Personal Computer AT, more commonly known as the
IBM AT and
also sometimes called the PC AT or PC/AT, was IBM's second-generation
PC, designed around the 6 MHz
Intel 80286 microprocessor and
released in 1984 as System Unit 5170. The name AT stood for "Advanced
Technology," and was chosen because the AT offered various
technologies that were then new in personal computers; one such
advancement was that the 80286 processor supported protected mode.
IBM later released an 8 MHz version of the AT.
1 AT features
2 Power supply
6 See also
8 External links
AT bus: The AT motherboard had a 16-bit data bus and 24-bit address
bus (16 MB) that was backward compatible with PC-style expansion cards
(which were 8-bit data, 20-bit address).
Fifteen IRQs and seven DMA channels, expanded from eight IRQs and four
DMA channels for the PC (and XT). The doubling of the IRQs was
achieved by adding another 8259A. IRQs 8–15 (from the second 8259A)
are cascaded through IRQ 2 of the first 8259A, which leaves 15
available instead of 16. Similarly, the number of DMA channels was
increased by adding another 8237A, also in master-slave configuration.
DMA channel 4 is reserved for cascading 0–3 leaving seven channels
active. Some IRQ and some DMA channels are used on the motherboard
and not exposed on the expansion bus.
16 MB maximum memory (because of the 24-bit address bus of the 286),
compared to the PC's 640 KB maximum (the remainder of the 8088's
1024 kB addressable memory space was reserved for ROM and video
Battery backed real-time clock (RTC) on motherboard with 50 bytes CMOS
memory available for power-off storage of
BIOS parameters. (The basic
PC had required either manual setting of its software clock using Time
and Date commands, or the addition of an accessory expansion card with
real-time clock, to avoid the default 01-01-80 file date.)
Additionally the AT RTC had a 1024-Hz timer (on IRQ 8), which was a
much finer resolution compared to the 18-Hz RTC used by
IBM PC XT (IRQ
0). The AT timer was accessible via INT 70h. The RTC was
implemented using a Motorola MC146818 integrated circuit.
BIOS setup program took the place of the DIP switches on
PCs and PC XTs. Most AT clones would have the setup program in ROM
rather than on a disk.
AT keyboard layout: the 84th key being <SysRq> i.e.
System request; numerical keypad now clearly separated from main key
group; also added indicator LEDs for Caps lock/Scroll lock/Num lock.
AT keyboard uses the same 5-pin
DIN connector as the PC keyboard,
but it uses a different, bidirectional interface (the PC and PC XT
keyboard interface is unidirectional) and generates different keyboard
scan codes. The bidirectional interface allows the computer to set the
LED indicators on the keyboard, reset the keyboard, set the typematic
rate, etc. Later ATs had 101-key keyboards which featured integrated
numeric keypad with
Num Lock key.
1.2 MB 135 mm (5¼ inch) floppy disk drive (15 sectors of
512 bytes, 80 tracks, two sides) stored over three times as much data
as the 360 KB PC floppy disk (nine sectors of 512 bytes, 40 tracks,
two sides). However, they had compatibility problems with 360k disks.
90mm (3½ inch) floppy drives became available in later ATs.
A 20 MB hard disk drive, although the early drives manufactured by
Computer Memories were very unreliable. This was attributed partly
to failure to automatically retract the read/write heads when the
computer was powered off, and partly to a bug in the DOS 3.0 FAT
ATs could be equipped with CGA, MDA, EGA, or PGA video cards.
8250 UART from the XT was upgraded to the 16450, although this
chip still had only a one byte buffer, so high-speed serial
communication was just as problematic as with the XT.
PC DOS 3.0 was released to support the new AT features, including
preliminary kernel support for networking (which was fully supported
in a later version 3.x release)
The AT was equipped with a physical lock that could be used to prevent
access to the computer by disabling the keyboard.
Just like its
IBM PC predecessor, the PC AT supported an optional math
co-processor chip, the Intel 80287, for faster execution of floating
IBM PC AT came with a 192-watt switching power supply. According
to IBM's documentation, in order to function properly, the AT power
supply needed a load of at least 7.0 amperes on the +5V line and a
minimum of 2.5 amperes was on its +12V line. In practice, the AT power
supply would randomly fail to start unless these minimum load
requirements were met. Because the AT motherboard didn't provide much
load on the +12V line, entry-level
IBM AT models that didn't have a
hard drive were shipped with a 5-ohm, 50-watt (maximum power) sandbar
resistor connected on the +12V line of the hard disk power connector.
In normal operation this resistor drew 2.4 amperes (28.8 watts),
getting fairly hot.
In addition to the unreliable hard disk drive, the high-density
floppy disk drives turned out to be problematic. Some ATs came with
one high-density (HD) disk drive and one double-density (DD)
360 kB drive. High-density floppy diskette media were compatible
only with high-density drives. There was no way for the disk drive to
detect what kind of floppy disk was inserted, and the only clue the
user had was the disk label and an asterisk molded into the
360 kB disk drive faceplate. If the user accidentally used a
high-density diskette in the 360 kB drive, it would sometimes
work, for a while, but the high-coercivity oxide would take a very
weak magnetization from the 360 kB write heads, so reading the
diskette would be problematic.
A different problem occurred when using a double-density diskette in
the 1.2 MB drive; the high-density drive's heads had a track
width half that of the 360 kB drive, so they were incapable of
fully erasing and overwriting tracks written by a 360 kB drive.
Therefore, overwriting a DD disk that had been written to in a DD
drive with an HD drive would result in a disk perfectly readable on an
HD drive, but producing many read errors in a DD drive. Whereas a HD
read head would only pick up the half track that drive had written,
the wider DD read head would pick up the half-track written by the HD
drive mixed with the unerased half-track remnant of the track written
earlier by a DD drive. Thus, the DD drive would end up reading both
new and old information together, causing it to "see" garbled data.
The combination of the faster clock rate, fewer clock cycles per
instruction, and the 16-bit bus led to a computer that was in the
marketing sense too fast.
IBM was protective of their lucrative
mainframe and minicomputer businesses and consequently ran the
original PC AT (139 version) at a very conservative 6 MHz with
one wait state. They also used a three-to-one interleave on the hard
disk, even though the controller supported two to one. Many customers
replaced the 12 MHz crystal (which ran the processor at
6 MHz) with a 16 MHz crystal (costing about five dollars
IBM introduced the PC AT 239 which would not boot the
computer at any speed faster than 6 MHz, by adding a speed loop
in the ROM. Previously sold AT 139s were subsequently offered an
upgrade costing $300 USD to the 8 MHz clock rate, merely by
replacing the crystal and ROM; apparently the DRAM was engineered from
the start for 8 MHz. This upgrade offering was, by design, quite
profitable for IBM. The final PC AT, the 339, ran the processor at
8 MHz with one wait state, and was built as IBM's flagship
microcomputer until the 1987 introduction of the
IBM's efforts to trademark the name AT largely failed, and most
286-based PCs were modeled after it. The label also became a standard
term in reference to PCs that used the same type of power supply,
case, and motherboard layout as the 5170. Even further, "AT-class"
became a term describing any machine which supported the BIOS
functions, 16-bit expansion slots, keyboard interface, and other
defining technical features of the
IBM PC AT; in the case of the
expansion slots, the term is largely synonymous with "ISA" (when the
latter is not applied as a retronym to XT-class machines, as in the
phrase "8-bit ISA slot".) As such, most systems with 486 and Pentium
CPUs, and at least some with Pentium Pro and Pentium II processors,
were describable as AT-class.
As of 2011, modern PCs still maintain nearly complete backwards
compatibility with the PC AT from a software perspective, but AT
mechanical and electrical compatibility is extremely rare. The AT
power supply pins and its connectors, the AT motherboard form factor,
and the physical ISA bus slots are no longer present on modern PCs
outside of specialized embedded designs. The
ATX standard from Intel
has completely replaced the original AT power supply and motherboard
design. Modern motherboards do not have ISA expansion bus connectors
any more, but a functionally equivalent bus lives on as the modern LPC
bus for software compatibility. Nearly all PC
BIOS ROMs, even modern
UEFI based ROMs, include code which is backwards compatible with the
BIOS interrupt calls. Even the 0xaa55 signature in the
master boot record is still required by many BIOSes to be present on
an attached hard disk for it to be recognized as a valid boot device.
PS/2 successor to the
AT keyboard interface still survives in the
modern market, though it is increasingly being replaced by USB in new
PS/2 keyboard interface is identical to the AT
keyboard interface except for the connector; the AT uses a 5-pin DIN
connector, while the
PS/2 uses a 6-pin mini-DIN.
The AT had three
BIOS versions dated January 10, 1984, June 10, 1985,
and November 15, 1985. Original models supported 15 hard disk types,
with this being expanded to 23 in the second and third BIOSes. The
BIOS fixed some bugs and added support for 720k 3.5" floppy
drives while the November 1985
BIOS added support for 101-key
keyboards and 1.44MB 3.5" floppies. ATs with the older BIOSes will
nominally work with 101-key keyboards, but the extra keys are ignored
unless the user writes his or her own code to read them.
If 3.5" 720k floppy drives are used on ATs with the January 1984 BIOS,
they are assumed to be 360k 5.25" floppies and the FORMAT command in
DOS will attempt to format them as such. In addition, DOS cannot
access anything but the first 40 tracks of the diskette. To solve this
problem, two separate utilities were provided with DOS 3.x, DRVPARM
and DRIVER.SYS, which modify the
BIOS parameter table and inform the
operating system that a 720k drive is present. Software on
self-booting diskettes (mainly games) does not have this problem since
the diskettes have their own internal disk access code. This same
situation also applies to using 1.44MB disk drives on the older AT
BIOSes, except that they are assumed to be 1.2MB disks.
Industry Standard Architecture
AT (form factor)
^ Somerson, Paul (Nov 13, 1984). "AT the Party". PC Magazine: 123.
Retrieved 5 July 2014.
IBM PC AT at Vintage Computer
IBM PC AT". www.vintage-computer.com.
^ Ziff Davis, Inc. (26 December 1989). PC Mag. Ziff Davis, Inc.
pp. 53–. ISSN 0888-8507.
^ N. MATHIVANAN (2007). PC-BASED INSTRUMENTATION: CONCEPTS AND
PRACTICE. PHI Learning Pvt. Ltd. pp. 227–229.
^ Howard Austerlitz (2002). Data Acquisition Techniques Using PCs.
Academic Press. pp. 90–91. ISBN 978-0-08-053025-3.
^ Brendan Horan (26 March 2013). Practical Raspberry Pi. Apress.
p. 146. ISBN 978-1-4302-4972-6.
^ Tom Shanley; Don Anderson (1995). ISA System Architecture.
Addison-Wesley Professional. pp. 441–444.
^ Dickinson, John (1985-06-25). "The AT's Slipped Disk". PC Magazine.
p. 55. Retrieved 28 October 2013.
^ Richard W. D. Nickalls; R. Ramasubramanian (1995). Interfacing the
IBM-PC to Medical Equipment: The Art of Serial Communication.
Cambridge University Press. p. 25.
^ Scott M. Mueller (2011). Upgrading and Repairing PCs (20 ed.). Que
Publishing. p. 882. ISBN 978-0-13-268218-3.
^ IBM's official 1986 response to "What percentage of the 20 MB drives
in PC ATs have failed?" was "We consider that information to be
confidential. However, based on the several customer surveys on the AT
that we have conducted for IBM, an overwhelming percentage of AT
owners tell us they're satisfied with the system." (questions on page
110, answers on page 111, PC Magazine, April 29, 1986). The article's
opening sentence, which reads "If you own an
IBM PC AT and your hard
disk hasn't crashed yet, don't worry -- it probably will."
http://www.highbeam.com/doc/1P2-3760999.html was described as "a
rarity in computer journalism" by the Chicago Sun-Times
http://www.encyclopedia.com/doc/1P2-3760999.html and the Sun-Times
called it a "badly flawed 20-megabyte" disk drive.
Personal Computer Hardware Reference Library: Guide to
Personal Computer XT Model 286.
IBM Part Number 68X2523.
PC AT entry at old-computers.com
IBM brings out the big guns, PC Mag 13 Nov 1984,
Wiki entry for PC AT at the Vintage Computer Forums
Historycorner.de - The
IBM PC AT (
IBM 5170) German
IBM 5170 information at www.mi