IBM Personal System/2

With certain models to the IBM PS/2 line, Micro Channel Architecture (MCA) was also introduced.[4] 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[5] 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 BIOS management of hardware were all benefits of MCA. (One book from the year 2000 writes: "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."[6] MCA never gained wide acceptance outside of the PS/2 line due to IBM's anti-clone practices and incompatibilities with ISA. IBM offered to sell an MCA license to anyone who could afford the royalty. However, royalties were required for every MCA-compatible machine sold and a payment for every IBM-compatible machine the particular maker had made in the past. There was nothing unique in IBM insisting on payment of royalties on the use of its patents applied to Micro Channel-based machines. Up until that time, some companies had failed to pay IBM for the use of its patents on the earlier generation of Personal Computer.

The PS/2 IBM Model M keyboard used the same 101-key layout of the previous IBM PC/AT Extended keyboard, itself derived from the original IBM PC keyboard.[4] European variants had 102 keys with the addition of an extra key to the right of the left Shift key. The Model M, using a buckling spring mechanism, is still being manufactured by Unicomp.

The original IBM PS/2 mouse

PS/2 connection ports (later colored purple for keyboard and green for mouse, according to PC 97) were once commonly used for connecting input devices.

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 70 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.

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.[4] VGA increased graphics memory to 256 KB and provided for resolutions of 640×480 with 16 colors, and 320×200 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:

• 8504: 12″, 640×480, 60 Hz non-interlaced, 1991, monochrome
• 8507: 19″, 1024×768, 43.5 Hz interlaced, 1988, monochrome
• 8511: 14″, 640×480, 60 Hz non-interlaced, 1987
• 8512: 14″, 640×480, 60 Hz non-interlaced, 1987
• 8513: 12″, 640×480, 60 Hz non-interlaced, 1987
• 8514: 16″, 1024×768, 43.5 Hz interlaced, 1987
• 8515: 14″, 1024×768, 43.5 Hz interlaced, 1991
• 8516: 14″, 1024×768, 43.5 Hz interlaced, 1991
• 8518: 14″, 640×480, 75 Hz non-interlaced, 1992
• 9515: 14″, 1024×768, 43.5 Hz interlaced, 1992
• 9517: 16″, 1280×1024, 53 Hz interlaced, 1991
• 9518: 14″, 640×480, non-interlaced, 1992
• 38F4737: 10", 640×480, non-interlaced, 1989, amber monochrome plasma screen; this display was exclusive to models P70 and P75

In truth, all "XGA" 1024×768 monitors are multimode, as XGA works as an add-on card to a built-in VGA and transparently passes-thru the VGA signal when not operating in a high resolution mode. All of the listed 85xx displays can therefore sync 640×480 at 60 Hz (or 720×400 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 640×480 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 640×480/720×400 and 1024×768, an (e.g.) 8514 cannot sync the otherwise common intermediate 800×600 "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 1024×768 pixel resolution was subsequently widely adopted as a standard by other manufacturers, and "XGA" became a synonym for this screen resolution. The lone exception 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 320x200x256 and 640x480x2 mode, but not EGA or color 640x480.

MCA IBM XGA-2 Graphics Card

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 (like 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.

Some PS/2 models used a quick-attachment socket on the back of the floppy drive which is incompatible with a standard 5.25" floppy connector.

Close-up of unusual 72-pin MCA internal hard drive connector.

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.[7] However IBM made available optional external 5.25" drives, with internal adapters for the early PS/2 models, to enable data transfer.

3.5" DD and HD floppies

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 identify 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.[8]

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 by using the optional SCSI interface.

Later PS/2 models introduced the 72-pin SIMM[9] which became the de facto standard for RAM modules by the mid-1990s in mid-to-late 486 and nearly all Pentium desktop systems. 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). 72-pin SIMMs were also made with greater capacities (starting at 1mb and ultimately reaching 128mb, vs 256kb to 16mb and more commonly no more than 4mb for 30-pin) and in a more finely graduated range (powers of 2, instead of powers of 4).

Many PS/2 models also used proprietary IBM SIMMs and could not be fitted with commonly available types. However industry standard SIMMs could be modified to work in PS/2 machines if the presence and type detection bridges or associated contacts were correctly rewired.

At launch, the PS/2 family comprised the Model 30, 50, 60 and 80;[4] the Model 25 was launched a few months later.

The PS/2 Models 25 and 30 (IBM 8525 and 8530, respectively) were the lowest-end models in the lineup and meant to replace the IBM PC and XT. Model 25s came with either an 8086 CPU running at 8 MHz, 512 KB of RAM, and 720 KB floppy disks, or 80286 CPU. The 8086s had ISA expansion slots and a built-in MCGA monitor, which could be either color or monochrome, while the 80286 models came with VGA monitor and ISA expansion slots. A cut-down Model M with no numeric keypad was standard, with the normal keyboard being an extra-cost option. There was a very rare later model called the PS/2 Model 25-SX which sported either a 16 MHz or 20 MHz 386 CPU, up to 12 MB of memory, IDE hard drive, VGA Monitor and 16 bit ISA slots making it the highest available model 25 available denoted by model number 8525-L41.

Case badge on model 25-SX 386-20 8525-L41

The Model 30 had either an 8086 or 286 CPU and sported the full 101-key keyboard and standalone monitor along with three 8-bit ISA expansion slots. 8086 models had 720 KB floppies while 286 models had 1440 KB ones. Both the Model 25 and 30 could have an optional 20 MB ST-506 hard disk (which in the Model 25 took the place of the second floppy drive if so equipped and used a proprietary 3.5" form factor). 286-based Model 30s are otherwise a full AT-class machine and support up to 4 MB of RAM.

IBM Personal System/2 Model 25

Later ISA PS/2 models comprised the Model 30-286 (a Model 30 with an Intel 286 CPU), Model 35 (IBM 8535) and Model 40 (IBM 8540) with Intel 386SX or IBM 386SLC processors.

The higher-numbered models (above 50) were equipped with the Micro Channel bus and mostly ESDI or SCSI hard drives (models 60-041 and 80-041 had MFM hard drives). PS/2 Models 50 (IBM 8550) and 60 (IBM 8560) used the Intel 286 processor, the PS/2 Models 70 (IBM 8570) and 80 used the 386DX, while the mid-range PS/2 Model 55SX (IBM 8555-081) and used the 16/32-bit 386SX processor. The Model 50 was revised to the Model 50Z still with 10MHz 80286 processor, but with memory run at zero wait state, and a switch to ESDI hard drives. Later Model 70 and 80 variants (B-xx) also used 25 MHz Intel 486 processors, in a complex called the Power Platform.

The externally very similar Models 60 and 80 next to each other

IBM Model 70 (case open over case closed)

The PS/2 Models 90 (IBM 8590/9590) and 95 (IBM 8595/9595/9595A) used Processor Complex daughterboards holding the CPU, memory controller, MCA interface, and other system components. The available Processor Complex options ranged from the 20 MHz Intel 486 to the 90 MHz Pentium and were fully interchangeable. The IBM PC Server 500, which has a motherboard identical to the 9595A, also uses Processor Complexes.

Other later Micro Channel PS/2 models included the Model 65SX with a 16 MHz 386SX; various Model 53 (IBM 9553), 56 (IBM 8556) and 57 (IBM 8557) variants with 386SX, 386SLC or 486SLC2 processors; the Models 76 and 77 (IBM 9576/9577) with 486SX or 486DX2 processors respectively; and the 486-based Model 85 (IBM 9585).

The IBM PS/2E (IBM 9533) was the first Energy Star compliant personal computer. It had a 50 MHz IBM 486SLC processor, an ISA bus, four PC card slots, and an IDE hard drive interface. The environmentally friendly PC borrowed many components from the ThinkPad line and was composed of recycled plastics, designed to be easily recycled at the end of its life, and used very little power.

IBM also produced several portable and laptop PS/2s, including the Model L40 (ISA-bus 386SX), N33 (IBM's first notebook-format computer from year 1991, Model 8533, 386SX), N51 (386SX/SLC), P70 (386DX) and P75 (486DX2).

The IBM PS/2 Server 195 and 295 (IBM 8600) were 486-based dual-bus MCA network servers supporting asymmetric multiprocessing, designed by Parallan Computer Inc.

The IBM PC Server 720 (IBM 8642) was the largest MCA-based server made by IBM, although it was not, strictly speaking, a PS/2 model. It could be fitted with up to six Intel Pentium processors interconnected by the Corollary C-bus and up to eighteen SCSI hard disks. This model was equipped with seven combination MCA/PCI slots.

The IBM ThinkPad 700C, aside from being labeled "700C PS/2" on the case, featured MCA and a 486SLC CPU.

The 6152 Academic System was a workstation computer developed by IBM's Academic Information Systems (ACIS) division for the university market introduced in February 1988. The 6152 was based on the PS/2 Model 60, adding a RISC Adapter Card on the Micro Channel bus. This card was a co-processor that enabled the 6152 to run ROMP software compiled for IBM's Academic Operating System (AOS), a version of BSD UNIX for the ROMP that was only available to select colleges and universities.[10]

The RISC Adapter Card contained the ROMP-C microprocessor (an enhanced version of the ROMP that first appeared in the IBM RT PC workstations), a memory management unit (the ROMP had virtual memory), a floating-point coprocessor, and up to 8 MB of memory for use by the ROMP.[11] The 6152 was the first computer to use the ROMP-C, which would later be introduced in new RT PC models.[12]

• Burton, Greg. IBM PC and PS/2 pocket reference. NDD (the old dealer channel), 1991.
• Byers, T.J. IBM PS/2: A Reference Guide. Intertext Publications, 1989. ISBN 0-07-009525-6.
• Dalton, Richard and Mueller, Scott. IBM PS/2 Handbook . Que Publications, 1989. ISBN 0-88022-334-0.
• Held, Gilbert. IBM PS/2: User's Reference Manual. John Wiley & Sons Inc., 1989. ISBN 0-471-62150-1.
• Hoskins, Jim. IBM PS/2. John Wiley & Sons Inc., fifth revised edition, 1992. ISBN 0-471-55195-3.
• Leghart, Paul M. The IBM PS/2 in-depth report. Pachogue, NY: Computer Technology Research Corporation, 1988.
• Newcom, Kerry. A Closer Look at IBM PS/2 Microchannel Architecture. New York: McGraw-Hill, 1988.
• Norton, Peter. Inside the IBM PC and PS/2. Brady Publishing, fourth edition 1991. ISBN 0-13-465634-2.
• Outside the IBM PC and PS/2: Access to New Technology. Brady Publishing, 1992. ISBN 0-13-643586-6.
• Shanley, Tom. IBM PS/2 from the Inside Out. Addison-Wesley, 1991. ISBN 0-201-57056-4.