IBM Personal Computer XT

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 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] InfoWorld observed in 1985 that "by the time you've installed all the 'extras', which are really necessities, you're lucky if you have even one slot left open";[3] almost every PC requires 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.[4] 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.[1] XTs with 240 V-compatible power supplies were later sold in international markets. Both were rated at 130 watts.[5]

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.

A striking but largely undocumented feature of the IBM XT, was that the motherboard could be configured for several different memory-chip types. By changing some jumpers, you could choose between 16Kb chips, 64Kb chips, 128Kb chips, and first half 256Kb plus second half 64Kb chips. Machines sold from IBM initially only used the standard 64Kb configuration for a maximum of 256KB onboard, however in 1986 stock machines configured for the 64Kb/256Kb combo and a maximum 640Kb onboard were sold. The machines with the alternate configuration were marketed as "256K-640K" boards to distinguish them from the "64K-256K" boards, allthough the two board "versions" are technically similar in almost every way except for the configuration and type of installed memory. Upgrading the older boards to "256K-640K" boards is as simple as changing some jumpers, adding an IC to an empty socket, and replacing the first two banks of RAM chips with 256Kb parts.

There were however two or three revisions of the motherboard. The differences between these are more subtle, and serves to fix minor hardware-issues. The first revision (used for submodel 084) solves a race-condition between the CPU and DMA controller using a time-delay circuit.[6] This revision is notable for having an empty IC-designator labled "SPARE" just next to the memory. In the spring of 1984 the second revision (used in submodel 083) was introduced, which rearranged some components and also incorporated a better solution to the DMA race condition. This new solution did away with the time-delay and instead included IC "U90" in place of the old "SPARE". At this time, most of the remaining first-revision boards were modified into second-revision boards using patch-wires. Otherwise, from a marketing standpoint, the only real difference a consumer would notice between submodels 084 and 083 was that they came from the factory with different amount of RAM installed (128KB vs the full onboard 256KB respectively). All later "64KB-256KB" boards would use the second revision motherboard, the same as submodel 083. When it comes to the submodels using the "256KB-640KB" board, they might use a third revision of the motherboard. The only difference between this third and the second revision would be added support for a full-size 64KB BIOS (the older boards used a 40KB BIOS).

Beginning in 1985, the XT was offered in floppy-only models without a hard drive (submodel 068 and 078) and the new Enhanced Graphics Adapter and Professional Graphics Adapter became available as video card options[1] In 1986, the 256–640 KB motherboard models were launched, which switched to half-height drives and submodels 268, 278 and 089 came with 101-key keyboards (essentially the IBM Model M, but in a modified variant that used the XT's keyboard protocol and lacked LEDs). Submodels 267, 277 and 088 had the original keyboard. 3.5" floppy drives became available and 20MB Seagate ST-225 hard disks in 5.25" half-height size replaced the full-height 10MB drives.

IBM PC XT 5160 Dual Floppy Drives with 5153 Color Monitor

. Subodel 788 was the only XT sold with the Color Graphics Adapter as a standard feature.[5]

Submodels 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.[5] 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.[7]

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.

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).[8] 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.

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 two 256 KB SIMMs) and with an AT-style 1.2 MB high density diskette drive plus a 20 MB hard disk.[9][10] Despite these features, an InfoWorld review from March 1987 declared it a poor market value.[11]

The XT 286 used a 157 watt power supply, which could internally switch between 115 or 230 V AC operation.[9]

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.[4]

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

Notes

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