Bus?

What Is an Expansion Bus?

There is no IT veteran that hasn’t heard of an expansion bus. If you are a newbie, this is one of the first terms you learn as you go with the basics of computer architecture.

Expansion bus, or external bus, as it is also called, is used as an instrument of communication between the CPU and other components. It is called external because it is external to the CPU and is also expanding the CPU’s range of communication to the peripherals.

To understand how it works, why it is so important, and what are the different types of an expansion bus, stick with me while I explain everything step-by-step.

How Does Expansion Bus Work?

As already said, an expansion bus is used to create a path of data flow between the CPU and other components. In order for this to be possible, there are certain data bus paths on the motherboard.

Expansion buses can be observed as a medium that allows the data to travel from one location to another. When you look at the motherboard’s structure, you will notice many different data bus paths that are created for different parts.

Every data bus path leads to an expansion bus, which is a narrow slot on the motherboard. Each of those slots you can notice have an integrated circuit, and each of those circuits is built differently so they can communicate with different devices such as CD-ROMs, monitors, printers, etc. These circuits are also known as cards.

Types of Expansion Buses

First of all, there are two types of buses in general. The first type is a system bus, and this is a path that is connecting two major parts on the motherboard – the CPU and the main memory. 

The second type is the expansion bus, or also known as the I/O bus (input/output bus). Expansion buses are connecting all the peripherals to the CPU. Expansion buses are further divided into several categories, and here I bring you a list of the most commonly used ones, starting from the first expansion bus ever made.


ISA or Industry Standard Architect bus was the most common type of expansion bus for many years because it was originally developed for IBM PC. The original ISA bus was an 8-bit bus, meaning it was able to transfer one byte of information in one go from output to the input device. 

With the development of the motherboards, the ISA bus was replaced with something called the AT bus – an advanced technology bus. It was called advanced because it was able to transfer 16-bit of data in one go, twice the information the ISA bus could transfer. 

The new AT bus was developed in such a way that it was still allowing a user to use 8-bit cards in new 16-bit expansion slots. In the beginning, both of them were operating at the same speed as the CPU – 4.77MHZ, but very soon AP buses were upgraded to operating at up to 8MHZ.


After the ISA bus, the MCA or Micro Channel Architecture bus was created. This bus was improved in many ways compared to its predecessor. The biggest upgrade was that the MCA was able to transfer data in 16-bit or 32-bit chunks. 

Also, the speed of the MCA bus was improved, so now they would transfer the information at 10MHZ and more.  

An innovation MCA buses brought was bus mastering. This is a term used to describe the technology of placing smaller processors on each expansion card, thus allowing them to control the data transfer and communication between the card and the CPU.

MCA buses are now part of history, mostly because they didn’t become popular since they were not compatible with the old ISA card types. So, the alternative has been developed – the EISA bus or the Extended Industry Standard Architecture bus.


EISA bus was developed to replace the MCA buses and to be compatible with the old ISA cards that were still widely in use. The EISA bus was operating with a 32-bit data path. The biggest difference is that the EISA bus was also offering the basic disk setup, like the MCA bus, but was made to work with the ISA technology as well.

Physically, the EISA slots were twice as thick as the ISA slot, so when you plug in an ISA card, it would not sit in place properly, reaching only the first and top row of the connectors. The EISA cards would, of course, reach to the bottom and utilize both rows of connectors.


VESA bus or Video Electronics Standard Association bus was developed for the sole purpose of enhancing the video properties of a computer. It was made to run at 25-32MHZ speed – the same speed the processor was running at.

The issues with VESA buses were growing alongside the increase of processor speed. They became too expensive for production and eventually were suspended completely. 

PCI Bus 

Peripheral Component Interconnect bus, or PCI bus, is what is nowadays being used in most computers out on the market. They are operating with both 32-bit and 64-bit data.

The biggest innovation and difference the PCI buses introduced was the plug-and-play system, which allowed even the laics to change and quickly configure their device. This system means that you can simply plug a card into its slot, and the system will identify and set up it on its own.


AGP or Accelerated Graphic Port is what makes the image on the computer be of good quality. It was developed alongside the demand for high-quality videos and graphics on PCs.

The most important feature is that it operates at the same speed as the processor, which means that the data transfer time would be optimal. This port is connected directly to the main memory, which technically gives the AGP access to unlimited memory.

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