IBM Personal Computer XT
|Release date||March 8, 1983|
|Operating system||IBM BASIC / PC DOS 2.0-3.20 / PC/IX / SCO Xenix / Minix|
|CPU||Intel 8088 @ 4.77 MHz|
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, factory equipped with a hard drive. It was released as IBM Machine Type number 5160 on March 8, 1983, and essentially the same 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, though later floppy-only models would effectively replace the original model 5150 PC. A corresponding 3270 PC featuring 3270 terminal emulation was released later in October 1983. XT stands for eXtended Technology.
The XT originally came with 128 kB of RAM, a 360 kB double-sided 5.25 in (133 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 an Intel 8088 microprocessor running at 4.77 MHz, with a socket for an 8087 math coprocessor.
The eight 8-bit ISA expansion slots were an increase over the five in the IBM PC, although slots were taken up by the floppy drive and hard drive adapters, the Async serial port card, and nearly always a display adapter board. The basic specification was soon upgraded to have 256 kB of RAM as standard. Expansion slots could be used for I/O devices or for main memory expansion.
Slot 8 on the XT motherboard was wired with an additional signal CARD SLCTD (pin B8), making it incompatible with some cards. This signal was provided for cards used in the 3270-PC. The modified slot is found only on the 5160 and 5155 Portable PC. It was commonplace for ISA cards to include a jumper allowing them to work in Slot 8. Video cards initially comprised the MDA and CGA, with EGA and PGC becoming available in 1984.
PC and XT keyboards are not compatible with those on later 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. They had switches on the bottom, usually under one of the flip-up risers. 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 IBM PC. It weighed 32 pounds, and was approximately 19.5 inches wide by 16 inches deep by 5.5 inches high. The power supply of the original XT sold in the US was configured for 120 V AC only and could not be used with 240 V mains supplies. XTs with 240V-compatible power supplies were later sold in international markets. Both were rated at 130 Watts.
The operating system usually sold with the XT was PC DOS 2.0 and above. Like the original PC, the XT came with IBM 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.
The XT was discontinued in the spring of 1987, replaced by the PS/2 Model 30.
Revisions and variants
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 cards. 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 BIOS with a faster booting time, as well as support for 101-key keyboards and 3.5 in (89 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 with minor differences between them. The first version is missing U90 (logic added to prevent a race condition between the CPU and DMA), and has some parts located at another place on the motherboard.. The change occurred in the spring of 1984 with Model 086 replacing the earlier Model 087. The motherboard revision and that Model 086 came with the full 256 kB RAM (instead of only 128 kB RAM) from the factory were the only differences between the two first Models.
Beginning in 1985, the XT was offered in floppy-only models without a hard drive (Model 068 and 078). In 1986, XTs with the "Late" 256-640 kB motherboard configuration became available. These 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).
Model 788 was the only XT with color display support.
Models 568, 588, and 589 were used as basis for the XT/370; they had an additional (co-)processor board that could execute System/370 instructions. An XT-based machine with a Series/1 co-processor board existed as well, but it had its own System Unit number, the IBM 4950.
5161 expansion unit
The 5161 was an expansion chassis that used an identical case and power supply as the XT, but had a different system board with 8 card slots and no microprocessor. The 5161 came with a 10 MB hard drive, and had room for another one. The 5161 was connected to the 5160 using an Extender Card in the system unit and a Receiver Card in the Expansion Unit, connected by a custom cable. The Expansion Unit could also contain extra memory. The Extender card inserted wait states for memory on the Expansion Unit. The 5161 could be connected to either an XT or to the earlier 5150 (the original IBM PC). This expansion feature was not available in later machines like the IBM PC AT.
IBM XT 286
In 1986, the XT 286 (IBM 5162) with a 6 MHz Intel 80286 processor was introduced. However, this system turned out to be faster than the ATs of the time using 6 MHz 286 processors because the XT 286 had zero wait state RAM, which could move data more quickly.:95 It came with standard 640 KB RAM on its system board (128 KB actually on board, plus 2 x 256 KB SIMMs) and with an AT-style 1.2 MB high density diskette plus a 20 MB hard disk. Despite these features an Infoworld review from March 1987 declared it a poor market value.
The XT 286 used a 157 Watt power supply, which could internally switch between 115 or 230 V AC operation.
- Scott Mueller, "Upgrading and Repairing PCs 2nd Edition" Que Books, 1992 ISBN 0-88022-856-3, pp. 59-79
- Personal Computer Family Service Information Manual (January 1989), IBM document SA38-0037-00, page 6-2
- Personal Computer Family Service Information Manual (January 1989), IBM document SA38-0037-00, page 14-1
- Personal Computer Family Service Information Manual (January 1989), IBM document SA38-0037-00, pages 7-1 to 7-3
- Personal Computer Family Service Information Manual (January 1989), IBM document SA38-0037-00, pages 8-1 to 8-2
- InfoWorld Media Group, Inc. (23 March 1987). InfoWorld. InfoWorld Media Group, Inc. p. 49. ISSN 01996649.
- IBM (1983). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT. IBM Part Number 6936831.
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IBM Personal Computer
|IBM Personal Computers||Succeeded by
IBM Personal Computer AT