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* [[IBM PS/2 Model 30]] — the desktop sibling
* [[IBM PS/2 Model 30]] — the desktop sibling
* [[IBM PC XT (5160)]] · [[IBM PCjr]]
* [[IBM PC XT (5160)]] · [[IBM PCjr]]
* [[IBM PS2 Model 25 Technical Reference (June 1987)]] — IBM's own reference, hosted here
* '''IBM documentation hosted here:''' [[IBM PS2 Model 25 Technical Reference (June 1987)]] · [[IBM Personal System 2 Hardware Maintenance Manual (S52G-9971-02, October 1994)]]
* '''IBM documentation hosted here:''' [[IBM PS2 Model 25 Technical Reference (June 1987)]] · [[IBM Personal System 2 Hardware Maintenance Manual (S52G-9971-02, October 1994)]]


Latest revision as of 00:20, 25 September 2026

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
Released4 August 1987 (Model 25, 8086); May 1990 (Model 25 286); April 1992 (Model 25 SX)
Discontinued17 November 1992 (Model 25 286)
CPUIntel 8086-2 @ 8 MHz, 0 wait states / Intel 80286 @ 10 MHz, 1 wait state / Intel 386SX @ 16 MHz
Memory512–640 KB (8086); 512 KB–4 MB on board (286); up to 12 MB (SX, non-K planars)
StorageOne or two 3.5" diskette drives; optional IBM fixed disk drive with its controller on the drive; 40-pin IDE on the SX
DisplayIntegrated 12" CRT, monochrome or colour. MCGA (8086), VGA (286), IBM SVGA (SX)
SoundPC speaker; earphone jack
Dimensions382 × 320 × 375 mm
Weight12.7 kg monochrome; 16.8 kg colour
OS / FirmwareIBM PC DOS 3.3 and later; OS/2 1.x on the 286
Predecessorentry-level PS/2, replacing the IBM PC (5150) and XT in the range
SuccessorIBM PS/1; IBM PS/ValuePoint
Model no.8525

The IBM Personal System/2 Model 25 (machine type 8525) is the entry point of the PS/2 range, announced on 4 August 1987, four months after IBM unveiled the rest of the line. It is the only PS/2 with an integrated CRT in an all-in-one case, and — like the Model 30 — it uses ISA expansion rather than Micro Channel.

Three generations were sold:

  • Model 25 — Intel 8086-2 at 8 MHz, MCGA video, 8-bit ISA, from August 1987.
  • Model 25 286 — 80286 at 10 MHz, VGA, 16-bit ISA, from May 1990 to November 1992.
  • Model 25 SX — 386SX, IBM SVGA, 16-bit ISA, sold to the K–12 education market from April 1992.

All three share the all-in-one chassis and the 12-inch display assembly, and all three take drive power and data through connectors on the planar rather than through Molex leads.

Submodels

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Selected IBM PS/2 Model 25 submodels
Submodel CPU Bus RAM (max on board) Diskette Fixed disk Display Notes
8525-001 8086-2, 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" monochrome US$2,095 list
8525-004 8086-2, 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none 12" colour US$2,595 list
8525-G01 / G04 8086-2, 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none mono / colour Enhanced 101-key keyboard
8525-C02 / C05 8086-2, 8 MHz ISA 8-bit 640 KB 2 × 720 KB none mono / colour Collegiate kit
8525-L01 / L04 8086-2, 8 MHz ISA 8-bit 512 KB (640 KB) 1 × 720 KB none mono / colour LS, with Token Ring
8525-006 / G06 80286, 10 MHz ISA 16-bit 512 KB (4 MB) 1 × 1.44 MB none 12" VGA colour
8525-036 / G36 80286, 10 MHz ISA 16-bit 1 MB (4 MB) 1 × 1.44 MB 30 MB 12" VGA colour
8525-K00 / K01 386SX ISA 16-bit up to 12 MB 1 × 1.44 MB IDE (optional) 12" SVGA colour School market; K01 adds Ethernet
8525-L02 386SX ISA 16-bit up to 12 MB 1 × 1.44 MB IDE (optional) 12" SVGA colour Adds Token Ring

Prices, weights and dimensions are IBM's own.[1][2] The 286 and SX machine types are listed in the ardent-tool planar pages.[3][4]

Architecture

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Model 25 (8086) planar

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Three planars exist — Type 1 (P/N 00F2073), Type 2 (FRU P/N 96F7390, P/N 64F0738) and a Japanese board (P/N 78X8943) built by Panasonic/Matsushita. The PCB layouts differ drastically, but the reference designators are largely consistent across all of them, and across the 8086-based Model 30 planars as well.[5]

Model 25 8086 planar — principal designators
Designator Device
U10 Intel 8086-2 CPU, 8 MHz
U13 Intel 8087 coprocessor socket
U8 Intel P8253-5 programmable interval timer
U9 / U11 / U12 / U14 / U15 IBM gate arrays: 72X8203, 72X8300 (MCGA memory controller), 72X8205 (MCGA video formatter), 72X8206 and 72X8202
U5 Inmos IMSG171P RAMDAC
U6, U7 HM53461P 64K×4 DRAM (MCGA video memory)
U38 Sony CXK5864BM-12L 8K×8 static RAM (MCGA)
U16, U22, U34, U35 NEC D4164C-10 DRAM
ZM1, ZM2 OKI MSM3764A-15 DRAM
U17, U20 ROM BIOS (two devices)
U18 NEC uPD765AC-2 diskette controller
U19 8237A DMA controller
U27 NS8250AV UART
U2, U3 / U4 DS1489AN line receivers / NS1488N line driver
F1 / F2 Mouse and keyboard fuses (combined into F1 on the Type 2 planar)
J1 / J2 Mouse / keyboard connectors
J9 Dual 30-pin SIMM socket
J11 44-pin fixed-disk connector
J12 40-pin diskette connector
J7 Power connector
ZM3 Earphone connector
Y1 / Y2 / Y3 14.31818 MHz / 25.175 MHz / 48.0 MHz oscillators

Note what is not on that list: no real-time clock and no 32.768 kHz crystal. The 8086 Model 25 has no hardware clock and no battery. Date and time are set at each boot, and there is nothing here to replace.[5]

The riser card (P/N 78X9031, "SYNTEC") carries two 8-bit ISA slots and no components other than a resistor pack — so there is no battery on the riser either, unlike the 8086 Model 30.[5]

The Model 25 8086 does not use the same power connector as the Model 30 8086, so although the two boards share a designator scheme they are not interchangeable machines.[5]

Model 25 286 planar

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FRU P/N 64F0178 (P/N 37F6675), built around the VLSI VL82CPCAT-QC ("PC/AT-compatible") chipset. It is very similar to the single-ROM Model 30 286 planar.[3]

  • ZM36 — 286-10 CPU socket; ZM1 — 287-10 coprocessor socket.
  • ZM2–ZM6 — VL82C100 peripheral controller, VL82C102A memory controller, VL82C103A address buffer, VL82C104 data buffer, VL82C101B system controller.
  • ZM9 — VL16C451 parallel port and UART.
  • ZM14 — 27F4094 "DISKGA" disk/diskette gate array.
  • ZM18 — 15F6864 VGA controller, with ZM23 IMSG171P-35 RAMDAC and eight 64K×4 DRAMs at ZM47–ZM54.
  • ZM19 — NEC uPD765BC diskette controller.
  • ZM33 — Intel 8042 keyboard controller.
  • ZM35 — Dallas DS1287 RTC/CMOS module, with the lithium cell sealed inside it.
  • ZM46 — BIOS ROM on later boards (earlier boards use two devices at ZM11/ZM12).
  • J9 / J10 — two 30-pin SIMM sockets, 100–150 ns with parity; 512 KB minimum, 4 MB maximum on the board, 16 MB using adapter cards.
  • J13 — password-override connector. J7 and J14 — power. F1 — combined keyboard/mouse fuse.

Model 25 SX planar

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FRU P/N 87F4769, in "K" and "L" variants.[4]

  • U2 — Intel i386SX. Most machines have a 20 MHz part underclocked to 16 MHz by the 32.000 MHz oscillator at OS1; some have a genuine 16 MHz chip.
  • U5 — 80387SX-16 socket.
  • U8 / U9 — VLSI VL82C304-QC and VL82C305-FC (84F9161).
  • U10 — Intel 8042 keyboard controller. This is worth knowing: on this board U10 is not memory or an RTC.
  • U25 — Intel 82077AA diskette controller. U26 — ROM BIOS 87F4794.
  • U42 — IBM SVGA 84F7985, with TC511665BZ-80 video RAM at U43/U44/U51/U54 and an IMSG171P RAMDAC at U45.
  • U34, U36 (and, on non-"K" planars, U11/U13/U33/U35/U48/U49) — HM514900JP8 512K×9 DRAM.
  • J6 — 72-pin SIMM socket ("MEM 3"). J4 is a set of pads for a second socket that IBM never fitted; attempts to add one have failed with memory errors.
  • J10 — 40-pin IDE connector. J14 — 4-pin Molex for hard-disk power. J2 — power-on password clear.
  • B1 — CR2032 coin cell backing the clock, which is inside the VLSI chipset and timed by the Y1 32.768 kHz crystal. It is not a Dallas module.

Storage

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Diskette

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Drive power and data are carried on a single cable from the planar's 40-pin diskette connector; there is no separate Molex lead.[6][7] Drives are ALPS, Mitsubishi, Sony or YE-Data. The 8086 Model 25 shipped only with 720 KB drives, but has undocumented 1.44 MB support — in practice only with Sony drives, and later planars are more tolerant than early ones.[5]

PS/2 3.5-inch drives are direct drive. The Mitsubishi MF355C specification states that the spindle is turned directly by a DC brushless motor at 300 rpm; there is no belt in these drives.[8] What does fail is the surface-mount electrolytics on the drive's logic board, and the corrosion they leave behind.[9]

Fixed disk

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The Model 25 does not have an ST-506 controller on the planar. IBM's Technical Reference describes the fixed-disk connector as "a dedicated I/O channel for the connection of the IBM Personal System/2 20MB Fixed Disk Drive and controller, or similar attachment": the signals are ordinary I/O-channel signals plus −DISK CS (decoded at hex 032x), −Disk Installed and address lines A0–A2 to select a register within the controller.[6] In other words the controller travels with the drive, on IBM's own 44-pin connector.[5]

The Model 25 286 submodels 8525-036 and G36 shipped with a 30 MB drive on the same planar connector. The Model 25 SX is the only Model 25 with a conventional 40-pin IDE interface, at J10.[4]

There is no standard hard-disk power connector on the 8086 machine; adding one is a known modification.[5]

Display

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The 8086 Model 25 uses MCGA, integrated on the planar with a 64 KB display buffer.[2] The palette is a 256-entry, 18-bit table driving three 6-bit DACs — so 256 simultaneous colours out of 262,144.[6] On a monochrome machine the BIOS sums colours to 64 shades of grey.[1]

MCGA video modes
Mode Resolution Colours Notes
0 / 1 40 × 25 text 16 CGA-compatible
2 / 3 80 × 25 text 16 CGA-compatible
4 / 5 320 × 200 4 CGA-compatible
6 640 × 200 2 CGA-compatible
hex 11 640 × 480 2 MCGA-specific
hex 13 320 × 200 256 MCGA-specific

MCGA is analogue only. It will not drive a TTL digital CGA or MDA monitor. EGA modes are not supported.[5]

The Model 25 286 has VGA (15F6864 gate array), and the Model 25 SX has IBM's SVGA controller (84F7985) — the same part used on the 512 KB SVGA/A adapter and on the Model 40 SX planar.[3][4] The display assembly itself is identical across every Model 25.[4]

Keyboard and Mouse

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The Model 25 introduced the 6-pin Mini-DIN keyboard and pointing-device connectors that became the PS/2 standard. On the 8086 machine the two are genuinely interchangeable: IBM's Technical Reference states that "the interface logic for the keyboard and the pointing device is the same, allowing the keyboard and pointing device to plug into either of the two 6-pin connectors at the rear of the system."[6] The interface is in the I/O support gate array, which also holds the chip-select logic and the interrupt controller.[6] The 286 and SX planars use a discrete 8042 instead.[3][4]

Each port is fuse-protected on the planar — F1 for the mouse and F2 for the keyboard on the Type 1 board, combined into F1 on the Type 2 and on the 286 and SX boards.[5][3] A 305 at POST is a keyboard voltage error, and IBM's first suggested FRU is the port fuse.[10]

Power Supply

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The supply is inside the display assembly and feeds the system board, the display, the options and the keyboard.[6]

  • Colour machines: 113 W, a single-phase, six-voltage-level supply. It delivers 7.0 A of +5 V, 1.7 A of +12 V, 100 mA of −5 V and 250 mA of −12 V to the system board, plus 450 mA of 115 V dc and 220 mA of +12 V dc to the display assembly.[6]
  • Monochrome machines: 90 W, a single-phase, five-voltage-level supply. The system-board rails are identical; the display assembly instead gets 800 mA of +39 V dc.[6]
  • Model 25 286 and Model 25 SX: 124.5 W, dual voltage with a switch on the rear.[11][12]

IBM's Canadian quick reference rounds the colour 8086 machine's supply to 115 W; the Technical Reference figure of 113 W is the one to quote.[1][6]

The system board and drives take about 4 A of +5 V, leaving roughly 3 A for the two expansion slots. −5 V exists for adapters; +12 V runs the drives; +12 V and −12 V drive the serial port's EIA line drivers.[6]

Model 25 — dc output and protection (IBM figures)
Rail Min load Max load Regulation Under-voltage sense
+5 Vdc 1.5 A 7.0 A +5% to −3% +3.2 V
−5 Vdc 0.0 A 0.11 A +10% to −8% −2.4 V
+12 Vdc 0.12 A 1.7 A +5% to −3% +10.5 V
−12 Vdc 0.015 A 0.25 A +10% to −8% −8.6 V

Vac input is 115 V nominal (90–137 V, 2.0 A max) or 230 V nominal (180–265 V, 1.0 A max), selected by a mechanical switch and protected by an internal fuse. If a dc over-current or under-voltage condition exists, the supply shuts down until it is corrected.[6] The 'power good' signal is active at 2.4 to 5.25 Vdc with a turn-on delay of at least 100 ms and no more than 500 ms.[6]

Power arrives at the planar on the 12-pin J7 connector: pin 1 Power Good, pins 2 and 5–8 ground, pin 3 +12 V, pin 4 −12 V, pin 9 −5 V, pins 10–12 +5 V.[6] Measurement limits for a working machine are published for the same connector.[13]

A Model 30 supply is not a drop-in replacement.[7]

ROM BIOS and Reference Diskette

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The PS/2 BIOS is in two layers: CBIOS for DOS compatibility and ABIOS for protected-mode OS/2. There is no built-in SETUP; configuration is done by booting the model-specific Reference Diskette, which also carries Advanced Diagnostics (Ctrl-A from the main menu) and the backup utility.

Reference Diskettes are model-specific. A Model 30 diskette will not configure a Model 25, and the 8086, 286 and SX machines each need their own. Cold-boot the Reference Diskette rather than warm-booting it: a soft boot can produce false errors, including a false report that a power-on password is set.[10]

General Maintenance

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Opening the machine, CRT safety, planar inspection, rail checks, RTC battery replacement on the 286 and SX and connector care are in IBM PS/2 Model 25 Maintenance Guide.

Troubleshooting

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Beep codes, the numeric POST code list, dead-system isolation, Reference Diskette procedure and display diagnostics are in IBM PS/2 Model 25 Troubleshooting Guide.

Capacitor Replacement

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Planar decoupling, the supply, the display assembly and the diskette drive logic board are in IBM PS/2 Model 25 Capacitor Replacement Guide.

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  • IBM PS/2 Model 25, early product photograph
    IBM PS/2 Model 25, early product photograph
  • Front view
    Front view
  • Angled view
    Angled view
  • Model 25 SX, sold to schools
    Model 25 SX, sold to schools
  • 8086 front badge
    8086 front badge
  • Model 25 286 front badge
    Model 25 286 front badge

References

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  1. ↑ 1.0 1.1 1.2 IBM Canada Ltd Quick Reference, "IBM PS/2 Model 25 (8525)", mirrored at ardent-tool.com. IBM-supplied product data: the announce date, the US$2,095 (8525-001) and US$2,595 (8525-004) list prices, the microprocessor and clock, the bus, the ports, the DIP and MMK 100–150 ns parity memory, the power-supply ratings, the weights (12.7 kg monochrome, 16.8 kg colour), the dimensions (382 × 320 × 375 mm) and the MCGA colour summing to 64 grey shades in BIOS on a monochrome display. Note that this document rounds the colour machine's supply to 115 W where IBM's own Technical Reference gives 113 W.
  2. ↑ 2.0 2.1 IBM Announcement Letter 187-148, IBM 8525, PS/2 Model 25. Gives the 12-inch colour or monochrome analog display, MCGA with a 64 KB display buffer, the 8525-001 (monochrome) and 8525-004 (colour) minimum configurations with 512 KB RAM and one 720 KB diskette drive, the 8525-G01 and G04 Enhanced Keyboard variants, the universal power supply with a rear switch for 90–137 Vac or 180–259 Vac at 50–60 Hz, and the system bolt-down capability.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Louis Ohland, Tomáš Slavotínek et al., "8525-286 Planar", Ardent Tool of Capitalism, https://www.ardent-tool.com/8525/Planar_286.html. Gives the 8525-286 planar designator map (FRU P/N 64F0178, P/N 37F6675; VL82CPCAT-QC chipset at ZM2–ZM6, VL16C451 at ZM9, DS1287 RTC/CMOS at ZM35, 8042 keyboard controller at ZM33, 286-10 CPU socket at ZM36, F1 combined keyboard/mouse fuse, J7 and J14 power-supply connectors, J13 password-override connector, J15 earphone connector), the earphone connector board part numbers, the memory expansion starting-address table, and the machine types 8525-006, G06, 036 and G36.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Louis Ohland, William R. Walsh, Tomáš Slavotínek et al., "8525 SX Planar", Ardent Tool of Capitalism, https://www.ardent-tool.com/8525/Planar_SX.html. Gives the Model 25 SX "K" and "L" planar designator maps (FRU P/N 87F4769; B1 CR2032 battery, U2 i386SX, U8 VL82C304, U9 VL82C305, U10 Intel i8042 keyboard controller, U25 82077AA diskette controller, U26 ROM BIOS 87F4794, U42 IBM SVGA 84F7985, J2 power-on password clear, J10 40-pin IDE, J11 40-pin diskette, J12 and J13 power, J14 4-pin Molex for hard-disk power, Y1 32.768 kHz RTC crystal), the machine types 8525-K00, K01 and L02, the flipped riser slot for an Ethernet or Token Ring adapter, the statement that the monitor assembly is identical to every other Model 25, and the 16 versus 20 MHz CPU question.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Louis Ohland, William R. Walsh, Tomáš Slavotínek et al., "8525 Planar", Ardent Tool of Capitalism, https://www.ardent-tool.com/8525/Planar.html. Gives the Model 25 Type 1 (P/N 00F2073), Type 2 (FRU P/N 96F7390, P/N 64F0738) and Japanese (P/N 78X8943) planar designator maps; the riser card P/N 78X9031 and its contents; the note that reference designators are largely consistent across all 8086-based Model 25 and Model 30 planars; the warning that the Model 25 8086 does not use the same power connector as the Model 30 8086; and William Walsh's account of PS/2 diskette drives ("you might as well plan on replacing every one of the surface mounted electrolytic capacitors on the drive's circuit board, and almost certainly cleaning up the corrosive mess left behind") and of undocumented 1.44 MB support with Sony drives.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 IBM, IBM Personal System/2 Model 25 Technical Reference, June 1987. File:IBM PS2 Model 25 Technical Reference Jun87.pdf. Section 3 "Power Supply", pages 3-3 to 3-6, gives the 113 W colour and 90 W monochrome ratings, the rails supplied to the system board and to the display assembly, the mechanical voltage switch on the rear, the Vac input requirements, the Vdc output regulation and ripple, the output protection sense levels, the power-good characteristics and the J7 pin assignments. Section 1 gives the earphone connector specification and the 40-pin diskette and 44-pin fixed-disk connector pin assignments, and pages 1-126 and 1-127 give the system board connector location drawings.
  7. ↑ 7.0 7.1 "IBM PS/2 Model 25", DOS Days, https://www.dosdays.co.uk/computers/IBM%20PS2%20Model%2025/ibm_ps2_model_25.php. Source for the single power-and-data cable with no Molex connectors, the absence of a dust shutter and the electrolyte corrosion on drive logic boards, the warning that a Model 30 supply is not a drop-in for a Model 25 because the planar connectors differ, the expansion slot counts for the three generations, and the Sony 1.44 MB drive compatibility note.
  8. ↑ Mitsubishi Electric, 3.5 inch Flexible Disk Drive Standard Specifications, MF355C, document UGD-0527A, February 1987. File:Mitsubishi MF355C 3.5 inch Diskette Drive Specifications (UGD-0527A, Feb 1987).pdf. Section 3 states that the disk rotation mechanism uses a DC brushless direct-drive motor to rotate the spindle directly at 300 rpm; the drive has no belt.
  9. ↑ PS/2 Surface Mount Capacitors, ibmfiles.com. Community teardown source; documents surface-mount electrolytic failure and pad corrosion on PS/2 Mitsubishi diskette drives and on the DBA hard-drive logic boards, and the technique of fitting through-hole parts to the surface-mount pads.
  10. ↑ 10.0 10.1 "PS/2 Error Codes", Ardent Tool of Capitalism, https://www.ardent-tool.com/trouble/ps2error.html, with the error code listings at https://www.ardent-tool.com/trouble/error_02.html (160–199) and https://www.ardent-tool.com/trouble/error_04.html (300–999). Gives the beep codes, the 161/162/163/167/168 battery and clock cluster with IBM's FRU order and the lithium battery caution, the 301–307 keyboard cluster including 305 "keyboard voltage error" with the keyboard port fuse as first FRU, the diskette cluster including 604 "unsupported drive or cable" and 662/663 wrong drive and media type, the dead-system isolation procedure, and the warning that soft-booting the Reference Diskette can produce false errors.
  11. ↑ IBM Announcement Letter 190-082, IBM PS/2 Model 25 286 (8525-006, G06, 036, G36). Gives the 124.5 watt dual-voltage power supply with a switch on the rear, the 100–125 Vac and 200–240 Vac input ranges, the 654 BTU/hr figure and the 16.0 kg weight of the diskette-only models.
  12. ↑ IBM Announcement Letter 192-019, IBM PS/2 Model 25 SX (8525-K00, K01, L02). Gives the 124.5 W power supply rating and DOS 5.0 as the shipped operating system.
  13. ↑ Louis Ohland, Tomáš Slavotínek et al., "8525 / 8530 Power", Ardent Tool of Capitalism, https://www.ardent-tool.com/8530/Power.html. Transcribes IBM's Hardware Maintenance Service data for the 8525 and 8530: the J7 connector measurement limits for the Models 25, 25 SX and 25-286, the minimum under-voltage sense levels, the power-good range and the dead-system isolation procedure, and the warning that the manually-switchable supply must be set before the cord is plugged in.
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