While a handful of devices were essentially " plug-n-play ", this was rare. This trouble with configuration eventually led to the creation of ISA PnP , 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.
In reality, ISA PnP could be troublesome and did not become well-supported until the architecture was in its final days.
PCI slots were the first physically-incompatible expansion ports to directly squeeze ISA off the motherboard. By the mids, the two slot types were roughly balanced, and ISA slots soon were in the minority of consumer systems.
Microsoft 's PC 99 specification recommended that ISA slots be removed entirely, though the system architecture still required ISA to be present in some vestigial way internally to handle the floppy drive , serial ports , etc.
In late , even floppy disk drives and serial ports were disappearing, and the extinction of vestigial ISA by then the LPC bus from chipsets was on the horizon.
Only one of the two connectors can be used in each slot at a time, but this allowed for greater flexibility.
This was at best awkward and at worst damaging to the motherboard, as ISA slots were not designed to support such heavy devices as HDDs.
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 slot.
ATA is basically a standardization of this arrangement plus a uniform command structure for software to interface with the HDC within the drive.
ATA has since been separated from the ISA bus and connected directly to the local bus, usually by integration into the chipset, for much higher clock rates and data throughput than ISA could support.
It has four DMA channels originally provided by the Intel , 3 of the DMA channels are brought out to the XT bus expansion slots; of these, 2 are normally already allocated to machine functions diskette drive and hard disk controller:.
It extends the XT-bus by adding a second shorter edge connector in-line with the eight-bit XT-bus connector, which is unchanged, retaining compatibility with most 8-bit cards.
The second connector adds four additional address lines for a total of 24, and 8 additional data lines for a total of However, with the popularity of the AT-architecture and the bit ISA bus, manufacturers introduced specialized pin connectors that integrated the two sockets into one unit.
These can be found in almost every AT-class PC manufactured after the mids. Motherboard devices have dedicated IRQs not present in the slots.
It is therefore possible to connect up to 6 devices that use one 8-bit IRQ each, or up to 5 devices that use one bit IRQ each.
At the same time, up to 4 devices may use one 8-bit DMA channel each, while up to 3 devices can use one bit DMA channel each.
When used at a higher bus frequency, some ISA cards certain Hercules-compatible video cards, for instance , could show significant performance improvements.
ISA is still used today for specialized industrial purposes. It is marketed to industrial and military users who have invested in expensive specialized ISA bus adaptors, which are not available in PCI bus versions.
In addition to the physical interface channel, ATA goes beyond and far outside the scope of ISA by also specifying a set of physical device registers to be implemented on every ATA IDE drive and a full set of protocols and device commands for controlling fixed disk drives using these registers.
The earliest versions of the ATA standard featured a few simple protocols and a basic command set comparable to the command sets of MFM and RLL controllers which preceded ATA controllers , but the latest ATA standards have much more complex protocols and instruction sets that include optional commands and protocols providing such advanced optional-use features as sizable hidden system storage areas, password security locking, and programmable geometry translation.
The XTA pinout was very similar to ATA, but only eight data lines and two address lines were used, and the physical device registers had completely different meanings.
AT-IDE type interfaces only entered the keyboard-cased Amiga line upon introduction of the A and A which have an integrated interface and 44 pin connector.
Many owners removed the 2,5 inch bracket and installed a 3,5 inch drive with an adapter cable. Although most modern computers do not have physical ISA buses, all IBM compatible computers — x86 , and x most non-mainframe, non-embedded — have ISA buses allocated in physical address space.
Embedded controller chips southbridge and CPUs themselves provide services such as temperature monitoring and voltage readings through these buses as ISA devices.
However, despite there even having been books published on the P specification, it never officially progressed past draft status.
Because of this, when the first -based system the Compaq Deskpro hit the market in , it still supported bit slots. Meanwhile, IBM began to worry that it was losing control of the industry it had created.
MCA included numerous enhancements over the bit AT bus, including bus mastering , burst mode , software configurable resources, and bit capabilities.
However, in an effort to reassert its dominant role, IBM patented the bus, and placed stringent licensing and royalty policies on its use.
Steve Gibson proposed that clone makers adopt NuBus. The EISA bus slot is a two-level staggered pin system, with the upper part of the slot corresponding to the standard ISA bus pin layout.
Additionally the lower part of the bus has five keying notches, so an ISA card with unusually long traces cannot accidentally extend down into the lower part of the slot.
Intel introduced their first EISA chipset and also their first chipset in the modern sense of the word as the in September It included such features as multiprocessing , hardware RAID , and bus-mastering network cards.
One of the benefits to come out of the EISA standard was a final codification of the standard to which ISA slots and cards should be held in particular, clock speed was fixed at an industry standard of 8.
Thus, even systems which didn't use the EISA bus gained the advantage of having the ISA standardized, which contributed to its longevity.
The Gang of Nine was the informal name given to the consortium of personal computer manufacturing companies that together created the EISA bus.
Rival members generally acknowledged Compaq's leadership, with one stating in that within the Gang of Nine "when you have 10 people sit down before a table to write a letter to the president, someone has to write the letter.
Compaq is sitting down at the typewriter". The user would boot into this utility, either from floppy disk or on a dedicated hard drive partition.
The utility software would detect all EISA cards in the system, and could configure any hardware resources interrupts , memory ports, etc. The user could also enter information about ISA cards in the system, allowing the utility to automatically reconfigure EISA cards to avoid resource conflicts.
Similarly, Windows 95 , with its Plug-and-Play capability, was not able to change the configuration of EISA cards, but it could detect the cards, read their configuration, and reconfigure Plug and Play hardware to avoid resource conflicts.
Windows 95 would also automatically attempt to install appropriate drivers for detected EISA cards. EISA's success was far from guaranteed. For example, Compaq actually produced prototype DeskPro systems using the bus.
However, these were never put into production, and when it was clear that MCA had lost, Compaq allowed its MCA license to expire the license actually cost relatively little; the primary costs associated with MCA, and at which the industry revolted, were royalties to be paid per system shipped.
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November and incorporated under the "relicensing" terms of the GFDL , version 1.
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