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IBM PS2 Model 70 Technical Reference (October 1990)

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The IBM Personal System/2 Model 70 386 Technical Reference, Fourth Edition (October 1990), is IBM's 96-page model-specific hardware and software interface reference for the PS/2 Model 70 (8570). It is the last of four editions; the May 1988 first edition is also hosted here.

What it contains

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  • Section 1, System Overview — model and submodel bytes, system board features and block diagram, the I/O address map, performance specifications by planar type, physical specifications, and the power supply section with component maximum currents and Micro Channel load currents.
  • Section 2, Programmable Option Select — the POS address map and the system board setup registers.
  • Section 3, System Board — Micro Channel implementation, the central arbiter, the processor complex and cache, the memory subsystem, the RT/CMOS RAM map and real-time clock bytes, system control ports A and B, and the power-on password.

Physical specifications: 360 mm wide, 420 mm deep, 140 mm high, 9.55 kg, heat output 215 W (735 BTU/hour). The May 1988 first edition gives 220 W for the same machine.

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 70 Technical Reference Oct90.pdf

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