IBM PS2 Model 25 Technical Reference (June 1987)

The IBM Personal System/2 Model 25 Technical Reference, First Edition (June 1987), is IBM's 349-page standalone technical reference for the PS/2 Model 25 (8525), the all-in-one 8086 machine with an integrated display. It is written in the pre-1988 one-volume format, so it carries the power supply, keyboard and instruction-set material that the later Micro Channel model references dropped.

What it contains

• Section 1, System Board (1-1) — the 8086, the system support gate array and I/O support gate array, DMA, interrupts and interrupt sharing, read/write memory and ROM, the I/O channel with connector pinouts, signal descriptions and signal timings (1-23 to 1-35), the video subsystem with block diagram, text and graphics modes, display formats, video storage organisation, the full video register set, initialisation tables and RAM-loadable fonts (1-36 to 1-77), the diskette drive interface with gate array and controller registers and commands (1-78 to 1-104), the serial port with its programmable baud-rate generator (1-105 to 1-119), the parallel port (1-120), the beeper and the earphone connector (1-125), and the connector and specification tables (1-126 to 1-130).
• Section 2, Coprocessor — the 8087-2.
• Section 3, Power Supply (3-1) — input and output power, output protection and the power good signal.
• Section 4, Keyboard; Section 5, System BIOS; Section 6, Instruction Set; Section 7, Characters and Keystrokes.

What it does not contain

No FRU part numbers, no removal procedures and no symptom-to-FRU index. No schematics and no component values: the integrated display, its flyback and its analogue board are outside the scope of this book entirely, and IBM published no component-level data for them. A Model 25 with a dead display is not diagnosable from this manual beyond the video signal interface.

What it settles

• The Model 25 has a −5 V rail. Figure 3-2 lists four DC outputs: +5 Vdc 1.5–7.0 A (+5 % / −3 %, 100 mV p-p ripple), −5 Vdc 0.0–0.11 A (+10 % / −8 %, 50 mV), +12 Vdc 0.12–1.7 A (+5 % / −3 %, 120 mV) and −12 Vdc 0.015–0.25 A (+10 % / −8 %, 120 mV). This matters because the Micro Channel PS/2 models do not have a −5 V rail, and the two rules have been confused on this wiki before.
• Mains input: 115 Vac nominal, 90–137 Vac, 2.0 A maximum; or 230 Vac nominal, 180–265 Vac, 1.0 A.
• Undervoltage sense levels (Figure 3-3): +5 at +3.2 V, −5 at −2.4 V, +12 at +10.5 V, −12 at −8.6 V.
• Power good: TTL, active 2.4–5.25 Vdc, inactive 0.0–0.4 Vdc, turn-on delay at least 100 ms and not more than 500 ms, sinks 2 mA or sources 100 µA.

Download it here: IBM PS2 Model 25 Technical Reference Jun87.pdf

Related pages

• IBM PS/2 Model 25 · IBM PS/2 Model 25 Maintenance Guide · IBM PS/2 Model 25 Troubleshooting Guide · IBM PS/2 Model 25 Capacitor Replacement Guide
• IBM PS2 Model 30 Technical Reference (January 1987) — the desktop sibling, same 8086 design
• IBM Personal System 2 Hardware Maintenance Manual (S52G-9971-02, October 1994)