IBM Personal Computer XT
Encyclopedia
The IBM
Personal Computer XT, often shortened to the IBM XT, PC XT, or simply XT, was IBM's successor to the original IBM PC. It was released as IBM Machine Type number 5160 on March 8, 1983, and came standard with a hard drive. It was based on essentially the same architecture as the original PC, with only incremental improvements; a new 16-bit bus architecture would follow in the AT. The XT was mainly intended as an enhanced machine for business use, and a corresponding 3270 PC
featuring 3270 terminal emulation was released later in October 1983. XT stands for X-tended Technology.
, a 360 kB double-sided 5.25 in (133.4 mm) full-height floppy disk
drive, a 10 MB Seagate
ST-412 hard drive with Xebec 1210 MFM
controller, an Asynchronous Adapter (serial
card with 8250 UART
) and a 130 watt
power supply
. The motherboard had eight 8-bit ISA
expansion slots, and an Intel 8088
microprocessor running at 4.77 MHz
(with a socket for an 8087
math coprocessor
); the operating system usually sold with it was PC DOS 2.0 and above. The eight expansion slots were an increase over the five in the IBM PC, although three were taken up by the floppy drive
and hard drive adapter
s, and the Async card. The basic specification was soon upgraded to have 256 kB of RAM as standard. Slot 8 on the XT motherboard was wired slightly different than the other slots, making it incompatible with some cards. This was done for cards designed to allow the XT to be connected to IBM mainframes. Video card
s initially comprised the MDA
and CGA
, with EGA
and PGC
becoming available in 1984.
There were two widely used configurations of the XT motherboard. The first could support up to 256 kB on the motherboard itself (four banks of 64 kB chips), with a maximum of 640 kB achieved by using expansion card
s. This was the configuration the XT originally shipped in. The second configuration—introduced in stock units in 1986—could support the whole 640 kB on the motherboard (two banks of 256 kB chips, two banks of 64 kB), had the later revision AT-compatible BIOS with a faster booting time, as well as support for 101-key keyboards and 3.5 in (88.9 mm) floppy drives. The earlier configuration could be adapted to "late" configuration after a couple of minor modifications.
There were also two or three revisions of the motherboard; however, there are only minor differences between them. Most notable is that the first revision is missing U90 and has some parts located at another place on the motherboard. The change occurred in the spring of 1984, when IBM stopped the production of submodel 087 and began producing the submodel 086 instead. However, the motherboard revision and the fact that 086 came with 256 kB RAM from the factory—whereas 087 came with only 128 kB RAM—are the only differences between the two first submodels.
Beginning in 1985, the XT was offered in floppy-only models without a hard drive (submodel 068 and 078). XTs with the 256-640 kB motherboard configuration came standard with half-height floppy drives in place of the full-height drives, as well as the option for a 20 MB ST-225 half-height hard drive and "enhanced" keyboard (essentially a Model M without the LED panel, and cross-compatibility between the AT and XT keyboard transfer protocol). The XT was discontinued in the spring of 1987, replaced by the PS/2
Model 30.
Like the original PC, the XT came with BASIC in its ROM
. Despite the lack of a cassette port on XTs, IBM's licensing agreement with Microsoft forced them to include BASIC on all their machines.
PC and XT keyboards are not compatible with those on more modern PCs (IBM AT or newer)—even with DIN to PS/2
mini-DIN plug adapters—because PC/XT keyboards have a different transfer protocol from PC/AT keyboards, as well as using different keyboard scan codes. Some keyboards are switchable between the two interfaces for compatibility with either computer. However, the newer 101-key keyboard based on the Model M will work with modern PCs.
The XT had a desktop case similar to that of the PC. It weighed 32 pounds, and was approximately 19.5 inches wide by 16 inches deep by 5.5 inches high. The power supply was configured for 120 V AC only and could not be used with 240 V supplies.
processor was introduced. However, this system turned out to be faster than the ATs of the time using 8 MHz 286 processors because the XT 286 had zero wait state
RAM, which could move data more quickly.
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
Personal Computer XT, often shortened to the IBM XT, PC XT, or simply XT, was IBM's successor to the original IBM PC. It was released as IBM Machine Type number 5160 on March 8, 1983, and came standard with a hard drive. It was based on essentially the same architecture as the original PC, with only incremental improvements; a new 16-bit bus architecture would follow in the AT. The XT was mainly intended as an enhanced machine for business use, and a corresponding 3270 PC
IBM 3270 PC
The IBM 3270 PC , released in October 1983, was an IBM PC XT containing additional hardware which could emulate the behaviour of an IBM 3270 terminal...
featuring 3270 terminal emulation was released later in October 1983. XT stands for X-tended Technology.
Features
The XT originally came with 128 kB of RAMRandom-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
, a 360 kB double-sided 5.25 in (133.4 mm) full-height floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
drive, a 10 MB Seagate
Seagate Technology
Seagate Technology is one of the world's largest manufacturers of hard disk drives. Incorporated in 1978 as Shugart Technology, Seagate is currently incorporated in Dublin, Ireland and has its principal executive offices in Scotts Valley, California, United States.-1970s:On November 1, 1979...
ST-412 hard drive with Xebec 1210 MFM
Modified Frequency Modulation
Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...
controller, an Asynchronous Adapter (serial
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
card with 8250 UART
Universal asynchronous receiver/transmitter
A universal asynchronous receiver/transmitter, abbreviated UART , is a type of "asynchronous receiver/transmitter", a piece of computer hardware that translates data between parallel and serial forms. UARTs are commonly used in conjunction with communication standards such as EIA RS-232, RS-422 or...
) and a 130 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
power supply
Power supply unit (computer)
A power supply unit converts mains AC to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode power supply...
. The motherboard had eight 8-bit ISA
Industry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...
expansion slots, and an Intel 8088
Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...
microprocessor running at 4.77 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(with a socket for an 8087
Intel 8087
The Intel 8087, announced in 1980, was the first floating-point coprocessor for the 8086 line of microprocessors. It had 45,000 transistors and was manufactured as a 3 μm depletion load HMOS circuit. The 8087 was built to be paired with the Intel 8088 or 8086 microprocessors...
math coprocessor
Coprocessor
A coprocessor is a computer processor used to supplement the functions of the primary processor . Operations performed by the coprocessor may be floating point arithmetic, graphics, signal processing, string processing, or encryption. By offloading processor-intensive tasks from the main processor,...
); the operating system usually sold with it was PC DOS 2.0 and above. The eight expansion slots were an increase over the five in the IBM PC, although three were taken up by the floppy drive
Floppy disk controller
A floppy disk controller is a special-purpose chip and associated disk controller circuitry that directs and controls reading from and writing to a computer's floppy disk drive . This article contains concepts common to FDCs based on the NEC µPD765 and Intel 8072A or 82072A and their descendants,...
and hard drive adapter
Disk controller
The disk controller is the circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.Early disk controllers were identified by their storage methods and data encoding. They were typically implemented on a separate controller card...
s, and the Async card. The basic specification was soon upgraded to have 256 kB of RAM as standard. Slot 8 on the XT motherboard was wired slightly different than the other slots, making it incompatible with some cards. This was done for cards designed to allow the XT to be connected to IBM mainframes. Video card
Video card
A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...
s initially comprised the MDA
Monochrome Display Adapter
The Monochrome Display Adapter introduced in 1981 was IBM's standard video display card and computer display standard for the PC. The MDA did not have any pixel-addressable graphics modes...
and CGA
Color Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....
, with EGA
Enhanced Graphics Adapter
The Enhanced Graphics Adapter is the IBM PC computer display standard specification which is between CGA and VGA in terms of color and space resolution. Introduced in October 1984 by IBM shortly after its new PC/AT, EGA produces a display of 16 simultaneous colors from a palette of 64 at a...
and PGC
Professional Graphics Controller
Professional Graphics Controller was a graphics card manufactured by IBM for the PC. It consisted of three interconnected PCBs, and contained its own processor and memory....
becoming available in 1984.
There were two widely used configurations of the XT motherboard. The first could support up to 256 kB on the motherboard itself (four banks of 64 kB chips), with a maximum of 640 kB achieved by using expansion card
Expansion card
The expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus.One edge of the expansion card holds the contacts that fit exactly into the slot...
s. This was the configuration the XT originally shipped in. The second configuration—introduced in stock units in 1986—could support the whole 640 kB on the motherboard (two banks of 256 kB chips, two banks of 64 kB), had the later revision AT-compatible BIOS with a faster booting time, as well as support for 101-key keyboards and 3.5 in (88.9 mm) floppy drives. The earlier configuration could be adapted to "late" configuration after a couple of minor modifications.
There were also two or three revisions of the motherboard; however, there are only minor differences between them. Most notable is that the first revision is missing U90 and has some parts located at another place on the motherboard. The change occurred in the spring of 1984, when IBM stopped the production of submodel 087 and began producing the submodel 086 instead. However, the motherboard revision and the fact that 086 came with 256 kB RAM from the factory—whereas 087 came with only 128 kB RAM—are the only differences between the two first submodels.
Beginning in 1985, the XT was offered in floppy-only models without a hard drive (submodel 068 and 078). XTs with the 256-640 kB motherboard configuration came standard with half-height floppy drives in place of the full-height drives, as well as the option for a 20 MB ST-225 half-height hard drive and "enhanced" keyboard (essentially a Model M without the LED panel, and cross-compatibility between the AT and XT keyboard transfer protocol). The XT was discontinued in the spring of 1987, replaced by the PS/2
IBM Personal System/2
The Personal System/2 or PS/2 was IBM's third generation of personal computers. The PS/2 line, released to the public in 1987, was created by IBM in an attempt to recapture control of the PC market by introducing an advanced proprietary architecture...
Model 30.
Like the original PC, the XT came with BASIC in its ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
. Despite the lack of a cassette port on XTs, IBM's licensing agreement with Microsoft forced them to include BASIC on all their machines.
PC and XT keyboards are not compatible with those on more modern PCs (IBM AT or newer)—even with DIN to PS/2
PS/2 connector
The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987...
mini-DIN plug adapters—because PC/XT keyboards have a different transfer protocol from PC/AT keyboards, as well as using different keyboard scan codes. Some keyboards are switchable between the two interfaces for compatibility with either computer. However, the newer 101-key keyboard based on the Model M will work with modern PCs.
The XT had a desktop case similar to that of the PC. It weighed 32 pounds, and was approximately 19.5 inches wide by 16 inches deep by 5.5 inches high. The power supply was configured for 120 V AC only and could not be used with 240 V supplies.
IBM XT 286
In 1986, the XT 286 (IBM 5162) with a 6 MHz Intel 80286Intel 80286
The Intel 80286 , introduced on 1 February 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and 8088...
processor was introduced. However, this system turned out to be faster than the ATs of the time using 8 MHz 286 processors because the XT 286 had zero wait state
Wait state
A wait state is a delay experienced by a computer processor when accessing external memory or another device that is slow to respond.As of late 2011, computer microprocessors run at very high speeds, while memory technology does not seem to be able to catch up: typical PC processors like the Intel...
RAM, which could move data more quickly.