Industry Standard Architecture

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One ISA slot.
From bottom to top: EISA , ISA and XT connectors
Single board computer with Intel 80386SX-40 -compatible ALi M6117C for the ISA bus and with connection for PC / 104

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

The bus protocol even allowed so-called bus mastering , although only the first 16  MiB of the main memory were available for direct access. With regard to the XT bus architecture, ISA is sometimes also referred to as AT bus architecture .

more details

The ISA bus is usually operated at 8.33 MHz and in its original version represents a simple lead-out of the system bus. A two-part slot with contacts 2.54 millimeters apart, the longer, 62-pin section being the XT Slot, while the shorter, 36-pin section contains the additional signals of the AT. As a result, XT plug-in cards could initially continue to be used.

The ISA bus was developed at IBM in the early 1980s by Mark E. Dean and Dennis Moeller .

After the development of the PCI bus in the mid-1990s, IBM PC motherboards were equipped with more and more PCI slots and - if at all - only a few ISA slots. They are controlled (at least in the case of main boards for processors from the Pentium ) via a PCI-ISA bridge, since the old PC system bus no longer exists from the Pentium. Practically no new PC has had ISA slots since the beginning of the 21st century. Old ISA plug-in cards ( sound cards , measurement cards, I / O cards or similar) can therefore no longer be used in newer computers. With the introduction of the PCI bus, its plug & play capability was also transferred to the ISA bus.

However, even in the latest PCs, ISA technology is still present on the motherboard, as the keyboard, mouse, floppy disk controller and some basic system components (such as the interrupt controller ) , for example, must be accessible via ISA during the operating system startup process for reasons of compatibility. The newer low-pin count bus is mostly used for this, which is fully compatible with ISA on the software side, but has a completely different, serial structure on the hardware side.

ISA buses were and are also used in industrial PCs or embedded systems . In contrast to the private customer area, the "old" systems are here because of the much longer product lifespan. Examples are the AT-96 and PC / 104 bus systems that use ISA bus signals. The mechanical connectors and the backplane are different . For industrial PCs, motherboards with at least one ISA slot are still produced (as of November 2018), which are offered at significantly higher prices than end-user motherboards, but come with a long delivery guarantee . Current CPUs are used on these boards, which leads to the somewhat strange combination of a processor with several GHz clock frequency and a peripheral bus with a clock frequency of 8 to 12 MHz.

ISA plug-in cards are available in two versions:

  • As a legacy ISA cards are called ISA expansion cards for the various system resources (IRQ, DMA, IO-base, Mem-Base) - if required - must be set by the user. This can be done at the hardware level, and then on the plug-in card by means of jumpers ( jumpers ) or switches DIP are set the values. This can also be done at the software level, whereby no settings have to be made on the plug-in card and the necessary system resources are transferred to the driver via software. There are also mixed solutions. With legacy ISA cards, the user must ensure that system resources are not used twice. Only serial interfaces ( RS-232 ) can share an IRQ, but then only one of these interfaces can be addressed at the same time.
  • As ISA PnP cards are called ISA expansion cards that receive the necessary system resources by a PnP-enabled BIOS. Operating systems that only need the BIOS for booting and then take over hardware management themselves can configure ISA PnP cards via software even if the BIOS is not PnP-capable.

Technical specifications

Pin assignment of the ISA bus
Bus width 16 bit
08 bit ISA ( XT bus ),
16 bit ISA
Number of pins 98 (62 + 36)
Pitch 2.54 mm (0.1 ")
Operating voltages +5 V, −5 V, +12 V, −12 V
Bus cycle 6 to 10 MHz a b c
data throughput
16 bit: 5.33 MByte / s at 8 MHz (AT, 1 W / S)
08 bit: 1.33 MByte / s at 8 MHz (AT, 4 W / S)
08 bit: 0.96 MByte / s at 4.77 MHz (XT, 1 W / S)
aoften adjustable to 4.77 MHz (emergency operation, derived from the timer oscillator) or derived from the CPU bus clock by whole number division: 8 MHz, 8.33 MHz, 10 MHz, 11 MHz, or 12 MHz. Clock rates from 10 MHz often lead to unstable systems.
b XT: 4, 4.77 or 5 MHz, usually with 1 waiting cycle (standard access takes 4 cycles), the card can request further waiting cycles
c AT: 6, 6.25, 8, 8.33, 10, 11, 12, 12.5, 16 or 20 MHz, with 0 to 5 (standard 1 for 16 bit, 3 for 8 bit) waiting cycles (standard access takes 2 Cycles), the card can request further waiting cycles

See also

Web links

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