Micro Channel architecture , or the Micro Channel bus, is a proprietary 16- or 32-bit parallel computer bus publicly introduced by IBM in 1987 which was used on PS/2 and other computers until the mid-1990s. Its name is commonly abbreviated as " MCA ", although not by IBM. In IBM products, it superseded the ISA bus and was itself subsequently superseded by the PCI bus architecture.
52-517: The development of Micro Channel was driven by both technical and business pressures. The IBM AT bus, which later became known as the Industry Standard Architecture (ISA) bus, had a number of technical design limitations, including: In addition, it suffered from other problems: These limitations became more serious as the range of tasks and peripherals, and the number of manufacturers for IBM PC-compatibles , grew. IBM
104-655: A 1024 Hz timer (on IRQ 8), a much finer resolution than the 18 Hz timer on the PC. In addition to keeping the time, the RTC includes 50 bytes of CMOS memory which is used to store software-adjustable BIOS parameters. A disk-based BIOS setup program which saved to this memory took the place of the DIP switches used to set system settings on PCs. Most AT clones have the setup program in ROM rather than on disk. The standard floppy drive
156-465: A 360 KB drive. Overwriting a DD disk that had been written in a DD drive with an HD drive would result in a disk that read on an HD drive, but produced read errors in a DD drive. Whereas a HD read head would only pick up the half track that drive had written, the wider DD read head would pick up the half-track written by the HD drive mixed with the unerased half-track remnant of the track written earlier by
208-583: A DD drive. Thus, the DD drive would end up reading both new and old information together, causing it to see garbled data. Due to a US antitrust consent decree with IBM, the PC AT architecture was functionally an open design, and IBM's efforts to trademark the AT name largely failed. Many 286-based PCs were modeled after it and marketed as AT-compatible . The label also became a standard term in reference to PCs that used
260-655: A few years of its arrival in 1992, PCI had largely superseded Micro Channel, EISA, and VLB. In response to the rise of EISA, IBM and thirteen Micro Channel card and peripheral manufacturers formed the Micro Channel Developers Association . This was a consortium that sought to consider and prioritize steps in the maturation of Micro Channel, as well as to explore better approaches to disseminating technical information about Micro Channel to third parties. In 1992, it reached 92 members, including IBM. Even after IBM discontinued MCA systems in 1995,
312-513: A minimum of 2.5 amperes on its +12 V line. The power supply would fail to start unless these minimum load requirements were met, but the AT motherboard did not provide much load on the +12 V line. To solve this problem, entry-level IBM AT models that did not have a hard drive were shipped with a 5-ohm, 50-watt resistor connected on the +12 V line of the hard disk power connector. In normal operation this resistor drew 2.4 amperes (dissipating 28.8 watts), getting fairly hot. In addition to
364-451: A modest increase in terms of clock rate, but the greater bus width, coupled with a dedicated bus controller that utilized burst mode transfers, meant that effective throughput was up to five times higher than ISA. For faster transfers the address bus could be reused for data, further increasing the effective width of the bus. While the 10 MHz rate allowed 40 MB/s of throughput at 32-bit width, later models of RS/6000 machines increased
416-538: A new card (video, printer, memory, network, modem, etc.) the user simply plugged in the MCA card and inserted a customized floppy disk (that came with the PC) to blend the new card into the original hardware automatically, rather than bringing in an expensively trained technician who could manually make all the needed changes. All choices for interrupts (an often perplexing problem) and other changes were accomplished automatically by
468-429: A newer card, causing an error at startup. In turn, this required IBM to release updated Reference Disks (The CMOS Setup Utility) on a regular basis. A fairly complete list of known IDs is available (see External links section). To accompany these reference disks were ADF files which were read by setup which in turn provided configuration information for the card. The ADF was a simple text file, containing information about
520-525: A number of additional optional connectors for memory cards which resulted in a huge number of physically incompatible cards for bus attached memory. In time, memory moved to the CPU's local bus , thereby eliminating the problem. On the upside, signal quality was greatly improved as Micro Channel added ground and power pins and arranged the pins to minimize interference; a ground or a supply was thereby located within 3 pins of every signal. Another connector extension
572-472: A prisoner of its standards as its competitors are. Once enough IBM machines have been bought, IBM cannot make sudden changes in their basic design; what might be useful for shedding competitors would shake off even more customers. Micro Channel architecture was publicly introduced at the launch of the PS/2 range in 1987, with three out of four of the new machines featuring it. IBM had actually discreetly introduced
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#1732780707643624-746: Is IBM PC compatible , with the most significant difference being a move to the 80286 processor from the 8088 processor of prior models. Like the IBM PC, the AT supported an optional math co-processor chip, the Intel 80287 , for faster execution of floating point operations . In addition, it introduced the AT bus , later known as the ISA bus, a 16-bit bus with backward compatibility with 8-bit PC-compatible expansion cards. The bus also offered fifteen IRQs and seven DMA channels, expanded from eight IRQs and four DMA channels for
676-576: Is passed more efficiently. Advanced interrupt handling refers to the use of level-sensitive interrupts to handle system requests. Rather than a dedicated interrupt line, several lines can be shared to provide more possible interrupts, addressing the ISA-bus interrupt line conflict problems. All interrupt request signals were "public" on Micro Channel architecture permitting any card on the bus to function as an I/O processor for direct service of I/O device interrupts. ISA had limited all such processing to just
728-471: Is taken for granted now, but at the time setup was a huge chore for ISA systems. POS was a simple system that included device IDs in firmware, which the drivers in the computer were supposed to interpret. (This type of software-configuration system is known as plug and play today.) The feature did not really live up to its promise; the automatic configuration was fine when it worked, but it frequently did not - resulting in an unbootable computer - and resolving
780-468: The AT keyboard , initially a new 84-key layout (the 84th key being SysRq ). The numerical keypad was now clearly separated from the main key group, and indicator LEDs were added for Caps Lock, Scroll Lock and Num Lock. The AT keyboard uses the same 5-pin DIN connector as the PC keyboard, but a different, bidirectional electrical interface with different keyboard scan codes . The bidirectional interface allows
832-513: The Chips and Technologies P82C612 MCA interface controller; allowing MCA implementations to become a lot easier. The Micro Channel was primarily a 32-bit bus, but the system also supported a 16-bit mode designed to lower the cost of connectors and logic in Intel -based machines like the IBM PS/2 . The situation was never that simple, however, as both the 32-bit and 16-bit versions initially had
884-957: The IBM 9370 systems - smallest members of the System/370 range. IBM licensed the architecture to other companies for one to five percent of revenue. Tandy Corporation was the first to ship a Micro Channel-based computer, the 5000 MC, but company head John Roach said "I'm surprised anybody at all would want it"; Tandy only sold the computer, he said, because there was some demand for it. NCR Corporation adopted Micro Channel comprehensively - they designed and built high-performance personal computer, workstation and server platforms supporting it, including their own Micro Channel architecture-based logic componentry, including SCSI, graphics, networking, and audio. A small number of other manufacturers, including Apricot , Dell , Research Machines , and Olivetti adopted it, but only for part of their PC range. Despite
936-511: The Tandy 3000 , Compaq Deskpro 286, HP Vectra, Zenith Z-286, Epson Equity Models II+ and III, and Commodore PC-30 and PC-40 . In Europe, on the other hand, most AT-clones sold were more or less anonymous. The AT bus became the de facto ISA ( Industry Standard Architecture ), while PC XT slots were retroactively named 8-bit ISA . The disk interface for the AT, originally a Seagate ST506 compatible interface on IBM's disk controller card,
988-540: The 16-bit AT bus, (embraced and renamed as ISA to avoid IBM's "AT" trademark) and manual configuration, although the VESA Local Bus (VLB) was briefly popular for Intel '486 machines. For servers the technical limitations of the old ISA were too great, and, in late 1988, the " Gang of Nine ", led by Compaq , announced a rival high-performance bus - Extended Industry Standard Architecture (EISA). This offered similar performance benefits to Micro Channel, but with
1040-528: The 286 expands RAM capacity to 16 MB . PC DOS 3.0 was included with support for the new AT features, including preliminary kernel support for networking (which was fully supported in a later version 3.x release). The motherboard includes a battery-backed real-time clock (RTC) using the Motorola MC146818. This was an improvement from the PC, which required setting the clock manually or installing an RTC expansion card. The RTC also included
1092-477: The 360 KB disk drive faceplate. If the user accidentally used a high-density diskette in the 360 KB drive, it would sometimes work, for a while, but the high- coercivity oxide would take a very weak magnetization from the 360 KB write heads, so reading the diskette would be problematic. Conversely, the high-density drive's heads had a track width half that of the 360 KB drive, so they were incapable of fully erasing and overwriting tracks written by
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#17327807076431144-584: The AT as the best desktop computer when "price is no object" for 1984, describing it as "an innovative, state-of-the-art computer that has the competition gasping for breath". An industry analyst wrote in Computerworld in 1985 that the AT's power was evidence of IBM's belief that personal computers were more important for the company than minicomputers. On April 2, 1987, IBM announced the Personal System/2 (PS/2) line, which they marketed as
1196-434: The AT was seen as an unusual move for the company, which typically waited for competitors to release new products before producing its own models. At $ 4,000–6,000, it was only slightly more expensive than considerably slower IBM models. The announcement surprised rival executives, who admitted that matching IBM's prices would be difficult. No major competitor showed a comparable computer at COMDEX Las Vegas that year. The AT
1248-676: The Micro Channel architecture in October 1986, half a year before the introduction of the IBM PS/2, as part of their "Gearbox" Industrial Computer 7552 series. These computers were rack-mountable, ruggedized, modular industrial PCs . They featured a hybrid 16-bit MCA and ISA bus, with certain ISA signal lines disabled. The use of MCA in IBM spread to the RS/6000 , AS/400 , and eventually to
1300-446: The PC reading the old configuration from the floppy disk, which made necessary changes in software, then wrote the new configuration to the floppy disk. In practice, however, this meant that the user must keep that same floppy disk matched to that PC . For a small company with a few PCs, this was annoying, but practical. But for large organizations with hundreds or even thousands of PCs, permanently matching each PC with its own floppy disk
1352-464: The PC was upgraded to the 16450 , but since both chips had single-byte buffers, high-speed serial communication was problematic as with the XT. The IBM PC AT came with a 192-watt switching power supply , significantly higher than the 130-watt XT power supply. According to IBM's documentation, in order to function properly, the AT power supply needed a load of at least 7.0 amperes on the +5 V line and
1404-434: The PC, achieved by adding another 8259A IRQ controller and another 8237A DMA controller. Some IRQ and DMA channels are used by the motherboard and not exposed on the expansion bus. Both dual IRQ and DMA chipsets are cascading which shares the primary pair. In addition to these chipsets, Intel 82284 Clock Driver and Ready Interface and Intel 82288 Bus Controller are to support the microprocessor. The 24-bit address bus of
1456-432: The card's memory addressing and interrupts. Although MCA cards cost nearly double the price of comparable non-MCA cards, the marketing stressed that it was simple for any user to upgrade or add more cards to their PC, thus saving the considerable expense of a technician. In this critical area, Micro Channel architecture's biggest advantage was also its greatest disadvantage, and one of the major reasons for its demise. To add
1508-539: The computer to set the LED indicators on the keyboard, reset the keyboard, set the typematic rate, and other features. Later ATs included 101-key keyboards, e.g. the Model M keyboard . The AT is also equipped with a physical lock that prevents access to the computer by disabling the keyboard and holding the system unit's cover in place. ATs could be equipped with CGA , MDA , EGA , or PGA video cards. The 8250 UART from
1560-402: The consortium still held meetings and maintained a catalog of MCA devices online. A number of non-PS/2 computers were manufactured between the late 1980s and early 1990s. Such third-party computers were also referred to as PS/2 clones or MCA clones . The first third-party Micro Channel–based computer was Tandy Corporation 's 5000 MC in 1988. Despite expensive research and development costs on
1612-405: The data rate to 20 MHz, and the throughput to 80 MB/s. Some higher throughput functions of the Micro Channel bus were available to RS/6000 platform only, and were not initially supported on cards operating on an Intel platform. With bus mastering , each card could talk to another directly. This allowed performance that was independent of the CPU. One potential drawback of multi-master design
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1664-506: The fact that MCA was a huge technical improvement over ISA, it soon became clear that its introduction and marketing by IBM was poorly handled. IBM had strong patents on Micro Channel architecture system features, and required Micro Channel system manufacturers to pay a licence fee - and actively pursued patents to block third parties from selling unlicensed implementations of it. The PC clone market did not want to pay royalties to IBM in order to use this new technology, and stayed largely with
1716-535: The form of the PS/2 Model 30 , the AT did not. Users either had to forgo all their 16-bit ISA expansion cards and switch to the proprietary Micro Channel architecture , or settle for a clone if they wanted to upgrade their machine while keeping their expansions. Eventually, in September 1988, IBM announced the PS/2 Model 30 286 , which featured an Intel 80286 processor and 16-bit ISA expansion slots, serving as
1768-523: The means of introducing Micro Channel to the general public) in 1992, NCR Corporation remained the one of a few remaining committed vendors of MCA clones. Expansion cards for the Micro Channel bus typically targeted high-end graphic workstation or server requirements, with SCSI , Token Ring , Ethernet , IBM 5250 and IBM 3270 connections. Very few MCA sound cards were ever produced. Some examples include: IBM AT The IBM Personal Computer AT (model 5170, abbreviated as IBM AT or PC/AT )
1820-487: The new IRQ for a new device—if a suitable one was available—for ISA was no fun at all, and beyond many users... it is obvious why the attempt was made to move to software-arbitrated configuration, and why this was to later succeed in the form of PnP .) In November 1983 The Economist stated that the IBM PC standard's dominance of the personal computer market was not a problem because "it can help competition to flourish". The magazine predicted that IBM will soon be as much
1872-452: The on-board graphics system when needed, and allowing a single system board connector for graphics that could be upgraded. Micro Channel cards also featured a unique, 16-bit software-readable ID, which formed the basis of an early plug and play system. The BIOS and/or OS can read IDs, compare against a list of known cards, and perform automatic system configuration to suit. This led to boot failures whereby an older BIOS would not recognize
1924-410: The part of third-party manufacturers of Micro Channel computers—in part due to the expensive licensing fees incurred by IBM in order to allow legal use of the Micro Channel technology—by 1990 most MCA clones were not fully compatible with the Micro Channel architecture or expansion cards based on Micro Channel. By the time IBM was winding down the PS/2 line of personal computers (which in 1987 acted as
1976-441: The problem by manual intervention was much more difficult than configuring an ISA system, not least because the documentation for the MCA device would tend to assume that the automatic configuration would work and so did not provide the necessary information to set it up by hand, unlike ISA device documentation which by necessity provided full details (however having to physically remove and check all IRQ settings, then find and set
2028-528: The same type of power supply, case, and motherboard layout as the 5170. AT-class became a term describing any machine which supported the same BIOS functions, 80286 or greater processor, 16-bit expansion slots, keyboard interface, 1.2 MB 5 + 1 ⁄ 4 inch floppy disk drives and other defining technical features of the IBM PC AT. In the United States, popular brands of AT clones included
2080-565: The second-generation of IBM PC. The company promised to continue manufacturing certain models of the first-generation PC, including the AT, for the coming months. In June 1987, they announced the full withdrawal of the PC/XT and the imminent discontinuation of the PC/AT. The last units of PC/AT (model 339) rolled off the assembly line in July. While the PC/XT received a directly compatible replacement in
2132-462: The system bus. Micro Channel bus-master-capable devices can even use the bus to talk directly to each other ( peer-to-peer ) at speeds faster than the system CPU, without any other system intervention. In theory, Micro Channel architecture systems could be expanded, like mainframes , with only the addition of intelligent masters, without periodic need to upgrade the central processor. Arbitration enhancement ensures better system throughput since control
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2184-542: The system's CPU. Likewise, bus master request and grant signals were public, such that bus attached devices could monitor latency to control internal buffering for I/O processors. These features were not adopted for PCI, requiring all I/O support to come uniquely from the system board processor. The final major Micro Channel architecture improvement was POS , the Programmable Option Select , which allowed all setup to take place in software. This feature
2236-413: The twin advantage of being able to accept older ISA boards and being free from IBM's control. For several years EISA and Micro Channel battled it out in the server arena, but, in 1996, IBM effectively conceded defeat, when they themselves produced some EISA-bus servers. In 2001 IBM executive Robert Moffat said that of the company's mistakes in the PC market, "the most obvious one is Micro Channel". Within
2288-439: The unreliable hard disk drive, the high-density floppy disk drives turned out to be problematic. Some ATs came with one high-density (HD) disk drive and one double-density (DD) 360 KB drive. High-density floppy diskette media were compatible only with high-density drives. There was no way for the disk drive to detect what kind of floppy disk was inserted, and the drives were not distinguished except by an asterisk molded into
2340-448: Was already investigating the use of RISC processors in desktop machines, and could, in theory, save considerable money if a single well-documented bus could be used across their entire computer lineup. It was thought that by creating a new standard, IBM would regain control of standards via the required licensing. As patents can take three years or more to be granted, however, only those relating to ISA could be licensed when Micro Channel
2392-425: Was announced. Patents on important Micro Channel features, such as Plug and Play automatic configuration, were not granted to IBM until after PCI had replaced Micro Channel in the marketplace. The overall reception was tepid and the impact of Micro Channel in the worldwide PC market was minor. The Micro Channel architecture was designed by engineer Chet Heath. A lot of the Micro Channel cards that were developed used
2444-475: Was included for graphics cards . This extension was used for analog output from the video card, which was then routed through the system board to the system's own monitor output. The advantage of this was that Micro Channel system boards could have a basic VGA or MCGA graphics system on board, and higher-level graphics ( XGA or other accelerator cards) could then share the same port. The add-on cards were then able to be free of ' legacy ' VGA modes, making use of
2496-440: Was logistically unlikely or impossible. Without the original, updated floppy disk, no changes could be made to the PC's cards. After this experience repeated itself thousands of times, business leaders realized their dream scenario for upgrade simplicity did not work in the corporate world, and they sought a better process. The basic data rate of the Micro Channel was increased from ISA's 8 MHz to 10 MHz. This may have been
2548-570: Was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 80286 microprocessor . IBM did not specify an expanded form of AT on the machine, press releases, brochures or documentation, but some sources expand the term as Advanced Technology , including at least one internal IBM document. IBM 's 1984 introduction of
2600-550: Was the possible collisions when more than one card would try to bus master, but Micro Channel included an arbitration feature to correct for these situations, and also allowed a master to use a burst-mode . Micro Channel cards had complete control for up to 12 milliseconds . This was long enough to permit the maximum number of other devices on the bus to buffer inbound data from over-runnable devices like tape and communications. Multiple bus-master support and improved arbitration mean that several such devices could coexist and share
2652-463: Was updated and standardized as ATA ("AT Attachment") by Western Digital and Compaq in 1986, and later renamed PATA ( Parallel AT Attachment ). The ATA interface was also known as IDE, because the drive controller, instead of being on the interface card, was integrated into the drive ( Integrated Drive Electronics ). As of January 1985 AT sales were so strong that IBM and its suppliers could not keep up with demand. Creative Computing chose
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#17327807076432704-523: Was upgraded to a 1.2 MB 5 + 1 ⁄ 4 inch floppy disk drive (15 sectors of 512 bytes, 80 tracks, two sides), which stored over three times as much data as the 360 KB PC floppy disk, but had compatibility problems with 360k disks (see Problems below). 3 + 1 ⁄ 2 inch floppy drives became available in later ATs. A 20 MB hard disk drive was included as standard. Early drives were manufactured by Computer Memories and were found to be very unreliable. The AT included
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