IBM Personal Computer XT
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.
|Also known as||IBM XT, PC XT, 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|
|Memory||128–640 KB RAM|
|Predecessor||IBM Personal Computer|
|Successor||IBM Personal Computer/AT|
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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. 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. 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.
When IBM announced a successor product to the PC in early 1983, initial speculations were that it would be a next-generation machine possibly based on the Intel 8086 or include other advanced features. When the XT was actually unveiled however, there was mild disappointment that the new machine was merely an incremental improvement of the PC based on the same 8088 CPU and would in fact not replace it at all. A BYTE Magazine article commented that "DOS 2.0 is more revolutionary and advanced than the computer itself."
A Seagate ST-412 hard disk was standard equipment, and the XT was not offered in a floppy-only model for its first two years on the market, although the standard ribbon cable with two floppy connectors was still included. The only way to purchase an XT with factory-installed dual floppy drives was if the user also bought the optional 5161 expansion chassis and placed the hard disk in that, which in effect amounted to purchasing two hard disks as the 5161 also came with one standard. Unlike many hard disk systems on microcomputers at the time, the XT was able to boot directly off the drive and did not require a boot floppy. Aside from the hard disk, a serial port card was also standard equipment on the XT, all other cards being optional.
By the end of 1983, the XT was neck-and-neck with the original PC for sales and IBM were selling every one that they made.
The XT had eight slots. Two were behind the floppy drive, and shorter than the original PC's slots. The other six fit into the same space as the original PC's five slots. Most PC cards would not fit into the two short slots, and some would not fit into the six standard-length, but narrower, slots, particularly cards with double boards on them. 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. Available Video cards were initially 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 (15 kg) and was approximately 19.5 inches (500 mm) wide by 16 inches (410 mm) deep by 5.5 inches (140 mm) 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 240 V-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 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 variantsEdit
XT motherboards came in two different versions. The original (1983–85) had 64 KB of 4164 RAM (64kx1 chips) socketed on it with further 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 1983-84 shipped from the factory with 128 KB and in 1985, 256k. The second version (1986–87) had 256 KB socketed on it and could accommodate the entire 640 KB. Later XTs used 4164 DRAMs only for the first 256k and the remainder of system memory consisted of larger 41256 DRAMs (256kx1). As a result, it took only 44 RAM chips to reach 640 kB versus the 80 chips needed on the original model XT.
There were also two or three revisions of the motherboard with minor differences between them. The first version (Model 084) incorporates a 470 ohm resistor to fix a race condition between the CPU and DMA controller which created the possibility of the system locking up. In the spring of 1984, a revised motherboard (Model 083) was introduced which rearranged some components and also incorporated a better solution to the DMA race condition by utilizing a 74LS44 gate (most XT clones instead used a group of 74LS04 gates), however most of the remaining Model 084 motherboards were modified for this before the revised Model 083 appeared. 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. 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 IBM Portable PC 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 In 1986, the 256–640 KB 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.
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.
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 256–640 KB 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.
The XT BIOS added a few technical improvements over the 5150 BIOS, including displaying a memory count during the POST instead of a blank screen. In the interest of faster booting (a fully loaded 5150 with 640 KB could take over half a minute to complete the POST memory scan), the XT BIOS only attempts to read one bit each from the leftmost chip in each bank and if so, assumes that bank is filled.
While the 5150 had two DIP switch blocks, one for RAM size and the other for system configuration, the XT has one DIP block and just three possible RAM settings (128 kB, 128-256 kB, and greater than 256 kB).
5161 expansion unitEdit
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). This expansion feature was not available for later machines like the IBM PC AT and in fact the 5161 is completely incompatible with anything but an 8088-based PC.
IBM XT 286Edit
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.:95 It came with standard 640 KB RAM on its system board (128 KB actually on board, plus two 256 KB SIMMs) and with an AT-style 1.2 MB high density diskette drive 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.
Byte in November 1983 stated that DOS 2.0 had changed more compared to its predecessor than the XT had. The magazine advised consumers to make sure that expansion cards fit the computer's shorter and narrower slots, and concluded that the XT was "competitively priced" and "a solidly engineered, although technically modest, encore to the" IBM PC.
- Scott Mueller, "Upgrading and Repairing PCs 2nd Edition" Que Books, 1992 ISBN 0-88022-856-3, pp. 59-79
- Dievendorff, Dick (1981). IBM Personal Computer Questions and Answers. IBM. pp. 9–10.
- Archer, Rowland Jr. (November 1983). "The IBM PC XT and DOS 2.00". BYTE. pp. 294–304. Retrieved 19 March 2016.
- Personal Computer Family Service Information Manual (January 1989), IBM document SA38-0037-00, page 6-2
- "Purpose of U90 in XT second revision board". Vintage Computer Forum. Retrieved 2017-08-06.
- 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
- The AT Clone from IBM, PC Magazine, January 13, 1987
- InfoWorld Media Group, Inc. (23 March 1987). InfoWorld. InfoWorld Media Group, Inc. p. 49. ISSN 0199-6649.
- IBM (1983). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT. IBM Part Number 6936831.