IBM Personal Computer XT

IBM Personal Computer XT

Also known as	IBM XT, PC XT, XT
Type	Personal computer
Release date	March 8, 1983 (1983-03-08)
Discontinued	April 1987 (1987-04)
Operating system	IBM BASIC / PC DOS 2.0-3.20 / PC/IX / SCO Xenix / Minix
CPU	Intel 8088 @ 4.77 MHz
Memory	128–640 KB RAM

The IBM Personal Computer XT, often shortened to the IBM XT, PC XT, or simply XT, is a version of the IBM PC with a built-in hard drive. It was released as IBM Machine Type number 5160 on March 8, 1983. Apart from the hard drive, it was essentially the same as the original PC, with only minor improvements. The XT was mainly intended as an enhanced IBM PC for business users. 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.

Features

The IBM Personal Computer XT originally came with 128 KB of RAM, a 360 KB double-sided 5¼ inch floppy disk drive, a 10 MB Seagate ST-412 hard drive with Xebec 1210 Modified Frequency Modulation (MFM) controller, an Asynchronous Adapter (serial card with 8250 UART), and a 130-watt power supply.^[1] The motherboard had an Intel 8088 microprocessor running at 4.77 MHz, with a socket for an optional 8087 math coprocessor.

IBM recognized soon after the IBM PC's release in 1981 that its five 8-bit "I/O channel" expansion slots (later named Industry Standard Architecture (ISA) by IBM's competitors) were insufficient. An internal IBM publication stated in October 1981 about the number that "In my opinion, it could be a problem", reporting that others within IBM advised swapping cards if necessary.^[2] Virtually every PC required at least a display adapter card and a floppy disk controller card, leaving only three slots available for a parallel printer port card (if the Color Graphics Adapter was used), a serial port card, memory expansion boards, a 3rd-party hard disk controller card, a second display adapter card, or possible other special adapter cards. The XT had eight slots, although the floppy and hard drive adapters, the serial port card, and nearly always a display adapter board occupied slots. 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 memory expansion. Video cards initially comprised the Monochrome Display Adapter (MDA) and Color Graphics Adapter (CGA) (the same two cards that were available for the type 5150 PC), with Enhanced Graphics Adapter (EGA) and Professional Graphics Controller (PGC) becoming available in 1984.

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.^[1] XTs with 240V-compatible power supplies were later sold in international markets. Both were rated at 130 Watts.^[3]

The operating system usually sold with the XT was PC DOS 2.0 or, by the time the XT was discontinued in early 1987, DOS 3.2. 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 PCs, and the BASICA program that was included with DOS depended on the BASIC ROM. The XT BIOS also displayed a memory count during the POST, unlike the PC.

The XT was discontinued in the spring of 1987, replaced by the PS/2 Model 30.

Revisions and variants

XT motherboards came in two different versions. The original (1983–1985) had 64 KB of RAM soldered to it with sockets to support up to 256 KB and any more RAM had to be put on an expansion card, of which the AST Research Six Pak was the most widespread and popular. XTs produced in 1983-1984 shipped from the factory with 128 KB and in 1985, 256k. The second version (1986–1987) had 256 KB soldered to it and could accommodate the entire 640 KB.^[1]

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.^[4] 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. Unlike the IBM PC motherboard, the XT lacked the trimmer capacitor used to adjust the NTSC color burst for CGA video cards or the second set of DIP switches used to configure memory size. Slot 8 (the one closest to the power supply) was wired slightly differently for cards used to connect IBM 3270 terminals. As a result, some ISA cards will not work in Slot 8 and others included a DIP switch to accommodate it. This feature is unique to the XT and not found on the IBM PC, AT, or any clones.

Beginning in 1985, the XT was offered in floppy-only models without a hard drive (Model 068 and 078) and the new Enhanced Graphics Adapter and Professional Graphics Adapter became available as video card options^[1] In 1986, the 256–640KB motherboard models were launched, which switched to half-height drives and 101-key keyboards (essentially the IBM Model M, but in a modified variant that used the XT's keyboard protocol and lacked LEDs). 3.5" floppy drives became available and 20 MB Seagate ST-225 hard disks in 5.25" half-height size replaced the full-height 10 MB drives.

Model 788 was the only XT with color display support.^[3]

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.^[3] 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.^[5]

The XT had three BIOS revisions; the original dated November 8, 1982, the second dated January 10, 1986, and the third dated May 9, 1986. The latter two were the versions found on 256–640KB motherboards and added support for 3.5" floppies, 101 key keyboards, and a few other minor things such as a joystick input function (INT 15h Function 84h) and faster POST. The May 1986 BIOS was identical to the January 1986 revision except for bug fixes pertaining to keyboard input. XTs with the 1982 BIOS may be upgraded to the newer revision.

5161 expansion unit

The 5161 was an expansion chassis that used an identical case and power supply as the XT but had instead of a system board a backplane 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, but the Extender card inserted wait states for memory in the Expansion Unit, so it was preferable to install memory onto the XT system board or on memory cards in slots plugged into the XT system board directly. The 5161 could be connected to either an XT or to the earlier 5150 (the original IBM PC).^[6] This expansion feature was not available for later machines like the IBM PC AT.

IBM XT 286

The IBM XT came with documentation giving the schematic diagrams, BIOS listing, character set, specifications, and other detailed technical information.

In 1986, the XT 286 (IBM 5162) with a 6 MHz Intel 80286 processor was introduced. Despite being marketed as a lower-tier model than the IBM AT, this system actually ran many applications faster than the ATs of the time with 6 MHz 286 processors, because the XT 286 had zero wait state RAM, which could move data more quickly.^[1]: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 drive plus a 20 MB hard disk.^[7][8] Despite these features, an Infoworld review from March 1987 declared it a poor market value.^[9]

The XT 286 used a 157 Watt power supply, which could internally switch between 115 or 230 V AC operation.^[7]

See also

• Amiga Sidecar
• PC-based IBM-compatible mainframes#Personal Computer XT/370

References

Notes

External links

Wikimedia Commons has media related to IBM XT.

• IBM 5160 information at www.minuszerodegrees.net

Photo galleries:

• XT with 256 KB on system board
• XT 286