ISA (Industry Standard Architecture) is the most common bus in the PC world. ISA is still used as a mainstay even in the newest computers. The ISA bus, mainly was used to add cards within the computer, dates back to the original IBM personal computer of the early 1980s

ISA Card	Mixed ISA Slots : Earlier motherboards came with a mix of 8-bit (left) and 16-bit (right) slots such as this one

On the picture below it is a motherboard both with PCI and ISA bus slots. An ISA Card that you can see on the first picture gets connected exactly to one of the corresponding slots on the motherboard. In the new versions of the computers you can see that the number of PCI buses founded more predominantly in compared to the earlier ones.

ISA Slots

Introduction

The original XT bus was characterized by the simplicity of its design. It didn't provide any particularly innovative features and in a lot of respects it was limited by this simplicity (e.g. very limited number of Interrupt and DMA channels; limited maximum addressable space; narrow 8-bit data path and lack of bus master support), however it was well suited to the XT and performed quite well with the 8-bit 8088 processor around which the XT was built.

An enhanced version of the XT bus was designed for use in the IBM-AT ('Advanced Technology') which was introduced in 1984. Importantly, while this bus provided significant enhancements to the original XT bus architecture, it retained backward compatibility with the original XT bus allowing old cards to be used in ATs using the new bus which was retrospectively described as ISA (Industry Standard Architecture). The major improvements to the XT bus that led to ISA are described below, most of these were implemented using an optional, additional connector (thus facilitating backwards compatibility).

· A 16-bit data bus was implemented with the addition of 8 extra data lines.

· Maximum Addressable space was increased to 16MB with an additional 4 address lines (to give a total of 24).

· 5 additional edge-triggered IRQ lines were added.

· Partial support for multiple bus masters was provided with the introduction of the signal.

· Maximum bandwidth of 5.3 MB/s

· 8 MHz bus clock speed.

Building it from XT bus:

It is important to note that IBM made an effort to ensure backwards compatibility with the XT-bus. As mentioned, most of the new features were implemented by adding on another connector to the bus which allowed extra features to be added without disrupting existing (XT) bus functions. The AT was built around the Intel 80286 which ran significantly faster than the original 8088 so a wait-state generator was added to lengthen the bus cycle. A bypass was provided via one unused line (pin 88) on the original bus (recalling that the XT bus used a 62 pin connector but only had 53 signal lines and 8 power lines). This pin became the zero wait state line (). When this line is being pulled low, some or all of the wait states generated by the AT motherboard are removed. By putting this signal on the 62-pin connector, IBM allowed manufacturers to make fast new 8-bit boards as well as fast 16-bit boards.

Limitations of ISA

The capabilities of the ISA are relatively limited. As a result of that ISA buses become more and more obsolete. The bus mastering was not a complete or perfect implementation due to certain limitations such as a request by a Bus master for 'Bus hand-off' requiring several cycles for completion and the master having to relinquish the bus periodically to allow memory refresh (or do the refresh itself). Due to the lack of high speed, plug and play ability and bus mastering capacity the use of ISA, there was need for something with more speed and ability.

The new connector added 4 new address lines (A20-A23), plus copies of three lower address lines (LA17-LA19). This duplication was necessary because the address lines on the XT bus were latched (the address signals were tied to flip-flops which maintained the address lines logic level until explicitly set to a different value) and this latching process caused propagation delays that would slow down peripheral boards.

Applications:

The main reasons that keep ISA on the market are, truly enormous base of existing peripherals using the standard and the fact that there are many devices for which ISA is still more then sufficient. The ISA bus expanded to 16 bits in 1984. Despite the fact, that almost two decades have passed it is remained largely unchanged. Most Computers produced today include ISA buses usually the ISA buses used for slower devices such as Modem, Hard disk controllers and Speakers.

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