Jump to content

IBM PC AT (5170)

From RetroTechCollection
Revision as of 11:18, 21 May 2026 by Josh (talk | contribs) (Create/rewrite IBM PC AT (5170) and IBM PCjr (4860) page set with verified facts, real images, and embedded content per RTC style guide)
(diff) โ† Older revision | Latest revision (diff) | Newer revision โ†’ (diff)


IBM Personal Computer AT
IBM Personal Computer AT (5170)
Specifications
DeveloperIBM Entry Systems Division, Boca Raton
ManufacturerIBM
TypeDesktop personal computer
ReleasedAugust 14, 1984
DiscontinuedApril 2, 1987 (final units of model 339 left the assembly line in July 1987)
CPUIntel 80286 @ 6 MHz (Type 1/2 motherboards) or 8 MHz (Type 3)
Optional Intel 80287 FPU
Memory256 KB or 512 KB on-board (depends on motherboard revision); expandable to 16 MB total via 16-bit ISA memory cards
Storage1.2 MB 5.25" high-density floppy drive; optional 20 MB or 30 MB MFM hard drive (CMI 5616 or CMI 6426 on early units; later Seagate ST-225/ST-251)
DisplayMDA + IBM 5151, CGA + IBM 5153, EGA, or PGA (separate adapter card)
SoundInternal PC speaker, driven by Intel 8254 timer
Dimensions21.3" W ร— 17.3" D ร— 6.4" H (54 ร— 44 ร— 16 cm)
Weight~43 lb (19.5 kg) without keyboard or monitor
OS / FirmwareIBM PC DOS 3.0 and later; OS/2 1.x; PC/IX 1.1; IBM and SCO Xenix; Windows 1.0 – 3.1
PredecessorIBM PC XT
SuccessorIBM Personal System/2 (no direct AT replacement until the IBM PS/2 Model 30 286 in 1988)
Model no.5170

The IBM Personal Computer AT (model 5170, also known as the IBM AT or PC/AT) was IBM's fourth Personal Computer, announced on August 14, 1984. It moved the IBM PC line from the 8088 to the Intel 80286, introduced the 16-bit AT bus (later standardised as ISA), and replaced the XT-era keyboard protocol and DIP-switch configuration with a bidirectional AT keyboard protocol and a battery-backed CMOS/RTC. The AT remained in production until early 1987 and was officially withdrawn on April 2, 1987, the day IBM announced the PS/2.

IBM never expanded the "AT" abbreviation on product packaging; it is generally taken to stand for "Advanced Technology."

Architecture and Processor

The AT uses an Intel 80286 CPU. Type 1 and Type 2 motherboards run at 6 MHz (12 MHz crystal); the Type 3 motherboard, introduced in April 1986, runs at 8 MHz (16 MHz crystal). A second socket accepts the matching Intel 80287 floating-point coprocessor.

The 80286 has a 24-bit address bus that raises the addressable RAM ceiling from 1 MB (8088) to 16 MB, and it can operate in either real mode or protected mode.

The AT chip set on the motherboard includes:

  • Two Intel 8259A programmable interrupt controllers, cascaded, providing 15 hardware IRQs.
  • Two Intel 8237A DMA controllers, cascaded, providing 7 DMA channels.
  • Intel 8254-2 programmable interval timer (system tick, refresh trigger, speaker).
  • Intel 82284 clock generator and Intel 82288 bus controller.
  • Motorola MC146818 real-time clock with 50 bytes of battery-backed CMOS RAM. Setup parameters (memory size, floppy and HDD type, etc.) are stored here in place of the DIP switches used on the 5150 and 5160.
  • Intel 8042 keyboard controller (an 8042-class microcontroller, also produced by IBM and others as the 8742).

The 5150/5160 cassette interface is removed entirely on the AT.

Motherboard Revisions

There are three motherboard revisions, identified in IBM service documentation as Type 1, Type 2 and Type 3.

IBM 5170 motherboard revisions
Type CPU speed Form factor On-board RAM BIOS shipped Main crystal Submodels fitted
Type 1 6 MHz 13.8" ร— 12" (full size AT) 2 banks ร— 256 KB (stacked 41128) 01/10/84 12 MHz 5170-068, 5170-099
Type 2 6 MHz 13.8" ร— 9.4" (Baby AT) 1 bank ร— 512 KB (41256) 06/10/85 12 MHz 5170-239
Type 3 8 MHz 13.8" ร— 9.4" (Baby AT) 1 bank ร— 512 KB (41256) 11/15/85 16 MHz 5170-319, 5170-339

The Type 1 motherboard can operate with only bank 0 populated (256 KB); switching between single-bank and dual-bank operation also requires the BIOS to be reconfigured via SETUP and may need jumper J18 changed.

BIOS revisions

The 5170 BIOS (including ROM BASIC) is 64 KB. On very early motherboards it shipped as four 16 KB chips (27128) in sockets U17/U27/U37/U47. Later production used two 32 KB chips (27256) in sockets U27 and U47. On Type 1 boards, shunt block U131 selects which ROM type is installed.

IBM 5170 BIOS revisions
Date Mask ROM (U27/U47) EPROM part numbers Motherboard Floppy support HDD support
01/10/84 U27=6181028, U47=6181029 U17=6181024, U27=6181025, U37=6181026, U47=6181027 Type 1 360 KB and 1.2 MB 20 MB (type 1) only
06/10/85 U27=6480090, U47=6480091 U27=6448896, U47=6448897 Type 2 360 KB and 1.2 MB; 720 KB 20 MB and 30 MB (type 20)
11/15/85 U27=62X0820, U47=62X0821 U27=61X9266, U47=61X9265 Type 3 Adds 1.44 MB drive support As above

IBM officially supported an upgrade from the 01/10/84 BIOS to the 06/10/85 revision (the 30 MB hard drive upgrade kit included the 06/10/85 ROMs because the 30 MB drive required the new HDD code). The 11/15/85 BIOS is desirable for 1.44 MB floppy support but was designed for 8 MHz motherboards; running it on a 6 MHz Type 1 or Type 2 has been reported as functional by experienced users.

Storage and Expansion

  • Eight expansion slots on a 16-bit AT bus (later standardised as ISA). The slot connector is a two-piece edge: an 8-bit XT-style portion plus a shorter 16-bit extension. Cards built for the XT (8-bit) fit in either piece, leaving the 16-bit portion unused.
  • Floppy drive: 1.2 MB 5.25" half-height high-density drive. The 1.2 MB drive cannot reliably write to 360 KB diskettes for later use in a 360 KB drive, because the 1.2 MB drive's head track width is half that of a 360 KB drive. The drive itself cannot detect the diskette type, so the user must use the correct media.
  • Hard drive: Optional 20 MB or 30 MB MFM hard drive on most submodels, connected through the IBM Fixed Disk and Diskette Drive Adapter (the "combo" controller, which combines the floppy and hard disk controllers on a single 16-bit card).
  • Keyboard lock: A keyed mechanical lock on the front panel disables the keyboard and holds the system unit's cover in place.

Display

The AT has no on-board video. Common adapter cards used with the AT are MDA, CGA, EGA and PGA. The AT shipped most commonly with EGA from 1985 onwards.

Keyboard

The AT shipped with a new 84-key Model F AT keyboard. The numeric keypad is clearly separated from the main key group and three LEDs are present (Caps Lock / Scroll Lock / Num Lock). The connector is the same DIN-5 as the XT keyboard, but the electrical interface is bidirectional โ€” the host can control the LEDs, reset the keyboard, set the typematic rate, etc. The scan code set is also different from the XT's. As a result, an XT keyboard cannot be used on an AT and vice versa without a protocol converter.

From April 1986 (Type 3 motherboard era), the AT shipped with the 101-key Enhanced Keyboard (Model M).

Power Supply

The 5170 includes a 192 W switching power supply with two motherboard connectors (P8/P9). It requires a minimum load of 7.0 A on +5 V and 2.5 A on +12 V to start. Because the AT motherboard provides little +12 V load on its own, entry-level submodels that did not include a hard drive shipped with a 5 ฮฉ 50 W resistor fitted to the +12 V line of the hard drive power connector to satisfy the minimum load. The resistor draws roughly 2.4 A (about 29 W) in normal operation and gets fairly hot.

The AT serial port is built around the 16450 UART, which (like the 8250 in the PC/XT) has only a single-byte buffer; high-speed serial work is problematic without a 16550-equivalent replacement.

Submodels

The following list of AT submodels is taken from the IBM Personal Computer Family Service Information Manual and the IBM Personal System/2 and IBM Personal Computer Publication Reference.

Selected IBM PC AT submodels
Submodel Clock Stock RAM HDD Notes
5170-068 6 MHz 256 KB none Base diskette-only configuration
5170-099 6 MHz 512 KB 20 MB Original "Enhanced" configuration
5170-239 6 MHz 512 KB 30 MB Type 2 motherboard
5170-319 8 MHz 512 KB 30 MB Type 3 motherboard, original Model F keyboard
5170-339 8 MHz 512 KB 30 MB Type 3 motherboard, ships with the Enhanced (101-key) Keyboard
5170-839 6 MHz 512 KB two 30 MB Includes controllers for the IBM 4680 Store System (point-of-sale)
5170-899 6 MHz 512 KB two 20 MB As 839 with 20 MB drives
5170-599 / -739 / -919 / -939 6 MHz 512 KB 20 MB or 30 MB AT/370 โ€” System/370 host emulation board
5171-168 / -339 8 MHz 512 KB none or 30 MB TEMPEST-shielded variants

General Maintenance

PSU voltage checks, RTC battery replacement, cleaning and slot care for the AT are documented in IBM PC AT Maintenance Guide.

Troubleshooting

POST beep codes, 1xx system-board errors, 161/162/163 CMOS battery errors, the 601 combo-card error, CMI hard-drive symptoms and other AT-specific known problems are documented in IBM PC AT Troubleshooting Guide.

Capacitor Replacement

Tantalum capacitor advice for the 5170 motherboard and the AT switching PSU is documented in IBM PC AT Capacitor Replacement Guide.

  • IBM Personal Computer AT (5170)
    IBM Personal Computer AT (5170)
  • Front view of an IBM 5170
    Front view of an IBM 5170
  • Back view of an IBM 5170 showing keyboard, parallel, serial and video connectors
    Back view of an IBM 5170 showing keyboard, parallel, serial and video connectors
  • IBM 5170 motherboard, Type 2 (Baby AT form factor, 6 MHz)
    IBM 5170 motherboard, Type 2 (Baby AT form factor, 6 MHz)

References


Retrieved from "https://wiki.retrotechcollection.com/index.php?title=IBM_PC_AT_(5170)&oldid=10499"