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Industry Standard Architecture

Industry Standard Architecture (ISA) is the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s. The bus was (largely) backward compatible with the 8-bit bus of the 8088-based IBM PC, including the IBM PC/XT as well as IBM PC compatibles.

ISA
Industry Standard Architecture
One 8-bit and five 16-bit ISA slots on a motherboard
Year created1981; 42 years ago (1981)
Created byIBM
Superseded byPCI, LPC (1993, 1998)
Width in bits8 or 16
No. of devicesup to 6 devices
SpeedHalf-duplex 8 MB/s or 16 MB/s[1]
StyleParallel
Hotplugging interfaceNo
External interfaceNo

Originally referred to as the PC bus (8-bit) or AT bus (16-bit), it was also termed I/O Channel by IBM. The ISA term was coined as a retronym by IBM PC clone manufacturers in the late 1980s or early 1990s as a reaction to IBM attempts to replace the AT-bus with its new and incompatible Micro Channel architecture.

The 16-bit ISA bus was also used with 32-bit processors for several years. An attempt to extend it to 32 bits, called Extended Industry Standard Architecture (EISA), was not very successful, however. Later buses such as VESA Local Bus and PCI were used instead, often along with ISA slots on the same mainboard. Derivatives of the AT bus structure were and still are used in ATA/IDE, the PCMCIA standard, CompactFlash, the PC/104 bus, and internally within Super I/O chips.

Even though ISA disappeared from consumer desktops many years ago, it is still used in industrial PCs, where certain specialized expansion cards that never transitioned to PCI and PCI Express are used.

History

 
8-bit XT, 16-bit ISA, EISA (top to bottom)
 
8-bit XT: Adlib FM Sound card
 
16-bit ISA: Madge 4/16 Mbps Token Ring NIC
 
16-bit ISA: Ethernet 10Base-5/2 NIC
 
8-bit XT: US Robotics 56k Modem

The original PC bus was developed by a team led by Mark Dean at IBM as part of the IBM PC project in 1981.[2] It was an 8-bit bus based on the I/O bus of the IBM System/23 Datamaster system - it used the same physical connector, and a similar signal protocol and pinout.[3] A 16-bit version, the IBM AT bus, was introduced with the release of the IBM PC/AT in 1984. The AT bus was a mostly backward compatible extension of the PC bus—the AT bus connector was a superset of the PC bus connector. In 1988, the 32-bit EISA standard was proposed by the "Gang of Nine" group of PC-compatible manufacturers that included Compaq. Compaq created the term "Industry Standard Architecture" (ISA) to replace "PC compatible".[4] In the process, they retroactively renamed the AT bus to "ISA" to avoid infringing IBM's trademark on its PC and PC/AT systems (and to avoid giving their major competitor, IBM, free advertisement).

IBM designed the 8-bit version as a buffered interface to the motherboard buses of the Intel 8088 (16/8 bit) CPU in the IBM PC and PC/XT, augmented with prioritized interrupts and DMA channels. The 16-bit version was an upgrade for the motherboard buses of the Intel 80286 CPU (and expanded interrupt and DMA facilities) used in the IBM AT, with improved support for bus mastering. The ISA bus was therefore synchronous with the CPU clock, until sophisticated buffering methods were implemented by chipsets to interface ISA to much faster CPUs.

ISA was designed to connect peripheral cards to the motherboard and allows for bus mastering. Only the first 16 MB of main memory is addressable. The original 8-bit bus ran from the 4.77 MHz clock of the 8088 CPU in the IBM PC and PC/XT. The original 16-bit bus ran from the CPU clock of the 80286 in IBM PC/AT computers, which was 6 MHz in the first models and 8 MHz in later models. The IBM RT PC also used the 16-bit bus. ISA was also used in some non-IBM compatible machines such as Motorola 68k-based Apollo (68020) and Amiga 3000 (68030) workstations, the short-lived AT&T Hobbit and the later PowerPC-based BeBox.

Companies like Dell improved the AT bus's performance[5] but in 1987, IBM replaced the AT bus with its proprietary Micro Channel Architecture (MCA). MCA overcame many of the limitations then apparent in ISA but was also an effort by IBM to regain control of the PC architecture and the PC market. MCA was far more advanced than ISA and had many features that would later appear in PCI. However, MCA was also a closed standard whereas IBM had released full specifications and circuit schematics for ISA. Computer manufacturers responded to MCA by developing the Extended Industry Standard Architecture (EISA) and the later VESA Local Bus (VLB). VLB used some electronic parts originally intended for MCA because component manufacturers already were equipped to manufacture them. Both EISA and VLB were backwards-compatible expansions of the AT (ISA) bus.

Users of ISA-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 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.

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. At first, motherboards were largely ISA, including a few PCI slots. By the mid-1990s, 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., which was why the software compatible LPC bus was created. ISA 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 the central processing unit, an array of PCI slots, and one or two ISA slots near the end. In late 2008, 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.

PCI slots are "rotated" compared to their ISA counterparts—PCI cards were essentially inserted "upside-down," allowing ISA 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 the 16-bit ISA of the PC/AT. ATA has its origins in the IBM Personal Computer Fixed Disk and Diskette Adapter, the standard dual-function floppy disk controller and hard disk controller card for the IBM PC AT; the fixed disk controller on this card implemented the register set and the basic command set which became the basis of the ATA interface (and which differed greatly from the interface of IBM's fixed disk controller card for the PC XT). Direct precursors to ATA were third-party ISA hardcards that integrated a hard disk drive (HDD) and a hard disk controller (HDC) onto one card. 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. ATA has clear characteristics of 16-bit ISA, such as a 16-bit transfer size, signal timing in the PIO modes and the interrupt and DMA mechanisms.

ISA bus architecture


 
 

The PC/XT-bus is an eight-bit ISA bus used by Intel 8086 and Intel 8088 systems in the IBM PC and IBM PC XT in the 1980s. Among its 62 pins were demultiplexed and electrically buffered versions of the 8 data and 20 address lines of the 8088 processor, along with power lines, clocks, read/write strobes, interrupt lines, etc. Power lines included −5 V and ±12 V in order to directly support pMOS and enhancement mode nMOS circuits such as dynamic RAMs among other things. The XT bus architecture uses a single Intel 8259 PIC, giving eight vectorized and prioritized interrupt lines. It has four DMA channels originally provided by the Intel 8237. Three 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):

DMA channel Expansion Standard function
0 No Dynamic random-access memory refresh
1 Yes Add-on cards
2 Yes Floppy disk controller
3 Yes Hard disk controller

The PC/AT-bus, a 16-bit (or 80286-) version of the PC/XT bus, was introduced with the IBM PC/AT. This bus was officially termed I/O Channel by IBM.[citation needed] 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 16. It also adds new interrupt lines connected to a second 8259 PIC (connected to one of the lines of the first) and 4 × 16-bit DMA channels, as well as control lines to select 8- or 16-bit transfers.

The 16-bit AT bus slot originally used two standard edge connector sockets in early IBM PC/AT machines. However, with the popularity of the AT-architecture and the 16-bit ISA bus, manufacturers introduced specialized 98-pin connectors that integrated the two sockets into one unit. These can be found in almost every AT-class PC manufactured after the mid-1980s. The ISA slot connector is typically black (distinguishing it from the brown EISA connectors and white PCI connectors).

Number of devices

Motherboard devices have dedicated IRQs (not present in the slots). 16-bit devices can use either PC-bus or PC/AT-bus IRQs. It is therefore possible to connect up to 6 devices that use one 8-bit IRQ each and up to 5 devices that use one 16-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 16-bit DMA channel each.

Varying bus speeds

Originally, the bus clock was synchronous with the CPU clock, resulting in varying bus clock frequencies among the many different IBM "clones" on the market (sometimes as high as 16 or 20 MHz), leading to software or electrical timing problems for certain ISA cards at bus speeds they were not designed for. Later motherboards or integrated chipsets used a separate clock generator, or a clock divider which either fixed the ISA bus frequency at 4, 6, or 8 MHz or allowed the user to adjust the frequency via the BIOS setup. When used at a higher bus frequency, some ISA cards (certain Hercules-compatible video cards, for instance), could show significant performance improvements.

8/16-bit incompatibilities

Memory address decoding for the selection of 8 or 16-bit transfer mode was limited to 128 KiB sections, leading to problems when mixing 8- and 16-bit cards as they could not co-exist in the same 128 KiB area. This is because the MEMCS16 line is required to be set based on the value of LA17-23 only.

Past and current use

ISA is still used today for specialized industrial purposes. In 2008 IEI Technologies released a modern motherboard for Intel Core 2 Duo processors which, in addition to other special I/O features, is equipped with two ISA slots. 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.[6]

Similarly, ADEK Industrial Computers is releasing a motherboard in early 2013 for Intel Core i3/i5/i7 processors, which contains one (non-DMA) ISA slot.[7]

The PC/104 bus, used in industrial and embedded applications, is a derivative of the ISA bus, utilizing the same signal lines with different connectors. The LPC bus has replaced the ISA bus as the connection to the legacy I/O devices on recent motherboards; while physically quite different, LPC looks just like ISA to software, so that the peculiarities of ISA such as the 16 MiB DMA limit (which corresponds to the full address space of the Intel 80286 CPU used in the original IBM AT) are likely to stick around for a while.

ATA

As explained in the History section, ISA was the basis for development of the ATA interface, used for ATA (a.k.a. IDE) hard disks. Physically, ATA is essentially a simple subset of ISA, with 16 data bits, support for exactly one IRQ and one DMA channel, and 3 address bits. To this ISA subset, ATA adds two IDE address select ("chip select") lines (i.e. address decodes, effectively equivalent to address bits) and a few unique signal lines specific to ATA/IDE hard disks (such as the Cable Select/Spindle Sync. line.) 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 ATA device registers are accessed using the address bits and address select signals in the ATA physical interface channel, and all operations of ATA hard disks are performed using the ATA-specified protocols through the ATA command set. 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.

A further deviation between ISA and ATA is that while the ISA bus remained locked into a single standard clock rate (for backward hardware compatibility), the ATA interface offered many different speed modes, could select among them to match the maximum speed supported by the attached drives, and kept adding faster speeds with later versions of the ATA standard (up to 133 MB/s for ATA-6, the latest.) In most forms, ATA ran much faster than ISA, provided it was connected directly to a local bus (e.g. southbridge-integrated IDE interfaces) faster than the ISA bus.

XT-IDE

Before the 16-bit ATA/IDE interface, there was an 8-bit XT-IDE (also known as XTA) interface for hard disks. It was not nearly as popular as ATA has become, and XT-IDE hardware is now fairly hard to find. Some XT-IDE adapters were available as 8-bit ISA cards, and XTA sockets were also present on the motherboards of Amstrad's later XT clones as well as a short-lived line of Philips units. 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. A few hard drives (such as the Seagate ST351A/X) could support either type of interface, selected with a jumper.

Many later AT (and AT successor) motherboards had no integrated hard drive interface but relied on a separate hard drive interface plugged into an ISA/EISA/VLB slot. There were even a few 80486 based units shipped with MFM/RLL interfaces and drives instead of the increasingly common AT-IDE.

Commodore built the XT-IDE based peripheral hard drive / memory expansion unit A590 for their Amiga 500 and 500+ computers that also supported a SCSI drive. Later models – the A600, A1200, and the Amiga 4000 series – use AT-IDE drives.

PCMCIA

The PCMCIA specification can be seen as a superset of ATA. The standard for PCMCIA hard disk interfaces, which included PCMCIA flash drives, allows for the mutual configuration of the port and the drive in an ATA mode. As a de facto extension, most PCMCIA flash drives additionally allow for a simple ATA mode that is enabled by pulling a single pin low, so that PCMCIA hardware and firmware are unnecessary to use them as an ATA drive connected to an ATA port. PCMCIA flash drive to ATA adapters are thus simple and inexpensive, but are not guaranteed to work with any and every standard PCMCIA flash drive. Further, such adapters cannot be used as generic PCMCIA ports, as the PCMCIA interface is much more complex than ATA.

Emulation by embedded chips

Although most modern computers do not have physical ISA buses, almost all PCs — x86-32, and x86-64 — have ISA buses allocated in physical address space. some Southbridges and some CPUs themselves provide services such as temperature monitoring and voltage readings through ISA buses as ISA devices.[citation needed]

Standardization

IEEE started a standardization of the ISA bus in 1985, called the P996 specification. However, despite books being published on the P996 specification, it never officially progressed past draft status.[8]

Modern ISA cards

There still is an existing user base with old computers, so some ISA cards are still manufactured, e.g. with USB ports[9] or complete single-board computers based on modern processors, USB 3.0, and SATA.[10]

See also

References

  1. ^ Kyle Chapman. "The Wonderful World of Buses". Retrieved 2021-06-30.
  2. ^ Reilly, Edwin (2003). Milestones in computer science and information technology. Westport, Conn.: Greenwood Press. pp. 37. ISBN 1573565210. OCLC 51258496.
  3. ^ John Titus (2001-09-15). "Whence Came the IBM PC". edn.com. Retrieved 2020-10-13.
  4. ^ LaPlante, Alice; Furger, Roberta (1989-01-23). "Compaq Vying To Become the IBM of the '90s". InfoWorld. pp. 1, 8. Retrieved 17 March 2016.
  5. ^ Lewis, Peter H. (1988-04-24). "Introducing the First PS/2 Clones". The New York Times. from the original on 6 January 2015. Retrieved 6 January 2015.
  6. ^ IEI Technology Corp: IMBA-9654ISA User Manual, Rev. 1.00, May 2008
  7. ^ ADEK Industrial Computers: MS-98A9 Product Specifications
  8. ^ Graves, Michael W. (2005). A+ Guide to PC Hardware Maintenance and Repair, Volume 1. Thomson, Delmar Learning. p. 191. ISBN 1401852300.
  9. ^ "Lo-tech ISA USB Adapter - lo-tech.co.uk". www.lo-tech.co.uk. from the original on 9 April 2018. Retrieved 3 May 2018.
  10. ^ "PCA-6763". www.advantech.com. from the original on 24 October 2017. Retrieved 3 May 2018.

Further reading

  • Intel ISA Bus Specification and Application Notes - Rev 2.01; Intel; 73 pages; 1989.

External links

  • "Connector Bus ISA (Industry Standard Architecture)". Hardware Book.
  • Microsoft (1999-06-02). . Microsoft. Archived from the original (Microsoft Word) on 2012-03-11. Retrieved 2007-07-14.

industry, standard, architecture, other, uses, disambiguation, confused, with, instruction, architecture, this, article, includes, list, general, references, lacks, sufficient, corresponding, inline, citations, please, help, improve, this, article, introducing. For other uses of ISA see Isa disambiguation Not to be confused with Instruction set architecture This article includes a list of general references but it lacks sufficient corresponding inline citations Please help to improve this article by introducing more precise citations January 2014 Learn how and when to remove this template message Industry Standard Architecture ISA is the 16 bit internal bus of IBM PC AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s The bus was largely backward compatible with the 8 bit bus of the 8088 based IBM PC including the IBM PC XT as well as IBM PC compatibles ISAIndustry Standard ArchitectureOne 8 bit and five 16 bit ISA slots on a motherboardYear created1981 42 years ago 1981 Created byIBMSuperseded byPCI LPC 1993 1998 Width in bits8 or 16No of devicesup to 6 devicesSpeedHalf duplex 8 MB s or 16 MB s 1 StyleParallelHotplugging interfaceNoExternal interfaceNoOriginally referred to as the PC bus 8 bit or AT bus 16 bit it was also termed I O Channel by IBM The ISA term was coined as a retronym by IBM PC clone manufacturers in the late 1980s or early 1990s as a reaction to IBM attempts to replace the AT bus with its new and incompatible Micro Channel architecture The 16 bit ISA bus was also used with 32 bit processors for several years An attempt to extend it to 32 bits called Extended Industry Standard Architecture EISA was not very successful however Later buses such as VESA Local Bus and PCI were used instead often along with ISA slots on the same mainboard Derivatives of the AT bus structure were and still are used in ATA IDE the PCMCIA standard CompactFlash the PC 104 bus and internally within Super I O chips Even though ISA disappeared from consumer desktops many years ago it is still used in industrial PCs where certain specialized expansion cards that never transitioned to PCI and PCI Express are used Contents 1 History 2 ISA bus architecture 2 1 Number of devices 2 2 Varying bus speeds 2 3 8 16 bit incompatibilities 3 Past and current use 3 1 ATA 3 2 XT IDE 3 3 PCMCIA 4 Emulation by embedded chips 5 Standardization 6 Modern ISA cards 7 See also 8 References 9 Further reading 10 External linksHistory Edit 8 bit XT 16 bit ISA EISA top to bottom 8 bit XT Adlib FM Sound card 16 bit ISA Madge 4 16 Mbps Token Ring NIC 16 bit ISA Ethernet 10Base 5 2 NIC 8 bit XT US Robotics 56k Modem The original PC bus was developed by a team led by Mark Dean at IBM as part of the IBM PC project in 1981 2 It was an 8 bit bus based on the I O bus of the IBM System 23 Datamaster system it used the same physical connector and a similar signal protocol and pinout 3 A 16 bit version the IBM AT bus was introduced with the release of the IBM PC AT in 1984 The AT bus was a mostly backward compatible extension of the PC bus the AT bus connector was a superset of the PC bus connector In 1988 the 32 bit EISA standard was proposed by the Gang of Nine group of PC compatible manufacturers that included Compaq Compaq created the term Industry Standard Architecture ISA to replace PC compatible 4 In the process they retroactively renamed the AT bus to ISA to avoid infringing IBM s trademark on its PC and PC AT systems and to avoid giving their major competitor IBM free advertisement IBM designed the 8 bit version as a buffered interface to the motherboard buses of the Intel 8088 16 8 bit CPU in the IBM PC and PC XT augmented with prioritized interrupts and DMA channels The 16 bit version was an upgrade for the motherboard buses of the Intel 80286 CPU and expanded interrupt and DMA facilities used in the IBM AT with improved support for bus mastering The ISA bus was therefore synchronous with the CPU clock until sophisticated buffering methods were implemented by chipsets to interface ISA to much faster CPUs ISA was designed to connect peripheral cards to the motherboard and allows for bus mastering Only the first 16 MB of main memory is addressable The original 8 bit bus ran from the 4 77 MHz clock of the 8088 CPU in the IBM PC and PC XT The original 16 bit bus ran from the CPU clock of the 80286 in IBM PC AT computers which was 6 MHz in the first models and 8 MHz in later models The IBM RT PC also used the 16 bit bus ISA was also used in some non IBM compatible machines such as Motorola 68k based Apollo 68020 and Amiga 3000 68030 workstations the short lived AT amp T Hobbit and the later PowerPC based BeBox Companies like Dell improved the AT bus s performance 5 but in 1987 IBM replaced the AT bus with its proprietary Micro Channel Architecture MCA MCA overcame many of the limitations then apparent in ISA but was also an effort by IBM to regain control of the PC architecture and the PC market MCA was far more advanced than ISA and had many features that would later appear in PCI However MCA was also a closed standard whereas IBM had released full specifications and circuit schematics for ISA Computer manufacturers responded to MCA by developing the Extended Industry Standard Architecture EISA and the later VESA Local Bus VLB VLB used some electronic parts originally intended for MCA because component manufacturers already were equipped to manufacture them Both EISA and VLB were backwards compatible expansions of the AT ISA bus Users of ISA 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 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 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 At first motherboards were largely ISA including a few PCI slots By the mid 1990s 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 which was why the software compatible LPC bus was created ISA 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 the central processing unit an array of PCI slots and one or two ISA slots near the end In late 2008 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 PCI slots are rotated compared to their ISA counterparts PCI cards were essentially inserted upside down allowing ISA 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 the 16 bit ISA of the PC AT ATA has its origins in the IBM Personal Computer Fixed Disk and Diskette Adapter the standard dual function floppy disk controller and hard disk controller card for the IBM PC AT the fixed disk controller on this card implemented the register set and the basic command set which became the basis of the ATA interface and which differed greatly from the interface of IBM s fixed disk controller card for the PC XT Direct precursors to ATA were third party ISA hardcards that integrated a hard disk drive HDD and a hard disk controller HDC onto one card 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 ATA has clear characteristics of 16 bit ISA such as a 16 bit transfer size signal timing in the PIO modes and the interrupt and DMA mechanisms ISA bus architecture Edit The PC XT bus is an eight bit ISA bus used by Intel 8086 and Intel 8088 systems in the IBM PC and IBM PC XT in the 1980s Among its 62 pins were demultiplexed and electrically buffered versions of the 8 data and 20 address lines of the 8088 processor along with power lines clocks read write strobes interrupt lines etc Power lines included 5 V and 12 V in order to directly support pMOS and enhancement mode nMOS circuits such as dynamic RAMs among other things The XT bus architecture uses a single Intel 8259 PIC giving eight vectorized and prioritized interrupt lines It has four DMA channels originally provided by the Intel 8237 Three 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 DMA channel Expansion Standard function0 No Dynamic random access memory refresh1 Yes Add on cards2 Yes Floppy disk controller3 Yes Hard disk controllerThe PC AT bus a 16 bit or 80286 version of the PC XT bus was introduced with the IBM PC AT This bus was officially termed I O Channel by IBM citation needed 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 16 It also adds new interrupt lines connected to a second 8259 PIC connected to one of the lines of the first and 4 16 bit DMA channels as well as control lines to select 8 or 16 bit transfers The 16 bit AT bus slot originally used two standard edge connector sockets in early IBM PC AT machines However with the popularity of the AT architecture and the 16 bit ISA bus manufacturers introduced specialized 98 pin connectors that integrated the two sockets into one unit These can be found in almost every AT class PC manufactured after the mid 1980s The ISA slot connector is typically black distinguishing it from the brown EISA connectors and white PCI connectors Number of devices Edit Motherboard devices have dedicated IRQs not present in the slots 16 bit devices can use either PC bus or PC AT bus IRQs It is therefore possible to connect up to 6 devices that use one 8 bit IRQ each and up to 5 devices that use one 16 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 16 bit DMA channel each Varying bus speeds Edit Originally the bus clock was synchronous with the CPU clock resulting in varying bus clock frequencies among the many different IBM clones on the market sometimes as high as 16 or 20 MHz leading to software or electrical timing problems for certain ISA cards at bus speeds they were not designed for Later motherboards or integrated chipsets used a separate clock generator or a clock divider which either fixed the ISA bus frequency at 4 6 or 8 MHz or allowed the user to adjust the frequency via the BIOS setup When used at a higher bus frequency some ISA cards certain Hercules compatible video cards for instance could show significant performance improvements 8 16 bit incompatibilities Edit Memory address decoding for the selection of 8 or 16 bit transfer mode was limited to 128 KiB sections leading to problems when mixing 8 and 16 bit cards as they could not co exist in the same 128 KiB area This is because the MEMCS16 line is required to be set based on the value of LA17 23 only Past and current use EditThis section needs to be updated Please help update this article to reflect recent events or newly available information January 2020 ISA is still used today for specialized industrial purposes In 2008 IEI Technologies released a modern motherboard for Intel Core 2 Duo processors which in addition to other special I O features is equipped with two ISA slots 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 6 Similarly ADEK Industrial Computers is releasing a motherboard in early 2013 for Intel Core i3 i5 i7 processors which contains one non DMA ISA slot 7 The PC 104 bus used in industrial and embedded applications is a derivative of the ISA bus utilizing the same signal lines with different connectors The LPC bus has replaced the ISA bus as the connection to the legacy I O devices on recent motherboards while physically quite different LPC looks just like ISA to software so that the peculiarities of ISA such as the 16 MiB DMA limit which corresponds to the full address space of the Intel 80286 CPU used in the original IBM AT are likely to stick around for a while ATA Edit As explained in the History section ISA was the basis for development of the ATA interface used for ATA a k a IDE hard disks Physically ATA is essentially a simple subset of ISA with 16 data bits support for exactly one IRQ and one DMA channel and 3 address bits To this ISA subset ATA adds two IDE address select chip select lines i e address decodes effectively equivalent to address bits and a few unique signal lines specific to ATA IDE hard disks such as the Cable Select Spindle Sync line 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 ATA device registers are accessed using the address bits and address select signals in the ATA physical interface channel and all operations of ATA hard disks are performed using the ATA specified protocols through the ATA command set 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 A further deviation between ISA and ATA is that while the ISA bus remained locked into a single standard clock rate for backward hardware compatibility the ATA interface offered many different speed modes could select among them to match the maximum speed supported by the attached drives and kept adding faster speeds with later versions of the ATA standard up to 133 MB s for ATA 6 the latest In most forms ATA ran much faster than ISA provided it was connected directly to a local bus e g southbridge integrated IDE interfaces faster than the ISA bus XT IDE Edit Before the 16 bit ATA IDE interface there was an 8 bit XT IDE also known as XTA interface for hard disks It was not nearly as popular as ATA has become and XT IDE hardware is now fairly hard to find Some XT IDE adapters were available as 8 bit ISA cards and XTA sockets were also present on the motherboards of Amstrad s later XT clones as well as a short lived line of Philips units 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 A few hard drives such as the Seagate ST351A X could support either type of interface selected with a jumper Many later AT and AT successor motherboards had no integrated hard drive interface but relied on a separate hard drive interface plugged into an ISA EISA VLB slot There were even a few 80486 based units shipped with MFM RLL interfaces and drives instead of the increasingly common AT IDE Commodore built the XT IDE based peripheral hard drive memory expansion unit A590 for their Amiga 500 and 500 computers that also supported a SCSI drive Later models the A600 A1200 and the Amiga 4000 series use AT IDE drives PCMCIA Edit The PCMCIA specification can be seen as a superset of ATA The standard for PCMCIA hard disk interfaces which included PCMCIA flash drives allows for the mutual configuration of the port and the drive in an ATA mode As a de facto extension most PCMCIA flash drives additionally allow for a simple ATA mode that is enabled by pulling a single pin low so that PCMCIA hardware and firmware are unnecessary to use them as an ATA drive connected to an ATA port PCMCIA flash drive to ATA adapters are thus simple and inexpensive but are not guaranteed to work with any and every standard PCMCIA flash drive Further such adapters cannot be used as generic PCMCIA ports as the PCMCIA interface is much more complex than ATA Emulation by embedded chips EditAlthough most modern computers do not have physical ISA buses almost all PCs x86 32 and x86 64 have ISA buses allocated in physical address space some Southbridges and some CPUs themselves provide services such as temperature monitoring and voltage readings through ISA buses as ISA devices citation needed Standardization EditIEEE started a standardization of the ISA bus in 1985 called the P996 specification However despite books being published on the P996 specification it never officially progressed past draft status 8 Modern ISA cards EditThere still is an existing user base with old computers so some ISA cards are still manufactured e g with USB ports 9 or complete single board computers based on modern processors USB 3 0 and SATA 10 See also EditPC 104 Embedded variant of ISA Low Pin Count LPC Extended Industry Standard Architecture EISA Micro Channel architecture MCA VESA Local Bus VLB Peripheral Component Interconnect PCI Accelerated Graphics Port AGP PCI X PCI Express PCI E or PCIe List of computer bus interfaces Amiga Zorro II NuBus Switched fabric List of device bandwidths CompactPCI PC card Universal Serial Bus USB Legacy port BackplaneReferences Edit Kyle Chapman The Wonderful World of Buses Retrieved 2021 06 30 Reilly Edwin 2003 Milestones in computer science and information technology Westport Conn Greenwood Press pp 37 ISBN 1573565210 OCLC 51258496 John Titus 2001 09 15 Whence Came the IBM PC edn com Retrieved 2020 10 13 LaPlante Alice Furger Roberta 1989 01 23 Compaq Vying To Become the IBM of the 90s InfoWorld pp 1 8 Retrieved 17 March 2016 Lewis Peter H 1988 04 24 Introducing the First PS 2 Clones The New York Times Archived from the original on 6 January 2015 Retrieved 6 January 2015 IEI Technology Corp IMBA 9654ISA User Manual Rev 1 00 May 2008 ADEK Industrial Computers MS 98A9 Product Specifications Graves Michael W 2005 A Guide to PC Hardware Maintenance and Repair Volume 1 Thomson Delmar Learning p 191 ISBN 1401852300 Lo tech ISA USB Adapter lo tech co uk www lo tech co uk Archived from the original on 9 April 2018 Retrieved 3 May 2018 PCA 6763 www advantech com Archived from the original on 24 October 2017 Retrieved 3 May 2018 Further reading EditIntel ISA Bus Specification and Application Notes Rev 2 01 Intel 73 pages 1989 External links Edit Wikimedia Commons has media related to ISA Connector Bus ISA Industry Standard Architecture Hardware Book Microsoft 1999 06 02 Removing the ISA Architecture in Windows Based Platforms Microsoft Archived from the original Microsoft Word on 2012 03 11 Retrieved 2007 07 14 Retrieved from https en wikipedia org w index php title Industry Standard Architecture amp oldid 1146951819, wikipedia, wiki, book, books, library,

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