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In 1987, IBM moved to replace the AT bus with their proprietary Micro Channel Architecture (MCA) in an effort to regain control of the PC architecture and the PC market. (Note the relationship between the IBM term "I/O Channel" for the AT-bus and the name "Micro Channel" for IBM's intended replacement.) MCA had many features that would later appear in PCI, the successor of ISA Adapter, but MCA was a closed standard, unlike ISA Adapter (PC-bus and AT-bus) for which IBM had released full specifications and even circuit schematics. The system was far more advanced than the AT bus, and computer manufacturers responded with the Extended Industry Standard Architecture (EISA Adapter) and later, the VESA Local Bus (VLB). In fact, VLB used some electronic parts originally intended for MCA because component manufacturers already were equipped to manufacture them. Both EISA Adapter and VLB were backwards-compatible expansions of the AT (ISA Adapter) bus.
Users of ISA Adapter-based machines had to know special information about the hardware they were adding to the system. While a handful of devices were essentially "plug-n-play", this was rare. Users frequently had to configure several parameters when adding a new device, such as the IRQ line, I/O address, or DMA channel. MCA had done away with this complication, and PCI actually incorporated many of the ideas first explored with MCA (though it was more directly descended from EISA Adapter).
This trouble with configuration eventually led to the creation of ISA Adapter<strong> PnP</strong>, a plug-n-play system that used a combination of modifications to hardware, the system BIOS, and operating system software to automatically manage resource allocations. This required a system with an advanced programmable interrupt controller (APIC) replacing the Intel 8259 which the PC was born with. In reality, ISA Adapter PnP can be troublesome, and did not become well-supported until the architecture was in its final days since the APIC chip was a serendipitous addition to ISA Adapter by the PCI standard, which required an APIC.
PCI slots were the first physically incompatible expansion ports to directly squeeze ISA Adapter off the motherboard. At first, motherboards were largely ISA Adapter, including a few PCI slots. By the mid-1990s, the two slot types were roughly balanced, and ISA Adapter slots soon were in the minority of consumer systems. Microsoft's PC 97 specification recommended that ISA Adapter slots be removed entirely, though the system architecture still required ISA Adapter to be present in some vestigial way internally to handle the floppy drive,serial ports, etc., which was why the software compatible LPC bus was created. ISA Adapter slots remained for a few more years, and towards the turn of the century it was common to see systems with an Accelerated Graphics Port (AGP) sitting near thecentral processing unit, an array of PCI slots, and one or two ISA Adapter slots near the end. In late 2008, even floppy disk drives and serial ports were dISA Adapterppearing, and the extinction of vestigial ISA Adapter (by then the LPC bus) from chipsets was on the horizon.
It is also notable that PCI slots are "rotated" compared to their ISA Adapter counterparts—PCI cards were essentially inserted "upside-down," allowing ISA Adapter and PCI connectors to squeeze together on the motherboard. Only one of the two connectors can be used in each slot at a time, but this allowed for greater flexibility.
The AT Attachment (ATA) hard disk interface is directly descended from ISA Adapter (the AT bus). ATA has its origins in hardcards that integrated a hard disk controller (HDC) — usually with an ST-506/ST-412 interface — and a hard disk drive on the same ISA Adapter adapter. This was at best awkward from a mechanical structural standpoint, as ISA Adapter slots were not designed to support such heavy devices as hard disks (and the 3.5" form-factor hard disks of the time were about twice as tall and heavy as modern drives), so the next generation of Integrated Drive Electronics drives moved both the drive and controller to a drive bay and used a ribbon cable and a very simple interface board to connect it to an ISA Adapter slot. ATA, at its essence, is basically a standardization of this arrangement, combined with a uniform command structure for software to interface with the controller on a drive. ATA has since been separated from the ISA Adapter bus, and connected directly to the local bus (usually by integration into the chipset), to be clocked much much faster than ISA Adapter could support and with much higher throughput. (Notably when ISA Adapter was introduced as the AT bus, there <em>was</em> no distinction between a <em>local</em> and <em>extension</em> bus, and there were no chipsets.) Still, ATA retains details which reveal its relationship to ISA Adapter. The 16-bit transfer size is the most obvious example; the signal timing, particularly in the PIO modes, is also highly correlated, and the interrupt and DMA mechanisms are clearly from ISA Adapter. (The article about ATA has more detail about this history.)
Computers with ISA Adapter slot
At NIXSYS, we work with major corporations, universities, small businesses and the U.S. government to develop custom designed computers with ISA Adapter slot. We make sure you get the components you need, and we provide the processing power necessary for your applications that required legacy technology.
One of our most popular computers with ISA Adapter slot is the tower NIX-TISA Adapter. Depending on your needs, you can use a Celeron or Pentium 4 processor for this server and you can use up 2GB of memory. Our computers with ISA Adapter slot also offer the Silencer Kit option that make these a very quite computers or virtually silent. Contact us
us for more information.