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IBM PS/2 Model 25

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IBM Personal System/2 Model 25
IBM PS/2 Model 25 (8525)
Specifications
DeveloperIBM Entry Systems Division, Boca Raton
ManufacturerIBM
TypeAll-in-one personal computer
ReleasedAugust 4, 1987 (Model 25 8086); May 1990 (Model 25 286); April 1992 (Model 25 SX)
DiscontinuedNovember 17, 1992 (Model 25 286)
CPUIntel 8086 @ 8 MHz (Model 25) / Intel 80286 @ 10 MHz, 1 wait state (Model 25 286) / Intel 80386SX @ 16 or 20 MHz (Model 25 SX)
Memory512 KB – 640 KB (Model 25); 1 MB – 4 MB (Model 25 286); 1 MB – 16 MB (Model 25 SX)
StorageOne or two 3.5" 720 KB diskette drives (Model 25); one 1.44 MB diskette drive + optional 30 MB ST-506 MFM hard drive (Model 25 286); 1.44 MB diskette + IDE hard drive (Model 25 SX)
DisplayIntegrated 12" CRT (mono or colour). MCGA (Model 25), VGA (Model 25 286), Super VGA (Model 25 SX)
SoundPC speaker; 1/4" mono earphone jack on rear of system unit
Dimensions382 H × 320 W × 375 D mm (System unit + integrated display)
Weight12.7 kg (28 lb) monochrome (001); 16.8 kg (37 lb) colour (004)
OS / FirmwareIBM PC DOS 3.3 (Model 25); IBM PC DOS 3.3 / 4.0 / OS/2 EE 1.0/1.2 (Model 25 286); IBM PC DOS 5.0 (Model 25 SX)
Predecessornone (entry-level PS/2 replaces the IBM PC (5150)/XT in the IBM range)
SuccessorIBM PS/1 series (consumer market); IBM PS/ValuePoint
Model no.8525

The IBM Personal System/2 Model 25 (machine type 8525) is the lowest-end entry in the IBM Personal System/2 (PS/2) family of personal computers. It was announced on August 4, 1987 as the fifth PS/2 model, four months after IBM unveiled the rest of the PS/2 line on April 2, 1987. The Model 25 was IBM's first PS/2 sold into the home and education market and was the only PS/2 with an integrated CRT in a Macintosh-style all-in-one case. Unlike most other PS/2 models, the Model 25 uses ISA expansion slots (not Micro Channel), making it electrically compatible with PC/XT and PC/AT expansion cards but not with the higher PS/2 models.

The Model 25 was sold in three CPU generations:

  • Model 25 (8086 at 8 MHz, MCGA video, 8-bit ISA, August 1987–1990)
  • Model 25 286 (80286 at 10 MHz, VGA, 16-bit ISA, May 1990–November 1992)
  • Model 25 SX (80386SX at 16 or 20 MHz, Super VGA, 16-bit ISA, K–12 education market only, April 1992–)

All three generations share the all-in-one chassis, the 12" CRT, and the PS/2-family proprietary floppy and hard-disk power and data connectors. The Model 25 was never officially sold outside the United States in its original form, although Canadian-French keyboard variants (suffixed -F0x) were available, and the Collegiate variant was bundled with PC DOS 3.30 and Microsoft Windows 1.04.

History

IBM announced the PS/2 line on April 2, 1987 with the Model 30 (8086, ISA), Model 50 (80286, MCA), Model 60 (80286, MCA) and Model 80 (80386, MCA). The Model 25 followed on August 4, 1987 as a price-competitive entry aimed at home-office workers and students. Contemporary reviewers compared it favourably to Apple's original Macintosh because of its single-case form factor, although they criticised its lack of a standard hard drive.

The original announcement letter described the Model 25 as a "low price general-purpose computer system which provides enhanced function beyond that established by the IBM Personal Computer," and it was positioned as a direct retail replacement for the IBM PC (5150) and IBM PC XT (5160).

A Collegiate variant (8525-C02/C05/K02/K05) shipped with two floppy drives, 640 KB RAM, the IBM PS/2 Mouse, DOS 3.30 and Microsoft Windows 1.04. A Model 25 LS (8525-L01/L04) shipped with a Token Ring network card pre-installed.

The Model 25 286 was announced in May 1990 with a 80286 at 10 MHz, VGA video, a 1.44 MB diskette drive and (on the -036/-G36 submodels) a 30 MB ST-506 MFM hard drive. It was withdrawn from the IBM catalogue on November 17, 1992. The Model 25 SX was developed by IBM Boca Raton's team led by José García and sold only into K–12 schools from April 1992; it was the only Model 25 with an IDE hard drive interface.

Submodels

The following list of Model 25 submodels is taken from the IBM Personal Computer Product Reference (Version 4.0, September 1988) and the IBM Canada Quick Reference.

Selected IBM PS/2 Model 25 submodels
Submodel CPU Clock Bus RAM (max) FDD HDD Monitor Keyboard Notes
8525-001 8086 8 MHz (0 ws) ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" mono Space-Saving US $2,095 list
8525-G01 8086 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" mono Enhanced 101-key
8525-F01 8086 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" mono Cdn-French SSK
8525-004 8086 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" colour Space-Saving US $2,595 list
8525-G04 8086 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" colour Enhanced 101-key
8525-C02 / -K02 8086 8 MHz ISA 8-bit 640 KB 2 × 720 KB none 12" mono varies Collegiate kit
8525-C05 / -K05 8086 8 MHz ISA 8-bit 640 KB 2 × 720 KB none 12" colour varies Collegiate kit
8525-L01 / -L04 8086 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" mono/colour varies Token Ring (LS)
8525-006 80286 10 MHz (1 ws) ISA 16-bit 512 KB (4 MB) 1 × 1.44 MB none 12" VGA colour Space-Saving US $2,295 list
8525-G06 80286 10 MHz ISA 16-bit 512 KB (4 MB) 1 × 1.44 MB none 12" VGA colour Enhanced 101-key
8525-036 80286 10 MHz ISA 16-bit 1 MB (4 MB) 1 × 1.44 MB 30 MB ST-506 12" VGA colour Space-Saving US $2,995 list
8525-G36 80286 10 MHz ISA 16-bit 1 MB (4 MB) 1 × 1.44 MB 30 MB ST-506 12" VGA colour Enhanced 101-key
8525-K00 386SX 16 MHz (underclocked) ISA 16-bit 1 MB (16 MB) 1 × 1.44 MB IDE (opt.) 12" SVGA colour Enhanced School market
8525-K01 386SX 16 MHz ISA 16-bit 4 MB (16 MB) 1 × 1.44 MB IDE (opt.) 12" SVGA colour Enhanced + Ethernet
8525-L02 386SX 16 MHz ISA 16-bit 4 MB (16 MB) 1 × 1.44 MB IDE (opt.) 12" SVGA colour Enhanced + Token Ring

Architecture

Model 25 (8086) planar

The Model 25 8086 planar uses an Intel 8086-2 CPU at 8 MHz, zero wait states, with a socket at ZM1 (sic — not Uxx, per the IBM planar silkscreen) for an optional Intel 8087 math coprocessor. The chip complement is:

  • Intel 8086-2 (CPU) at 8 MHz.
  • IBM 72X8202 / 72X8203 / 72X8206 — three IBM-fabricated PS/2 gate-array support chips (CPU/bus support, planar I/O glue, video).
  • Intel P8253-5 — programmable interval timer (system tick, refresh, speaker), at U8.
  • Seiko Epson SLA64030J2L — clock generator / chip-set support (U11).
  • Seiko Epson SLA6330J1C — chip-set support (U15).
  • NEC D765AC-2 — floppy disk controller, at U18.
  • Siemens 8237A — DMA controller, at U19. (One 8237A on the Model 25, unlike the AT's pair.)
  • National Semiconductor NS8250AV — UART for the serial port, at U27.
  • Inmos IMSG171P — video RAMDAC for the integrated MCGA.
  • ROM BIOS — two 32 KB EPROMs in sockets U13/U17 (also marked U10/U20 on some silkscreen revisions), 64 KB total, splitting CBIOS (Compatible BIOS for backward compatibility with PC DOS) and ABIOS (Advanced BIOS for OS/2).
  • Sony CXK5864BM-12L — 8 KB static RAM, used for video character generator and BIOS work area, at U38.
  • DRAM: NEC D4164C-10 chips (×4) at U16/U33/U34/U35 plus OKI M3764A-15 chips at U44 and ZM1, giving 512 KB on-planar with parity. Expansion uses an IBM-pinout 30-pin DIP/SMK RAM module for 128 KB to reach 640 KB.
  • Crystals: Y1 = 14.318 18 MHz (software clock); Y2 = 25.175 MHz (video pixel clock); Y3 = 48.0 MHz (CPU/system clock master, divided by 6 for the 8086).
  • F1 fuse on the planar protects the keyboard/mouse +5 V rail.
  • The 8086 Model 25 has no hardware real-time clock — date and time must be set manually from DOS at each boot. (The Model 25 286 and 25 SX both have a hardware RTC backed by a battery.)

The Model 25 and the Model 30 share substantially the same planar layout; many sources confirm that an 8525 8086 planar will physically and electrically swap with a 8530 planar.

Model 25 286 planar

The Model 25 286 planar uses an Intel 80286 at 10 MHz with 1 wait state to system memory and a VLSI VL82C100 / VL82C101 / VL82C102 / VL82C103 / VL82C104 chipset (the "Popular AT" PC/AT-compatible chipset by VLSI). The system supports up to 4 MB of RAM on two 30-pin "MMK" parity SIMM sockets (J9/J10) with a proprietary IBM pinout, and includes a VGA graphics subsystem driven by the IBM 15F6864 VGA gate array (ZM18) and the Inmos IMSG171P-35 RAMDAC (ZM23).

Additional planar I/O on the Model 25 286:

  • VLSI VL16C451 — integrated UART + parallel port (ZM9).
  • NEC D765BC — floppy disk controller (ZM19).
  • Intel 8042 — keyboard controller (ZM33), supporting the PS/2 keyboard and pointing device (PS/2 mouse) ports.
  • 27F4094 DISKGA — IBM disk/diskette gate array (ZM14), supporting the proprietary PS/2 single-cable floppy interface and the optional ST-506 hard drive interface.
  • Crystals: Y1 = 14.318 MHz; Y2 = 25.175 MHz; Y3 = 20.322 MHz; Y4 = 20.0 MHz; Y5 = 48.0 MHz.
  • The 286 planar adds a 4-pin auxiliary power connector to feed the 16-bit ISA bus (not present on the 8086 Model 25), and a math coprocessor socket at ZM1 for an 80287.
  • Battery-backed CMOS via the MC146818-equivalent RTC integrated into the VLSI chipset.

Model 25 SX planar

The Model 25 SX planar uses an Intel 80386SX (typically a 386SX-16 or a 386SX-20 underclocked to 16 MHz) at U2, with an 80387SX socket at U5. The chipset is VLSI VL82C304 / VL82C305-FC. Additional features:

  • Intel 82077AA (U25) — floppy controller, supporting 1.44 MB diskettes natively.
  • IBM SVGA gate array 84F7985 (U42) with Toshiba TC511665BZ-80 video RAM at U43/U44/U51/U54, driven by an IMSG171P RAMDAC at U45.
  • Hitachi HM514900JP8 — system RAM, with additional 30-pin socket pads at U33/U32/U48/U49 populated on non-K (non-school) variants.
  • On-board IDE hard disk interface at J10 (the Model 25 and 25 286 use a proprietary ST-506 / PS/2 floppy interface; the 25 SX uses standard 40-pin IDE).
  • On the K01 and L02 submodels, a second flipside ISA slot on the riser card carries an Ethernet or Token Ring card aligned to a rear-panel cutout.

Memory

The Model 25 (8086) ships with 512 KB on-board and accepts a single 128 KB IBM memory module (IBM P/N 78X8955) at a proprietary 30-pin RAM socket to reach the architectural ceiling of 640 KB. The on-board DRAM is DIP 41464-class and OKI M3764-class chips with parity.

The Model 25 286 has two proprietary 30-pin MMK parity SIMM sockets (J9/J10). The MMK pinout is similar to the standard JEDEC 30-pin SIMM but not interchangeable: the IBM modules have different chip-select routing and require modules with the IBM-style edge connector. The largest supported configuration is 4 MB (2 × 2 MB).

The Model 25 SX has one 72-pin SIMM socket on the planar (in addition to soldered DRAM at U34 and optional sockets at U32/U33/U48/U49). The SIMM must be of an ECC parity type and must have the four presence-detect pins (67–70) configured for IBM size and speed encoding. Standard PC-style 72-pin parity SIMMs will not be recognised unless the presence-detect pins are tied to GND in the correct configuration.

Storage and Expansion

Expansion bus

  • Model 25 (8086): two 8-bit ISA expansion slots on a riser. The upper slot is partially obstructed by the CRT cage, preventing full-length cards from fitting; only the lower slot is fully usable for long cards.
  • Model 25 286: two 16-bit ISA slots on a riser, with an auxiliary 4-pin power connector for the 16-bit bus.
  • Model 25 SX: three 16-bit ISA slots on a riser, with a fourth (flipside) slot used by the optional network card on the K01/L02 submodels.

Floppy interface

The Model 25 uses the PS/2 single-cable floppy interface: power and data are carried in a single ribbon cable connecting the drive directly to the planar (no separate Molex 4-pin power lead). The interface is electrically different from PC/XT/AT 34-pin floppy interfaces. Drives are manufactured by ALPS, Mitsubishi, Sony, or YE-Data; Sony drives are the most likely to support 1.44 MB media on the original 8086 Model 25 even though it shipped only with 720 KB drives.

Hard drive interface

  • The 8086 Model 25 has no hard drive support from IBM at launch. After-market kits used Seagate ST-506 MFM drives in a 20 MB capacity, mounted in place of one of the floppy drives. The single-cable PS/2 interface includes the ST-506 lines.
  • The Model 25 286 supports a 30 MB ST-506 drive on the same single-cable interface (submodels 8525-036 / 8525-G36).
  • The Model 25 SX uses standard 40-pin IDE at J10, with hard drive power supplied through the planar.

Power supply

  • Model 25 (8086) monochrome (001, C02) — 90 W IBM PS/2 power supply.
  • Model 25 (8086) colour (004, C05) — 115 W IBM PS/2 power supply (higher current required by the colour CRT).
  • Both are 110/220 VAC switchable.
  • The PSU has no Molex 4-pin drive connectors; drive power is delivered through the planar via the same single-cable PS/2 interface as the data lines.
  • The Model 30 PSU is not a drop-in replacement for the Model 25 PSU — the planar connectors and pinouts differ.

Display

The Model 25 8086 uses MCGA (Multi-Color Graphics Array), the same video subsystem used on the Model 30 8086. MCGA is built into the planar and supports:

MCGA video modes (Model 25 8086)
Mode Resolution Colours Refresh Notes
0 / 1 40 × 25 text 16 70 Hz 8 × 16 character box
2 / 3 80 × 25 text 16 70 Hz 8 × 16 character box
4 / 5 320 × 200 4 70 Hz CGA-compatible
6 640 × 200 2 70 Hz CGA-compatible
0x11 640 × 480 2 60 Hz MCGA-specific (monochrome)
0x13 320 × 200 256 70 Hz MCGA-specific

The total displayable palette is 262,144 colours (18-bit). On a monochrome 12" CRT, colours are summed to 64 shades of grey by the BIOS. MCGA is analog-only (RGB-A on a 15-pin VGA-style connector); it will not drive a CGA TTL digital monitor, and digital displays are not supported.

The Model 25 286 and Model 25 SX have VGA (Model 25 286) or Super VGA (Model 25 SX), driven on the same integrated 12" CRT and exposed through a standard 15-pin VGA connector on the rear of the system unit.

Keyboard and Mouse

The Model 25 ships with either the IBM Space-Saving Keyboard (IBM P/N 1391472, 84-key Model M, no separate numeric keypad) or the IBM Enhanced Keyboard (IBM P/N 1391406, 101-key Model M). Both use the PS/2 keyboard protocol on a 6-pin Mini-DIN connector (the same connector used on PCs from 1996 onwards).

The Model 25 also introduces the IBM PS/2 Pointing Device port — the connector that became known as the "PS/2 mouse port." The keyboard and mouse ports on the 8525 are functionally identical at the planar level (both connect through the 8042 keyboard controller); IBM identifies them only by a raised keyboard icon over the keyboard port. The two are interchangeable in software, but plugging a mouse into the keyboard socket and vice versa will produce a 301 error at POST.

A 3 A fuse (F1 on the planar) protects the combined +5 V supply to the keyboard and pointing device.

ROM BIOS and Reference Diskette

The PS/2 line uses two parallel BIOS layers:

  • CBIOS (Compatible BIOS): backward-compatible with the PC/XT/AT BIOS, used by PC DOS.
  • ABIOS (Advanced BIOS): the PS/2 protected-mode / multitasking-aware BIOS, used by OS/2.

The Model 25 BIOS is held in two 32 KB EPROMs (64 KB total) in sockets U13/U17 (or U19/U20 depending on planar revision). Unlike the PC/AT, the Model 25 does not have IBM ROM BASIC in the BIOS — instead, a stub in the BIOS prompts for a boot diskette if none is found.

The Model 25 does not have a built-in SETUP utility. All configuration is performed from the IBM Personal System/2 Reference Diskette, a model-specific boot diskette supplied with the machine. The Reference Diskette includes:

  • SETUP — sets memory size, drive types, date/time (286/SX only), display, mouse, and adapter card definitions.
  • Advanced Diagnostics — full hardware test suite (entered with Ctrl-A from the main menu).
  • Backup utilities for copying the Reference Diskette to a working diskette.

Reference Diskettes are model-specific. A 5170 Diagnostics diskette will not work on a Model 25; a Model 30 Reference Diskette is also not interchangeable with the Model 25 Reference Diskette.

Cold boots of the Reference Diskette are recommended over warm (Ctrl-Alt-Del) boots, because a warm boot may produce false errors or falsely indicate that a power-on password is already set.

General Maintenance

CRT bezel removal, planar inspection, PSU rail checks, RTC battery replacement (Model 25 286 / 25 SX), keyboard / mouse port care and PS/2 single-cable connector care are documented in IBM PS/2 Model 25 Maintenance Guide.

Troubleshooting

Power-on audio beep codes, the complete 1xx through 25xx PS/2 numeric POST error code listing, the dead-system isolation procedure, Reference Diskette use, ECA 082 system board recall, keyboard and PS/2 mouse port diagnostics, MCGA / VGA CRT diagnostics and PSU fault isolation are documented in IBM PS/2 Model 25 Troubleshooting Guide.

Capacitor Replacement

Tantalum failure on the Model 25 planar and electrolytic ageing in the Model 25 PSU and integrated CRT subsystem are documented in IBM PS/2 Model 25 Capacitor Replacement Guide.

  • IBM PS/2 Model 25 (early Type 1 product photo)
    IBM PS/2 Model 25 (early Type 1 product photo)
  • IBM PS/2 Model 25 front view (Museo de Informática)
    IBM PS/2 Model 25 front view (Museo de Informática)
  • IBM PS/2 Model 25 angled view (Museo de Informática)
    IBM PS/2 Model 25 angled view (Museo de Informática)
  • IBM PS/2 Model 25 SX (Intel 386SX, sold to schools)
    IBM PS/2 Model 25 SX (Intel 386SX, sold to schools)
  • Front-panel badge from the 8086 Model 25
    Front-panel badge from the 8086 Model 25
  • Front-panel badge from the Model 25 286
    Front-panel badge from the Model 25 286

References

  • IBM PS/2 Model 25 — Wikipedia. Source for release/withdrawal dates, model line history, submodel list, MCGA/VGA video distinction.
  • IBM PS/2 Model 25 — DOS Days. Source for submodel pricing, planar IC layout, PSU wattage, hard drive options, BIOS layout (two 32 KB EPROMs in U19/U20), and proprietary memory pinouts.
  • IBM Canada Ltd Quick Reference — PS/2 Model 25 (8525). IBM-supplied technical reference: machine type, announce date, list price ($2,095 / $2,595), microprocessor and clock, BUS, ports, memory, PSU wattage and weight (12.7 kg mono / 16.8 kg colour), dimensions (382 × 320 × 375 mm), MCGA modes, configurations and options.
  • IBM Canada Ltd Quick Reference — PS/2 Model 25 286. Source for the Model 25 286 chipset (VLSI VL82C100–104), wait-state count, ISA 16-bit slot count, hard drive option, BIOS/RTC battery.
  • IBM, IBM Personal System/2 and IBM Personal Computer Product Reference, Version 4.0 (September 1988). Source for the submodel table.
  • PS/2 Error Codes, Ardent Tool of Capitalism. Reference for the PS/2 error code organisation (Major Error / Minor Diagnostic Error, the 4 + 4 format used on the Premium Line LED panel).


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