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IBM PS2 Model 55 Technical Reference (October 1990)

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The IBM Personal System/2 Model 55 SX Technical Reference, Second Edition (October 1990), is IBM's 64-page model-specific hardware and software interface reference for the PS/2 Model 55 SX (8555). It supersedes the February 1989 first edition also hosted here.

What it contains

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  • Section 1, System Overview — model and submodel bytes, system board features, the I/O address map, performance and physical specifications and acoustical readings.
  • Power Supply (1-9) — outputs, output protection and voltage sequencing. New in this edition.
  • Section 2, Programmable Option Select — the system board POS address map, card selected feedback (hex 0091), enable/setup (hex 0094) and POS registers 2 to 5 (hex 0102–0105), including the memory card ID bits, plus adapter setup (hex 0096).
  • Section 3, System Board — microprocessor and math coprocessor, Micro Channel implementation and exception reporting, the central arbiter, the bus adapter, the fixed disk and diskette drive connectors, the ROM and RAM subsystems with error recovery and memory error address reassignment, the system memory map and memory connectors, the RT/CMOS RAM map and real-time clock bytes, and system control ports A and B.

Physical specifications: 406 mm wide, 397 mm deep, 102 mm high, 8.64 kg with a fixed disk, heat output 140 W (480 BTU/hour).

What it does not contain

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This is a programming and interface reference, not a service manual. There are no FRU part numbers, no removal and replacement procedures, no symptom-to-FRU index and no component values — IBM put those in the model's Hardware Maintenance Reference and Hardware Maintenance Service pamphlets. There are no schematics; the planar is described as a block diagram and a set of register and connector tables.

It is also not self-contained. IBM split the PS/2 Technical Reference library into volumes, and this one carries only the model-specific part. Anything common to the family — Micro Channel architecture, the serial and parallel port controllers, the instruction sets, the keyboard scan codes — is in the Hardware Interface Technical Reference volumes, and BIOS in the BIOS Interface Technical Reference.

What it settles

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  • The system board has no −5 V rail. Section 1 states plainly that "the system-board voltages are +5 Vdc, +12 Vdc, and −12 Vdc" and that the drive voltages are +5 Vdc and +12 Vdc. Several repair pages on this wiki have had to correct an invented −5 V rail on Micro Channel PS/2 models; this is the primary source that settles it. Note that the ISA-bus Model 25 and Model 30 do have a −5 V rail — the distinction is Micro Channel versus ISA, not PS/2 versus PC.
  • Micro Channel per-slot load limits — 1.6 A at +5.0 V, 0.175 A at +12.0 V and 0.040 A at −12.0 V per connector.
  • Power supply protection behaviour — a short between any two outputs, or an internal overvoltage, latches every DC output off before any output exceeds 130 % of nominal, and the supply only restarts after the fault is cleared and the switch has been off for at least ten seconds. Outputs track within 50 ms of each other at power-on.

Download it here: IBM PS2 Model 55 Technical Reference Oct90.pdf

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