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IBM PC AT (5170)

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IBM Personal Computer AT
IBM Personal Computer AT (5170)
Specifications
DeveloperIBM Entry Systems Division, Boca Raton
ManufacturerIBM
TypeDesktop personal computer
ReleasedAugust 1984
Discontinued1987
CPUIntel 80286 @ 6 MHz (type 1 and 2 boards) or 8 MHz (type 3); optional Intel 80287
Memory256 KB or 512 KB on the system board; IBM supports expansion to 3 MB with its own 512 KB options
Storage1.2 MB 5.25" high-capacity diskette drive; optional 20 MB or 30 MB fixed disk
DisplayMDA, CGA, EGA or PGA on an adapter card
SoundSpeaker driven by the 8254 timer
Dimensions540 mm L × 439 mm D × 162 mm H
Weight19.05 kg (42 lb) per IBM's service manual; 20.0 kg (44 lb) per IBM's Technical Reference
OS / FirmwareIBM PC DOS 3.0 and later; OS/2 1.x; IBM and SCO Xenix; PC/IX
PredecessorIBM PC XT (5160)
SuccessorIBM Personal System/2
Model no.5170

The IBM Personal Computer AT (model 5170) moved the IBM PC line to the Intel 80286, introduced the 16-bit AT bus that was later standardised as ISA, and replaced the DIP switches and one-way keyboard protocol of the 5150 and 5160 with a battery-backed clock and configuration memory and a bidirectional keyboard.

IBM's own service manual summarises the machine as: 80286 processor, 7-channel DMA, 128 KB ROM, 256 KB or 512 KB of system-board RAM expandable to 3 MB with IBM's 512 KB Memory Expansion options, eight I/O slots, a 1.2 MB high-capacity diskette drive, a 20 MB or 30 MB fixed disk and a keylock.[1]

Base Models

IBM recognises five base models, and they are told apart by serial number range:[1]

IBM 5170 base models
Model Serial number range
068 5000000 – 5999999
099 0000001 – 0999999
239 6000000 – 6999999
319 / 339 7000000 – 7999999

An AT/370 variant, carrying System/370 emulation hardware, is documented separately in the same manual.[1]

System Board Revisions

Three revisions exist, which IBM's documentation calls type 1, type 2 and type 3:[2]

IBM 5170 system board revisions
Type Clock Form factor Main crystal RAM BIOS as supplied
1 6 MHz 13.8" × 12" 12 MHz Stacked/piggybacked 41128 128K-bit chips, 150 ns or faster 01/10/84
2 6 MHz 13.8" × 9.4" 12 MHz 41256 256K-bit chips, 150 ns or faster 06/10/85
3 8 MHz 13.8" × 9.4" 16 MHz As type 2 11/15/85

IBM's Technical Reference describes the memory arrangement by bank: the type 1 board has four banks of nine 128K-by-1 modules, and the type 2 board has two banks of nine 256K-by-1 modules — both totalling 512 KB with parity.[3] minuszerodegrees describes the type 1 board as two banks of 256 KB, counting the piggybacked 41128 pairs as single positions; the two descriptions are of the same board.[2]

A type 1 board can run with only bank 0 populated. Switching between single-bank and dual-bank operation needs the BIOS reconfigured through SETUP and may need jumper J18 moved.[2]

The memory module sockets on system boards P/N 8286112, P/N 6480170 and P/N 6480072 were redesigned: the contact surface sits below the top of the pin holes and is not visible from above. It is functionally equivalent to the old style, and a board that looks like it has no contacts is not faulty.[1]

BIOS Revisions

The BIOS, including cassette BASIC, is 64 KB. Very early boards carry four 16 KB 27128 devices at U17/U27/U37/U47; later production uses two 32 KB 27256 devices at U27 and U47. On type 1 boards, shunt block U131 selects which ROM type is fitted.[2]

IBM 5170 BIOS revisions
Date Mask ROM EPROM Board
01/10/84 U27 = 6181028, U47 = 6181029 U17 = 6181024, U27 = 6181025, U37 = 6181026, U47 = 6181027 Type 1
06/10/85 U27 = 6480090, U47 = 6480091 U27 = 6448896, U47 = 6448897 Type 2
11/15/85 U27 = 62X0820, U47 = 62X0821 U27 = 61X9266, U47 = 61X9265 Type 3

IBM supported the 01/10/84 to 06/10/85 upgrade: the 30 MB hard drive upgrade kit for type 1 machines included 06/10/85 ROMs, because the 30 MB drive needed the newer code. Whether IBM ever endorsed an upgrade to 11/15/85 is not known; it is desirable because it adds 1.44 MB diskette support, and owners who have tried it on 6 MHz boards report no problems, but it was designed for an 8 MHz board.[2]

The 601 that is not a fault

With the 06/10/85 and 11/15/85 BIOS, removing the IBM Fixed Disk and Diskette Drive Adapter and fitting a third-party replacement can produce a 601 at POST. Those revisions expect that adapter, or something that identifies itself the same way; the 601 is the POST telling you it did not find one. It does not stop the boot — a prompt to press F1 follows — although diskette operation may be compromised. The 01/10/84 BIOS does not do this. Fitting a card that behaves like IBM's, such as a Western Digital WD1003A-WA2, avoids it.[2]

Expansion

Eight I/O slots, but they are not all the same. IBM's Technical Reference specifies "six with a 36-pin and a 62-pin card-edge socket, two with only the 62-pin card-edge socket" — so six 16-bit slots and two 8-bit slots.[3] An 8-bit card fits either kind; a 16-bit card needs one of the six.

Storage

  • Diskette — a 1.2 MB high-capacity 5.25-inch drive. The 1.2 MB drive's head is half the track width of a 360 KB drive's, so a 360 KB diskette written in a 1.2 MB drive is often unreadable in a real 360 KB drive afterwards. The drive cannot detect media type; the user has to use the right media.
  • Fixed disk — an optional 20 MB or 30 MB drive on the IBM Fixed Disk and Diskette Drive Adapter, the 16-bit "combo" card that carries both controllers.[1]
  • Keylock — a keyed front-panel lock disables the keyboard and holds the cover on. If the key is lost, the cover can still be removed: with the unit flat on the bench and the five rear cover screws out, insert a screwdriver into the gap between frame and cover from underneath, in line with the lock, and lift — this raises the cover lip over the latch. To run diagnostics in that state, remove one wire from the lock switch to re-enable the keyboard.[1]

Display and Ports

There is no video on the system board; MDA, CGA, EGA and PGA cards were all used.

There is no serial or parallel port on the system board either. Both are on the separate IBM PC AT Serial/Parallel Adapter, whose serial side is built around an NS16450 UART — a single-byte-buffer part like the 8250 in the 5150, and the reason high-speed serial work on an AT wants a 16550-class replacement.[3]

Keyboard

The AT introduced the 84-key AT keyboard, with the numeric keypad separated from the main group and three lock lamps. The connector is the same DIN-5 as the XT's, but the interface is bidirectional and the scan code set is different, so an XT keyboard will not work on an AT and an AT keyboard will not work on a 5150 or 5160 without a converter.[4] From the type 3 board era the AT shipped with the 101-key Enhanced Keyboard instead.

Power Supply

Approximately 192 W.[2] IBM's own figures:[3][1]

IBM 5170 — dc load requirements and sense levels
Rail Minimum load Maximum load Regulation Minimum sense level
+5 Vdc 7.0 A 19.8 A +5% to −4% +4.5 V
−5 Vdc 0.0 A 0.3 A +10% to −8% −4.3 V
+12 Vdc 2.5 A 7.3 A +5% to −4% +10.8 V
−12 Vdc 0.0 A 0.3 A +10% to −9% −10.2 V

The supply also provides switched 115 Vac or 230 Vac for the IBM Monochrome Display.[3] Input is 100–125 Vac at up to 5 A, or 200–240 Vac at up to 3 A, selected by a switch above the power-cord socket; maximum in-rush current is 100 A.[3][1] Heat output is 1100 BTU/hr.[1]

Protection and behaviour:[3]

  • If any output becomes overloaded the supply switches off within 20 ms. An over-current condition does not damage it.
  • Output voltages track within 300 ms of each other at power-on and power-off, provided minimum loading is present.
  • No damage occurs with no load, but the supply may switch off and need a power-on reset.
  • Switch off for at least one second before switching on again, or 'power good' will not be generated. It has a turn-on delay of at least 100 ms and no more than 500 ms, and can drive six standard TTL loads.

The load resistor

The +12 V rail needs 2.5 A minimum. An AT system board provides very little +12 V load on its own, so machines shipped without a fixed disk carry a 5 ohm, 50 watt resistor on P10 in its place.[3] At 12 V that draws about 2.4 A — roughly 29 W — and it gets hot in normal operation. That heat is not a fault.[2]

A supply fault that only appears when warm

IBM's own service tip: some 5170 power supplies fail after two or more hours powered on, then work correctly again once the machine has been switched off and has cooled. IBM's instruction in that case is to replace the supply.[1]

The Battery and the 161 Error

The AT keeps its configuration in battery-backed CMOS alongside a Motorola MC146818 real-time clock. IBM's procedure for checking the battery:[1]

  1. Switch the system unit off and remove the cover.
  2. Disconnect the battery from the system board. Measuring it while connected gives varying readings because of the board's drain.
  3. Meter on the 12 V dc range, common lead to pin 1 and voltage lead to pin 4 of the battery connector.
  4. Look for 6.0 Vdc minimum.

A 161 ("battery defective or new battery installed") appears on the first POST after any battery change. If you have just fitted a new one, that is expected — run SETUP.[1]

Diagnosing Memory Errors

IBM's memory error reporting on the AT is more informative than it looks:[1]

  • Parity check 1 — a system board parity error. Parity check 2 — a Memory Expansion Adapter parity error.
  • A 10-character error code beginning 00, 01, 02, 03, 04, 05, 06 or 07 is a system board memory failure. The first two characters give the bank — 00 to 03 is bank 0, 04 to 07 is bank 1 — and the last four characters identify the failing module within it. For example, 04002 8000 points at a specific chip in bank 1.
  • If the first character of a parity check error is 8 or 9, replace the 18 memory modules of the base Memory Expansion Adapter; if the same code returns, replace the adapter.
  • If a diagnostic passes on bare system-board memory, refit Memory Expansion Adapters one at a time, running SETUP each time so the memory size is right.

Models 068 and 099 with system board P/N 8286112, an Enhanced Graphics Adapter and the 0.5 MB to 2 MB Memory Expansion Adapter produce a spurious Parity Check 2 message after a soft boot (Ctrl-Alt-Del). IBM's fix is ECA024, not a memory replacement.[1]

Blank Display, No Beep — Check the Coprocessor First

Some 5170 planars left the factory with the math coprocessor socket fitted the wrong way round, notch to the rear of the board. An 80287 installed to match the socket notch then fails power-on diagnostics: blank display, no beep. Installing the coprocessor with its notch towards the front of the planar corrects it.[1] It is worth checking before condemning a board, and it is worth remembering when moving a coprocessor to a replacement board — IBM's own manual reminds engineers not to forget to transfer it.[1]

General Maintenance

Rail checks, battery replacement, cleaning and slot care are in IBM PC AT Maintenance Guide.

Troubleshooting

Beep codes, 1xx system board errors, the 161/162/163 cluster, the 601 combo-card message, CMI hard drive symptoms and other AT-specific problems are in IBM PC AT Troubleshooting Guide.

Capacitor Replacement

Tantalum work on the system board and the switching supply is in IBM PC AT Capacitor Replacement Guide.

  • IBM Personal Computer AT (5170)
    IBM Personal Computer AT (5170)
  • Front view
    Front view
  • Rear view
    Rear view
  • Type 2 system board
    Type 2 system board

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 IBM, IBM Personal Computer Family Service Information Manual, SA38-0037-00 — hosted on this wiki as IBM Personal Computer Family Service Information Manual. Chapter 9 "5170 Personal Computer AT": Figure 9-1 (features), the base-model serial-number ranges, Figure 9-2 (specifications, including 162 × 540 × 439 mm and 19.05 kg, 1100 BTU/hr heat output and the electrical ranges), Figure 9-7 (input protection), Figure 9-8 (dc load requirements), Figure 9-9 (sense levels), the locked-cover removal procedure, the power-supply thermal-failure tip, the battery voltage check at the battery connector, the reversed math-coprocessor socket that gives a blank display and no beep, the parity check 1 and 2 distinction, the 10-character memory error code bank/module decode, ECA024 for the Parity Check 2 message on models 068 and 099, and the redesigned memory-module sockets on system boards P/N 8286112, 6480170 and 6480072.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 minuszerodegrees.net pages for the IBM 5170: Motherboard Versions for the type 1, 2 and 3 boards with their speeds, form factors, crystals, RAM types and the jumper J18 note; BIOS Revisions for the three BIOS dates, the mask-ROM and EPROM part numbers, the U131 shunt block, the IBM-supported 01/10/84 to 06/10/85 upgrade through the 30 MB hard drive kit, and the 601 POST error produced by the two 1985 BIOS revisions when the IBM Fixed Disk and Diskette Drive Adapter is not found; Power Supply for the approximate 192 W rating and the minimum loading; and Load Resistor for the 5 ohm 50 W resistor drawing 2.4 A from the +12 V line.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 IBM, IBM Personal Computer AT Technical Reference (September 1985) — hosted on this wiki as File:IBM PC AT 5170 Technical Reference Sep85.pdf and documented as IBM PC AT 5170 Technical Reference (September 1985); the March 1984 edition is also hosted. Section 1 "System Board" for the feature list, the eight I/O slots (six with a 36-pin and a 62-pin socket, two with only the 62-pin socket) and the type 1 and type 2 memory arrangements; Section 1 "Specifications" for the 540 × 439 × 162 mm size and the 20.0 kg weight; Section 3 "Power Supply", pages 3-3 to 3-7, for the input requirements, the dc load currents and regulation tolerances, the 20 ms output protection, output voltage sequencing, no-load operation, the 'power-good' signal and its one-second off requirement, and the 5 ohm 50 W load resistor on P10.
  4. ↑ IBM 5150 — Keyboards, minuszerodegrees.net. Source for IBM's own name for this keyboard ("83-key keyboard"), the type 1 / type 2 distinction in reset behaviour, the point that "Model F" is a family name covering keyboards that will not work with a 5150, the reassembly warning, the cable metering diagram, and the incompatibility of AT-protocol keyboards.


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