Industry Standard Architecture

Industry Standard Architecture (in practice almost always referred to only as ISA ) is a computer bus standard for IBM-compatible PCs that extends the XT bus architecture from 8 bits to 16 bits.

The bus protocol allowed even so-called bus mastering, although only the first 16 MB of main memory were available for direct access. With respect to the XT bus architecture ISA is sometimes occupied by the term AT- bus architecture.

The bus is usually operated with 8.33 MHz and is in its original version, a simple lead-out of the system bus dar. slot is used as a two-part slot with contacts at intervals of 2.54 mm, whichever is the longer, 62 -pin section of the XT slot equivalent, while the shorter, 36 -pin section contains the additional signals of the AT. This could initially be used XT plug-in cards.

After the development of the PCI bus mid-1990s IBM PC motherboards were always more PCI slots and - if at all - equipped only with a few ISA slots. Their control is done (at least for motherboards for processors from the Pentium ) via a PCI -ISA bridge, as the old PC system from the Pentium no longer exists. Since the beginning of the 21st century, virtually no new PC has more ISA slots. Old ISA expansion cards ( sound cards, data acquisition cards, I / O cards or similar ) can therefore be used no longer in newer computers. With the introduction of the PCI bus whose plug was transferred play capability on the ISA bus.

However, in the latest PCs on the motherboard ISA technology itself is still available, as for example, keyboard, mouse, floppy disk controller and some basic system components (such as the interrupt controller ) for compatibility during the operating system startup process through ISA be addressed. For this, the newer low- pin-count bus is mostly used, which is the software side fully compatible with ISA, but the hardware side a completely different, serial, construction has.

ISA buses were and are also used in industrial PCs or embedded systems. In contrast to the private customer area here keep the "old" systems because of the many times longer product life. As an example, the bus AT- 96 and PC/104 be mentioned, use the ISA bus signals. But are different from the mechanical connector and the backplane. For industrial PCs also still motherboards with at least one ISA slot are produced that are offered at much higher prices as the end user motherboards, but instead are associated with a long delivery guarantee. On these boards current CPUs are used, which leads to the somewhat strange combination of a processor with several GHz and a bus with a clock frequency 8-12 MHz.

ISA plug-in cards are available in two versions:

  • As legacy ISA cards ISA expansion cards are referred to, for the various system resources (IRQ, DMA, IO-Base, Mem - base) to the extent required to be set by the user. This can be done at the hardware level, in which case on the riser card by means of jumpers ( jumpers) or DIP switches are set the values. This can also be done at the software level, which are made ​​on the plug-in card, no settings and the necessary system resources by software are passed to the driver. There are also mixed solutions. For legacy ISA cards, the user has to ensure that system resources are not assigned twice. Only serial interfaces ( RS232) can share an IRQ, then being but only one of these interfaces must be addressed at the same time.
  • As ISA PnP cards ISA expansion cards are referred to that have been assigned by a PnP -compliant BIOS, the necessary system resources. Operating systems that need the BIOS only for booting and then even take over the hardware management, to configure by software ISA PnP cards even if the BIOS is not PnP.


(1 ) is often set to 4.77 MHz (emergency operation, derived from the timer oscillator ) or from the CPU bus clock by integer division derived: 8 MHz, 8.33 MHz, 10 MHz, 11 MHz, or 12 MHz. Clock rates of 10 MHz often cause unstable systems.