Industry Standard Architecture is one of the earliest standards for computer bus and expansion slots. The first ISA bus standard was developed for IBM compatibles in 1981. It had width for 8 bit data with 4.77 MHZ data rates. It enhanced to XT bus architecture in 1981 and to 16-bit in 1984. 16-bit architecture provided data rates of 6MHz to 8MHz.

ISA Enhancements

MCA (micro channel architecture)

Micro channel architecture (MCA) replaced ISA in 1987. It supported two widths 16-bit and 32-bit. 32-bit standard was the original and expensive. So a 16-bt standard was developed for IBM PS/2 to reduce cost. The plus point for MCA was that it improved the signal quality. The drawback was that it was incompatible with existing lower pin memory cards. But this didn't remain long as memory moved to the CPU-local bus. MCA allowed more than one cards to communicate with each other. This increased the performance but created a problem when more than one cards wanted to talk at the same time. MCA failed because it could not support and was not compatible with existing ISA bus architecture. Later it was replaced by EISA and VLB.

EISA (extended industry standard architecture)

EISA was an improvement over MCA. It removed the problem of compatibility with existing older ISA bus architecture. The cards could access 4 GB of memory. EISA provided 32-bit bus width with a data rate of 20 Mega Bytes per second. Moreover EISA configures the cards automatically. Just plug and play the card no need to be install extra drivers. Well it became very popular in manufacturers and mostly network card standards were developed based on EISA. But to resolve the limited bandwidth problem associated with ISA, VESA Local Bus (VLB) was developed.

VLB (VESA Local Bus)

The problem with ISA and its all other versions was the limited bandwidth. The issue was to provide video compatibility. S in 1992 Video Electronics Standard Association (VESA) introduced VLB standard especially to add on video functionality. VLB not only supports the ISA architecture but also allows the ISA cards. The bus width is 32-bit with a data rate of more than 33 MHz. The VLB card provided much more quality video than ISA. The week point with VLB was no feasibility of Plug and play. Also it couldn't support bus sharing and mastering in a proper way. Though it was quite popular when introduced but with the introduction of PCI standard by Pentium in 1994, it got terminated. Now you hardly find a system with ISA or its standard.