ISA Bus and ISA Slot: Industry Standard Architecture, Types, Working and Differences

ISA Bus and ISA Slot are important concepts in computer architecture and computer hardware. If you are studying CSE, BCA, MCA, computer organization or hardware, you will often encounter terms such as Industry Standard Architecture, ISA bus, ISA slot, 8-bit ISA, 16-bit ISA, IRQ and DMA. In this complete guide, we will understand what ISA is, how the ISA bus works, how ISA slots are designed, and how 8-bit ISA, 16-bit ISA and EISA differ.

ISA was one of the most important expansion-bus standards used in early IBM-compatible computers. Although it is now considered a legacy technology, understanding ISA is still useful for computer architecture examinations and for understanding the evolution from ISA to PCI and modern PCI Express.

Difference Between 8 bit ISA, 16 bit ISA and EISA

Characteristics	ISA( 8 Bit)	ISA (16 Bit)	EISA
Acronym for	Industry Standard Architecture	Industry Standard Architecture	Enhanced Industry Standard Architecture
Introduced In	1981	1984	1988
Introducer	IBM	IBM	Group of Nine Computer manufacturing Companies Led by Compaq(AST Research, Compaq Computer, Epson, Hewlett-Packard, NEC, Olivetti, Tandy, WYSE, Zenith Data Systems)
Data Bus	8 Bits	16 Bits	32 Bits
Address Bus	De-multiplexed 20 Bit address Bus	24 Bit Address Bus	32 Bit Address Lines
Memory Addressability	1.048576 MB	Up to 16 MB	Up to 4GB

8 Bit Industry Standard Architecture(ISA) or PC Bus

ISA (Industry Standard Architecture) was first used with the IBM PC in 1981. It was the first open system for PC architecture. No restriction was put on the use of this interconnection method by the IBM. IBM offered free licensing of this PC architecture technique which resulted in more popularity of this. Some main features of 8 bit ISA are:

1) It was introduced by IBM and was the first open system for PC architecture.
2) It provides data transfer rate up to 4.77 Mbps.
3) ISA interface contains 62 pins to interface add on cards. These 62 pins are arranged in two rows, each containing 31 pins.
4) Distance between each pin is 0.1 Inches.
5) 8 Bit ISA make use of Following signals:
    3 Ground Signals
    2 +5V DC Supply
   +12,12,-5V DC supply
    Bidirectional 8 bit data Bus.
   20 Address line, hence memory address space of 1MB (2²⁰=1MB)
   3 DMA request lines
   3 DMA acknowledge lines.
   Six Interrupt Support

Table given below illustrates all signals used by this connector along with their functions: