Second generation of personal computers by IBM
The Personal System/2 or PS/2 is IBM's second generation of personal computers. Released in 1987, it officially replaced the IBM PC, XT, AT, and PC Convertible in IBM's lineup. Many of the PS/2's innovations, such as the 16550 UART (serial port), 1440 KB 3.5-inch floppy disk format, 72-pin SIMMs, PS/2 port, and VGA video standard, went on to become standards in the broader PC market.
The PS/2 line was created by IBM partly in an attempt to recapture control of the PC market by introducing the advanced yet proprietary Micro Channel architecture (MCA) on higher-end models. These models were in the strange position of being incompatible with the hardware standards previously established by IBM and adopted in the IBM PC compatible industry. Most major PC manufacturers balked at IBM's licensing terms for MCA-compatible hardware, particularly the per-machine royalties. The OS/2 operating system was announced at the same time as the PS/2 line and was intended to be the primary operating system for models with Intel 80286 or later processors. However, at the time of the first shipments, only IBM PC DOS 3.3 was available. OS/2 1.0 (text-mode only) and Microsoft's Windows 2.0 became available several months later. IBM also released AIX PS/2, a UNIX operating system for PS/2 models with Intel 386 or later processors.
IBM's initial PS/2 computers were popular with target market corporate buyers, and by September 1988, IBM reported that it had sold 3 million PS/2 machines in the past 18 months. However, the PS/2 was unsuccessful in the consumer market since IBM failed to establish a link in the consumer's mind between the PS/2 MicroChannel architecture and the immature OS/2 1.x operating system (the more capable OS/2 version 2.0 was not released until 1992) to justify the PS/2's price premium, in contrast to rival IBM PC compatibles that stuck with industry-wide standard hardware while running Microsoft Windows. Rival manufacturers also teamed up to form the EISA bus standard in opposition to the Micro Channel. In 1992, "Macworld" stated that "IBM lost control of its own market and became a minor player with its own technology." IBM officially retired the PS/2 line in July 1995.
Technology.
IBM's PS/2 was designed to remain software compatible with their PC/AT/XT line of computers upon which the large PC clone market was built, but the hardware was quite different. PS/2 had two BIOSes: one named ABIOS (Advanced BIOS) which provided a new protected mode interface and was used by OS/2, and CBIOS (Compatible BIOS) which was included to be software compatible with the PC/XT/AT. CBIOS was so compatible that it even included Cassette BASIC. While IBM did not publish the BIOS source code, it did promise to publish BIOS entry points.
Micro Channel architecture.
With certain models to the IBM PS/2 line, Micro Channel Architecture (MCA) was also introduced. MCA was conceptually similar to the channel architecture of the IBM System/360 mainframes. MCA was technically superior to ISA and allowed for higher-speed communications within the system. The majority of MCA's features would be seen in later buses with the exception of: streaming-data procedure, channel-check reporting, error logging and internal bus-level video pass-through for devices like the IBM 8514. Transfer speeds were on par with the much later PCI standard. MCA allowed one-to-one, card-to-card, and multi-card to processor simultaneous transaction management which is a feature of the PCI-X bus format.
Bus mastering capability, bus arbitration, and a primitive form of plug-and-play management of hardware were all benefits of MCA. Gilbert Held in his 2000 book "Server Management" observes: "MCA used an early (and user-hostile) version of what we know now as 'Plug-N′-Play', requiring a special setup disk for each machine and each card." MCA never gained wide acceptance outside of the PS/2.
When setting up the card with its disk, all choices for interrupts and other changes were accomplished automatically by the PC reading the old configuration from the floppy disk. This made necessary changes, then recorded the new configuration to the floppy disk. This meant that the user must keep that same floppy disk matched to that particular PC. For a small organization with a few PCs, this was annoying, but less expensive and time-consuming than bringing in a PC technician to do installation. But for large organizations with hundreds or even thousands of PCs, permanently matching each PC with its own floppy disk was a logistical nightmare. Without the original, (and correctly updated) floppy disk, no changes could be made to the PC's cards.
Keyboard/mouse.
Layout.
The PS/2 IBM Model M keyboard used the same 101-key layout of the previous IBM PC/AT Extended keyboard. European variants had 102 keys with the addition of an extra key to the right of the left Shift key.
Interface.
PS/2 systems introduced a new specification for the keyboard and mouse interfaces, which are still in use today (though increasingly supplanted by USB devices) and are thus called "PS/2" interfaces. The PS/2 keyboard interface, inspired by Apple's ADB interface, was electronically identical to the long-established AT interface, but the cable connector was changed from the 5-pin DIN connector to the smaller 6-pin mini-DIN interface. The same connector and a similar synchronous serial interface was used for the PS/2 mouse port.
The initial desktop "Model 50" and "Model 60" also featured a new cableless internal design, based on use of interposer circuit boards to link the internal drives to the planar (motherboard). Additionally, these machines could be largely disassembled and reassembled for service without tools.
Additionally, the PS/2 introduced a new software data area known as the Extended BIOS Data Area (EBDA). Its primary use was to add a new buffer area for the dedicated mouse port. This also required making a change to the "traditional" BIOS Data Area (BDA) which was then required to point to the base address of the EBDA.
Another new PS/2 innovation was the introduction of bidirectional parallel ports which, in addition to their traditional use for connecting a printer, could now function as a high-speed data transfer interface. This allowed the use of new hardware such as parallel port scanners, CD-ROM drives, and also enhanced the capabilities of printers by allowing them to communicate with the host PC and send back signals instead of simply being a passive output device.
Graphics.
Most of the initial range of PS/2 models were equipped with a new frame buffer known as the Video Graphics Array, or VGA for short. This effectively replaced the previous EGA standard. VGA increased graphics memory to 256 KB and provided for resolutions of 640×480 with 16 colors, and with 256 colors. VGA also provided a palette of 262,144 colors (as opposed to the EGA palette of 64 colors). The IBM 8514 and later XGA computer display standards were also introduced on the PS/2 line.
Key monitors and their maximum resolutions:
In truth, all XGA monitors are multimode, as XGA works as an add-on card to a built-in VGA and transparently passes the VGA signal through when not operating in a high-resolution mode. All of the listed 85xx displays can therefore sync 640×480 at 60 Hz (or at 70 Hz) in addition to any higher mode they may also be capable of. This however is not true of the 95xx models (and some unlisted 85xx's), which are specialist workstation displays designed for use with the XGA-2 or Image Adapter/A cards, and whose fixed frequencies all exceed that of basic VGA – the lowest of their commonly available modes instead being at 75 Hz, if not something much higher still. It is also worth noting that these were still merely dual- or "multiple-frequency" monitors, not variable-frequency (also known as multisync); in particular, despite running happily at , and , an (e.g.) 8514 cannot sync the otherwise common intermediate SVGA resolution, even at the relatively low 50 to 56 Hz refresh rates initially used.
Although the design of these adapters did not become an industry standard as VGA did, their pixel resolution was subsequently widely adopted as a standard by other manufacturers, and XGA became a synonym for this screen resolution. The only exceptions were the bottom-rung 8086-based Model 25 and 30, which had a cut-down version of VGA referred to as MCGA; the 286 models came with VGA. This supported CGA graphics modes, VGA 256 color and monochrome mode, but not EGA or color .
VGA video connector.
All of the new PS/2 graphics systems (whether MCGA, VGA, 8514, or later XGA) used a 15-pin D-sub connector for video out. This used analog RGB signals, rather than four or six digital color signals as on previous CGA and EGA monitors. The digital signals limited the color gamut to a fixed 16- or 64-color palette with no room for expansion. In contrast, any color depth (bits per primary) can be encoded into the analog RGB signals so the color gamut can be increased arbitrarily by using wider (more bits per sample) DACs and a more sensitive monitor. The connector was also compatible with analog grayscale displays. Unlike earlier systems such as MDA and Hercules, this was transparent to software, so all programs supporting the new standards could run unmodified whichever type of display was attached. On the other hand, whether the display was color or monochrome was undetectable to software, so selection between application displays optimized for color or monochrome, in applications that supported both, required user intervention. These grayscale displays were relatively inexpensive during the first few years the PS/2 was available, and they were very commonly purchased with lower-end models.
The VGA connector became the de facto standard for connecting monitors and projectors on both PC and non-PC hardware over the course of the early 1990s, replacing a variety of earlier connectors.
Storage.
Apple had first popularized the 3.5" floppy on the Macintosh line and IBM brought them to the PC in 1986 with the PC Convertible. In addition, they could be had as an optional feature on the XT and AT. The PS/2 line used entirely 3.5" drives which assisted in their quick adoption by the industry, although the lack of 5.25" drive bays in the computers created problems later on in the 1990s as they could not accommodate internal CD-ROM drives. In addition, the lack of built-in 5.25" floppy drives meant that PS/2 users could not immediately run the large body of existing IBM-compatible software. However IBM made available optional external 5.25" drives, with internal adapters for the early PS/2 models, to enable data transfer.
In the initial lineup, IBM used 720 KB double density (DD) capacity drives on the 8086-based models and 1440 KB high density (HD) on the 80286-based and higher models. By the end of the PS/2 line they had moved to a somewhat standardized capacity of 2880 KB.
The PS/2 floppy drives lacked a capacity detector. 1440 KB floppies had a hole so that drives could distinguish them from 720 KB floppies, preventing users from formatting the smaller capacity disks to the higher capacity (doing so would work, but with a higher tendency of data loss). Clone manufacturers implemented the hole detection, but IBM did not. As a result of this a 720 KB floppy could be formatted to 1440 KB in a PS/2, but the resulting floppy would only be readable by a PS/2 machine.
PS/2s primarily used Mitsubishi floppy drives and did not use a separate Molex power connector; the data cable also contained the power supply lines. As the hardware aged the drives often malfunctioned due to bad quality capacitors.
The PS/2 used several different types of internal hard drives. Early models used MFM or ESDI drives. Some desktop models used combo power/data cables similar to the floppy drives. Later models used DBA ESDI or Parallel SCSI. Typically, desktop PS/2 models only permitted use of one hard drive inside the computer case. Additional storage could be attached externally using the optional SCSI interface.
Memory.
Later PS/2 models introduced the 72-pin SIMM which became the de facto standard for RAM modules by the mid-1990s in mid-to-late 486 and nearly all Pentium desktop systems. The 72-pin SIMMs were 32/36 bits wide and replaced the old 30-pin SIMM (8/9-bit) standard. The older SIMMs were much less convenient because they had to be installed in sets of two or four to match the width of the CPU's 16-bit (Intel 80286 and 80386SX) or 32-bit (80386 and 80486) data bus, and would have been extremely inconvenient to use in Pentium systems (which featured a 64-bit memory bus). The 72-pin SIMMs were also made with greater capacities (starting at 1 MB and