In this example, the system queried shows a BIOS date of 01/22/97.

XT Technical Specifications

Technical information for the XT system, described in this section, provides information about the system architecture, memory configurations and capacities, standard system features, disk storage, expansion slots, keyboard specifications, and also physical and environmental specifications. Figure 23.6 shows the layout and components on the XT motherboard.

FIG. 23.6  The XT motherboard.

System Architecture
Microprocessor	8088
Clock speed	4.77MHz
Bus type	ISA (Industry Standard Architecture)
Bus width	8-bit
Interrupt levels	8 (6 usable)
Type	Edge-triggered
Shareable	No
DMA channels	4 (3 usable)
Bus masters supported	No
Upgradable processor complex	No
Memory
Standard on system board	256K or 640K
Maximum on system board	256K or 640K
Maximum total memory	640K
Memory speed (ns) and type	200ns dynamic RAM chips
System board memory-socket type	16-pin DIP
Number of memory-module sockets	36 (4 banks of 9)
Memory used on system board	36 64Kx1-bit DRAM chips in 4 banks of 9, or 2 banks of 9 256Kx1-bit and 2 banks of 9 64Kx1-bit chips
Memory cache controller	No
Wait states:
System board	1
Adapter	1
Standard Features
ROM size	40K or 64K
ROM shadowing	No
Optional math coprocessor	8087
Coprocessor speed	4.77MHz
Standard graphics	None standard
RS232C serial ports	1 (some models)
UART chip used	NS8250B
Maximum speed (bits per second)	9,600 bps
Maximum number of ports supported	2
Pointing device (mouse) ports	None standard
Parallel printer ports	1 (some models)
Bi-directional	No
Maximum number of ports supported	3
CMOS real-time clock (RTC)	No
CMOS RAM	None
Disk Storage
Internal disk and tape drive bays	2 full-height or 4 half-height
Number of 3 1/2 or 5 1/4-inch bays	0/2 or 0/4
Standard floppy drives	1x360K
Optional floppy drives:
5 1/4-inch 360K	Optional
5 1/4-inch 1.2M	No
3 1/2-inch 720K	Optional
3 1/2-inch 1.44M	No
3 1/2-inch 2.88M	No
Hard disk controller included:	ST-506/412 (Xebec Model 1210)
ST-506/412 hard disks available	10/20M
Drive form factor	5 1/4-inch
Drive interface	ST-506/412
Drive capacity	10M	20M
Average access rate (ms)	85	65
Encoding scheme	MFM	MFM
BIOS drive type number	1	2
Cylinders	306	615
Disk storage
Heads	4	4
Sectors per track	17	17
Rotational speed (RPMs)	3600	3600
Interleave factor	6:1	6:1
Data transfer rate (kilobytes/second)	85	85
Automatic head parking	No	No
Expansion Slots
Total adapter slots	8
Number of long/short slots	6/2
Number of 8-/16-/32-bit slots	8/0/0
Available slots (with video)	4
Keyboard Specifications
101-key Enhanced keyboard	Yes
Fast keyboard speed setting	No
Keyboard cable length	6 feet
Physical Specifications
Footprint type	Desktop
Dimensions:
Height	5.5 inches
Width	19.5 inches
Depth	16.0 inches
Weight	32 pounds
Environmental Specifications
Power-supply output	130 watts
Worldwide (110v/60Hz, 220v/50Hz)	No
Auto-sensing/switching	No
Maximum current:
90-137 VAC	4.2 amps
Operating range:
Temperature	60-90 degrees F
Relative humidity	8-80 percent
Maximum operating altitude	7,000 feet
Heat (BTUs/hour)	717
Noise (Average db, operating, 1m)	56
FCC classification	Class B

Table 23.6 shows the XT motherboard switch settings. The XT motherboard uses a single eight-position switch block to control various functions, as detailed in the table.

Table 23.6  IBM PC/XT Motherboard Switch Settings

SWITCH BLOCK 1 (PC and XT)

	Switch 1	IBM PC Function (PC Only):
	Off	Boot From Floppy Drives
	On	Do Not Boot From Floppy Drives
	Switch 1	IBM XT Function (XT Only):
	Off	Normal POST (Power-On Self Test)
	On	Continuous Looping POST
	Switch 2	Math Coprocessor (PC/XT):
	Off	Installed
	On	Not Installed
Switch 3	Switch 4	Installed Motherboard Memory (PC/XT):
On	On	Bank 0 only
Off	On	Banks 0 and 1
On	Off	Banks 0, 1, and 2
Off	Off	All 4 Banks
Switch 5	Switch 6	Video Adapter Type (PC/XT):
Off	Off	Monochrome (MDA)
Off	On	Color (CGA); 40x25 mode
On	Off	Color (CGA); 80x25 mode
On	On	Any Video Card w/onboard BIOS (EGA/VGA)
Switch 7	Switch 8	Number of Floppy Drives (PC/XT):
On	On	1 floppy drive
Off	On	2 floppy drives
On	Off	3 floppy drives
Off	Off	4 floppy drives

Table 23.7 shows the part numbers of the XT system units.

Table 23.7  IBM XT Model Part Numbers

Description	Number
XT system unit/83-key keyboard, 256K:
one full-height 360K drive	5160068
one half-height 360K drive	5160267
two full-height 360K drives	5160078
two half-height 360K drives	5160277
XT system unit/101-key keyboard, 256K:
one half-height 360K drive	5160268
two half-height 360K drives	5160278
XT system unit/83-key keyboard, 256K, one serial,
one full-height 360K drive, 10M hard disk	5160086
XT system unit/83-key keyboard, 640K, one serial,
one half-height 360K drive, 20M fixed disk	5160088
XT system unit/101-key keyboard, 640K, one serial,
one half-height 360K drive, 20M fixed disk	5160089
Option Numbers
PC expansion-unit Model 002, 20M fixed disk	5161002
20M fixed disk drive	6450326
20M fixed disk adapter	6450327
10M fixed disk drive	1602500
10M fixed disk adapter	1602501
5 1/4-inch, half-height, 360K drive	6450325
5 1/4-inch, full-height, 360K drive	1503810
3 1/2-inch, half-height, 720K internal drive	6450258
3 1/2-inch, half-height, 720K external drive	2683190
8087 math coprocessor option	1501002
Asynchronous serial adapter	1502074
Enhanced Keyboard Accessories
Clear keycaps (60) with paper inserts	6341707
Blank light keycaps	1351710
Blank dark keycaps	1351728
Paper inserts (300)	6341704
Keycap-removal tools (6)	1351717

An Introduction to the Portable PC

IBM introduced the Portable PC on February 16, 1984. The IBM Portable PC, a "transportable" personal computer, is a small suitcase-sized system that has a built-in 9-inch, amber composite video monitor; one 5 1/4-inch, half-height floppy disk drive (with space for an optional second drive); an 83-key keyboard; two adapter cards; a floppy disk controller; and a CGA. The unit also has a universal-voltage power supply capable of overseas operation on 220-volt power. Figure 23.7 shows the Portable PC exterior.

FIG. 23.7  The IBM Portable PC

The system board used in the IBM Portable PC is the same board used in the original IBM XTs, with 256K of memory. Because the XT motherboard was used, eight expansion slots are available for the connection of adapter boards, although only two slots can accept a full-length adapter card due to internal space restrictions. The power supply is basically the same as an XT's, with physical changes for portability and a small amount of power drawn to run the built-in monitor. In function and performance, the Portable PC system unit has identical characteristics to an equivalently configured IBM PC XT system unit. Figure 23.8 shows the Portable PC interior view.

IBM withdrew the Portable PC from the market on April 2, 1986, a date that coincides with the introduction of the IBM Convertible laptop PC. The Portable PC is rare because not many were sold, although it compared to, and in many ways was better than, the highly successful Compaq Portable that was available at the time. The system was largely misunderstood by the trade press and user community. Most did not understand that the system was really a portable XT and had more to offer than the standard IBM PC. Maybe if IBM had called the system the Portable XT, it would have sold better!

FIG. 23.8  The IBM Portable PC interior.

The Portable PC system unit has these major functional components:

• Intel 8088 4.77MHz microprocessor
  
  
• ROM-based diagnostics (POST)
  
  
• BASIC language interpreter in ROM
  
  
• 256K of dynamic RAM
  
  
• Eight expansion slots (two long slots, one 3/4-length slot, and five short slots)
  
  
• Socket for 8087 math coprocessor
  
  
• Color/Graphics Monitor Adapter
  
  
• 9-inch, amber, composite video monitor
  
  
• Floppy disk interface
  
  
• One or two half-height 360K floppy drives
  
  
• 114-watt universal power supply (115-230V, 50-60Hz)
  
  
• Lightweight 83-key keyboard
  
  
• Enclosure with carrying handle
  
  
• Carrying bag for the system unit

Seen previously, Figure 23.6 showed the XT motherboard, which is also used in the Portable PC. The following is the technical data for the Portable PC system:

System Architecture
Microprocessor	8088
Clock speed	4.77MHz
Bus type	ISA (Industry Standard Architecture)
Bus width	8-bit
Interrupt levels	8 (6 usable)
Type	Edge-triggered
Shareable	No
DMA channels	4 (3 usable)
Bus masters supported	No
Upgradable processor complex	No
Memory
Standard on system board	256K
Maximum on system board	256K
Maximum total memory	640K
Memory speed (ns) and type	200ns dynamic RAM chips
System board memory-socket type	16-pin DIP
Number of memory-module sockets	36 (4 banks of 9)
Memory used on system board	36 64Kx1-bit DRAM chips in 4 banks of 9 chips
Memory cache controller	No
Wait states:
System board	1
Adapter	1
Standard Features
ROM size	40K
ROM shadowing	No
Optional math coprocessor	8087
Coprocessor speed	4.77MHz
Standard graphics	CGA adapter with built-in 9-inch amber CRT
RS232C serial ports	None standard
UART chip used	NS8250B
Maximum speed (bits per second)	9,600 bps
Maximum number of ports supported	2
Pointing device (mouse) ports	None standard
Parallel printer ports	None standard
Bi-directional	No
Maximum number of ports supported	3
CMOS real-time clock (RTC)	No
CMOS RAM	None
Disk Storage
Internal disk and tape drive bays	2 half-height
Number of 3 1/2-/5 1/4-inch bays	0/2
Standard floppy drives	1 or 2x360K
Optional floppy drives:
5 1/4-inch 360K	Optional
5 1/4-inch 1.2M	No
3 1/2-inch 720K	Optional
3 1/2-inch 1.44M	No
3 1/2-inch 2.88M	No
Hard disk controller included	None
Expansion Slots
Total adapter slots	8
Number of long/short slots	2/6
Number of 8-/16-/32-bit slots	8/0/0
Available slots (with video)	6
Keyboard Specifications
101-key Enhanced keyboard	No
Fast keyboard speed setting	No
Keyboard cable length	6 feet
Physical Specifications
Footprint type	Desktop
Dimensions:
Height	8.0 inches
Width	20.0 inches
Depth	17.0 inches
Weight	31 pounds
Environmental Specifications
Power-supply output	114 watts
Worldwide (110/60,220/50)	Yes
Auto-sensing/switching	No
Maximum current:
90-137 VAC	4.0 amps
Operating range:
Temperature	60-90° F
Relative humidity	8-80 percent
Maximum operating altitude	7,000 feet
Heat (BTUs/hour)	650
Noise (Average db, operating, 1m)	42
FCC classification	Class B

Table 23.8 shows the part numbers for the Portable PC.

Table 23.8  IBM Portable PC Model Part Numbers

Description	Number
256K, one 360K half-height drive	5155068
256K, two 360K half-height drives	5155076
Half-height 360K floppy disk drive	6450300

The disk drive used in the Portable PC was a half-height drive, the same unit specified for use in the PCjr. When the Portable PC was introduced, PCjr was the only other IBM sold with the same half-height drive.

An Introduction to the AT

IBM introduced the Personal Computer AT (Advanced Technology) on August 14, 1984. The IBM AT system included many features previously unavailable in IBM's PC systems such as increased performance, an advanced 16-bit microprocessor, high-density floppy disk and hard disk drives, larger memory space, and an advanced coprocessor. Despite its new design, the IBM AT incredibly retained compatibility with most existing hardware and software products for the earlier systems.

In most cases, IBM AT system performance was from three to five times faster than the IBM XT for single applications running DOS on both computers. The performance increase is due to the combination of a reduced cycle count for most instructions by the 80286 processor, an increased system clock rate, 16-bit memory, and faster hard disk and controller.

The AT system unit has been available in several models: a floppy-disk-equipped base model (068) and several hard-disk-enhanced models. Based on a high-performance, 16-bit, Intel 80286 microprocessor, each computer includes Cassette BASIC language in ROM and a CMOS (Complementary Metal Oxide Semiconductor) clock and calendar with battery backup. All models are equipped with a high-density (1.2M) floppy disk drive, a keyboard, and a lock. For standard memory, the base model offers 256K, and the enhanced models offer 512K. In addition, the enhanced models have a 20M or a 30M hard disk drive and a serial/parallel adapter. Each system can be expanded through customer-installable options. You can add memory (up to 512K) for the base model by adding chips to the system board. You can expand all models to 16M by installing memory cards.

Besides the standard drives included with the system, IBM only offered two different hard disks as upgrades for the AT:

• 30M hard disk drive
  
  
• 20M hard disk drive

IBM also offered only three different types of floppy drives for the AT:

• A second, high-density (1.2M) floppy disk drive
  
  
• A double-density (320/360K) floppy disk drive
  
  
• A new 3 1/2-inch 720K drive

The original 068 and 099 models of the AT did not support the 720K drive in the BIOS; you had to add a special driver (DRIVER.SYS--supplied with DOS) for the drive to work properly. The later model ATs supported the 720K drive directly in the BIOS, and also added support for the 1.44M high-density 3 1/2-inch floppy drive, although IBM never sold or officially supported such a drive in the AT.

You can install as many as two floppy disk drives and one hard disk drive or one floppy disk drive and two hard disk drives in the system unit. To use the high-density 5 1/4-inch floppy disk drives properly, you must have special floppy disks--5 1/4-inch, high-coercivity, double-sided, soft-sectored disks. Due to track width problems between the high-density (1.2M) drives and the double-density (360K) drives, a double-density floppy disk drive (320/360K) was available for compatibility with the standard PC or XT systems. You can exchange disks reliably between the 1.2M and the standard 360K drives if you use the proper method and understand the recording process. For transferring data between a system with a 1.2M drive to a system with a 360K drive, you must start with a blank (never previously formatted) 360K disk, which must be formatted and written only by the 1.2M drive. No special precautions are needed to transfer the data the other way. This information is covered in Chapter 13, "Floppy Disk Drives." For complete interchange reliability and to simplify the process however, IBM recommends you purchase the 360K drive.

The AT motherboard has eight slots that support cards for additional devices, features, or memory. Six slots support the advanced 16-bit or 8-bit cards. Two slots support only 8-bit cards. All system-unit models use one 16-bit slot for the fixed disk and floppy disk drive adapter. The enhanced models use an additional 8-bit slot for the serial/parallel adapter. The result is seven available expansion slots for the base model and six available expansion slots for enhanced models. Figure 23.9 shows the interior of an AT system unit.

All AT models include a 192-watt universal power supply; a temperature-controlled, variable-speed cooling fan; and a security lock with key. The user selects the power supply for a country's voltage range. The cooling fan significantly reduces the noise in most environments; the fan runs slower when the system unit is cool and faster when the system unit is hot. When the system is locked, no one can remove the system-unit cover, boot the system, or enter commands or data from the keyboard, thereby enhancing the system's security.

FIG. 23.9  The IBM AT unit interior.

The keyboard is attached to the system unit by a 9-foot coiled cable that enables the AT to adapt to a variety of workspace configurations. The keyboard includes key-location enhancements and mode indicators for improved keyboard usability. Figure 23.10 shows the rear panel of an AT.

FIG. 23.10  The IBM AT rear panel.

Every system unit for the AT models has these major functional components:

• Intel 80286 (6MHz or 8MHz) microprocessor
  
  
• Socket for 80287 math coprocessor
  
  
• Eight I/O expansion slots (six 16-bit, two 8-bit)
  
  
• 256K of dynamic RAM (base model)
  
  
• 512K of dynamic RAM (enhanced models)
  
  
• ROM-based diagnostics (POST)
  
  
• BASIC language interpreter in ROM
  
  
• Hard/floppy disk controller
  
  
• 1.2M hard disk floppy drive
  
  
• 20M or 30M hard disk drive (enhanced models)
  
  
• Serial/parallel interface (enhanced models)
  
  
• CMOS Clock-calendar and configuration with battery backup
  
  
• Keylock
  
  
• 84-key keyboard or Enhanced, 101-key keyboard (standard on newer models)
  
  
• Switchable worldwide power supply

AT Models and Features

Since the introduction of the AT, several models have become available. First, IBM announced two systems: a base model (068) and an enhanced model (099). The primary difference between the two systems is the standard hard disk that came with the enhanced model. IBM has introduced two other AT systems since the first systems, each offering new features.

The first generation of AT systems has a 6MHz system clock that dictates the processor cycle time. The cycle time, the system's smallest interval of time, represents the speed at which operations occur. Every operation in a computer takes at least one or (usually) several cycles to complete. Therefore, if two computers are the same in every way except for the clock speed, the system with the faster clock rate executes the same operations in a shorter time proportional to the difference in clock speed. Cycle time and clock speed are two different ways of describing the same thing. Discussions of clock speed are significant when you consider buying the AT because not all models have the same clock speed.

All models of the AT included a combination hard/floppy disk controller that was really two separate controllers on the same circuit board. The board was designed by IBM and Western Digital (WD), and manufactured for IBM by WD. This controller had no on-board ROM BIOS like the Xebec hard disk controller used in the XT. In the AT, IBM built full support for the hard disk controller directly into the motherboard ROM BIOS. To support different types of hard disks, IBM encoded a table into the motherboard ROM that listed the parameters of various drives that could be installed. In the first version of the AT, with a ROM BIOS dated 01/10/84, only the first 14 types in the table were filled in. Type 15 itself was reserved for internal reasons, and was not usable. Other table entries from 16 through 47 were left unused and were actually filled with zeros. Later versions of the AT added new drive types to the tables, starting from Type 16 and up.

The first two AT models were the 068 (base) model, which had 256K on the motherboard and a single 1.2M floppy disk drive; and the model 099 (enhanced), which had a 20M hard disk drive, a serial/parallel adapter, and 512K on the motherboard. IBM designated the motherboard on these computers as Type 1, which is larger than the later Type 2 board and used an unusual memory layout. The memory is configured as four banks of 128K chips--a total of 512K on the board. This configuration sounds reasonable until you realize that a 128K chip does not really exist in the physical form factor that IBM used. IBM actually created this type of memory device by stacking one 64K chip on top of another 64K chip and soldering the two together. My guess is that IBM had many 64K chips to use, and the AT was available to take them.

On October 2, 1985, IBM announced a new model of the AT, the Personal Computer AT Model 239. The system has all the standard features of the AT Model 099, but also has a 30M hard disk rather than a 20M hard disk. A second, optional 30M hard disk drive expands the Model 239's hard disk storage to 60M. This unit's motherboard, a second-generation design IBM calls Type 2, is about 25 percent smaller than the Type 1 but uses the same mounting locations for physical compatibility. All important items, such as the slots and connectors, remain in the same locations. Other major improvements in this board are in the memory. The 128K memory chips were replaced by 256K devices. Now only two banks of chips were needed to get the same 512K on the board.

The AT Model 239 includes these items:

• 512K of RAM (standard)
  
  
• 6MHz Type 2 motherboard with 256K memory chips
  
  
• Serial/parallel adapter (standard)
  
  
• 30M hard disk (standard)
  
  
• New ROM BIOS (dated 06/10/85). ROM supports 3 1/2-inch 720K floppy drives without using external driver programs, and 22 usable hard disk types (up to Type 23), including the supplied 30M disk. POST "fixes" clock rate to 6MHz.

The Type 2 motherboard's design is a big improvement on Type 1's; the Type 2 mother- board improved internal-circuit timing and layout. Improvements in the motherboard indicated that the system would be pushed to higher speeds--exactly what happened with the next round of introductions.

In addition to obvious physical differences, the Model 239 includes significantly different ROM software from the previous models. The new ROM supports more types of hard and floppy disks, and its new POST prevents alteration of the clock rate from the standard 6MHz models. Because support for the 30M hard disk is built into the new ROM, IBM sold a 30M hard disk upgrade kit that included the new ROM for the original AT systems. This $1,795 kit represented the only legal way to obtain the newer ROM.

The 30M hard disk drive upgrade kit for the Personal Computer AT Models 068 and 099 included all the features in the 30M hard disk drive announced for the AT Model 239. The upgrade kit also had a new basic input-output subsystem (BIOS), essential to AT operation. The new ROM BIOS supports 22 drive types (compared to the original 14 in earlier ATs), including the new 30M drive. To support the 30M hard disk drive, a new diagnostics floppy disk and an updated guide-to-operations manual are shipped with this kit.

The 30M update kit includes these items:

• 30M hard disk drive
  
  
• Two new ROM BIOS modules
  
  
• Channel keeper bar (a bracket for the fixed disk)
  
  
• Data cable for the hard disk
  
  
• Diagnostics and Setup disk
  
  
• An insert to the AT guide-to-operations manual

Some people were upset initially that IBM had "fixed" the microprocessor clock to 6MHz in the new model, thereby disallowing any possible "hot rod" or overclocking modifications. Many people realized that the clock crystal on the AT models was socketed so that the crystal could be replaced easily by a faster one. More importantly, because the AT circuit design is modular, changing the clock crystal does not have repercussions throughout the rest of the system, as is the case in the PC and PC XT. For the price of a new crystal (from $1 to $30) and the time needed to plug it in, someone easily could increase an AT's speed by 30 percent, and sometimes more. Unfortunately, due to the POST in the newer model's BIOS, you no longer can implement a simple speedup alteration without also changing the ROM BIOS as well.

Many people believed that this change was made to prevent the AT from being "too fast" and therefore competing with IBM's minicomputers. In reality, the earlier motherboard was run intentionally at 6MHz because IBM did not believe that the ROM BIOS software and critical system timing was fully operational at a higher speed. Also, IBM used some components that were rated only for 6MHz operation, starting of course with the CPU. Users who increased the speed of their early computers often received DOS error messages from timing problems, and in some cases, total system lockups due to components not functioning properly at the higher speeds.

Many companies selling speedup kits sold software to help smooth over some of these problems, but IBM's official solution was to improve the ROM BIOS software and motherboard circuitry, and to introduce a complete new system running at the faster speed. If you want increased speed no matter what model you have, several companies used to sell clock-crystal replacements that were frequency synthesizers rather than a fixed type of crystal. The units can wait until the POST is finished and change midstream to an increased operating speed. Unfortunately, I don't know of anyone who is still making or selling these upgrades.

If you really want to speed up your AT by installing a faster clock crystal, instructions on how to burn your own set of BIOS without the check can be found on the CD accompanying this book. However, it requires the use of a specialized PROM or EPROM burner, or access to one.

On April 2, 1986, IBM introduced the Personal Computer AT Models 319 and 339. These were the last and best AT models, and were an enhancement of the earlier Model 239. The primary difference from the Model 239 is a faster clock crystal that provides 8MHz operation. The Model 339 has a new keyboard--the Enhanced keyboard--with 101 keys rather than the usual 84. Model 319 is the same as Model 339, but includes the original keyboard.

Highlights of the Models 319 and 339 are shown in this list:

• Faster processor speed (8MHz)
  
  
• Type 2 motherboard, with 256K chips
  
  
• 512K of RAM (standard)
  
  
• Serial/parallel adapter (standard)
  
  
• 30M hard disk (standard)
  
  
• New ROM BIOS (dated 11/15/85). ROM support for 22 usable types (up to type 23) of hard disks, and 3 1/2-inch drives, at both 720K and 1.44M capacities. POST "fixes" clock rate to 8MHz.
  
  
• 101-key Enhanced keyboard (standard on Model 339)

The most significant physical difference in these new systems is the Enhanced keyboard on the Model 339. The keyboard, similar to a 3270 keyboard, has 101 keys. It could be called the IBM "corporate" keyboard because it is standard on all new desktop systems. The 84-key PC keyboard still was available, with a new 8MHz model, as the Model 319.

These new 8MHz systems were available only in an enhanced configuration with a standard 30M hard drive. If you wanted a hard disk larger than IBM's 30M, you could either add a second drive or simply replace the 30M unit with something larger.

ROM support for 3 1/2-inch disk drives at both 720K and 1.44M exists only in Models 339 and 319. In particular, the 1.44M drive, although definitely supported by the ROM BIOS and controller, was never offered as an option by IBM. This means that the IBM Setup program found on the Diagnostics and Setup disk did not offer the 1.44M floppy drive as a choice when configuring the system! Anybody adding such a drive had to use one of the many Setup replacement programs available in the public domain, or "borrow" one from an IBM-compatible system that used a floppy disk-based setup program. Adding the 1.44M drive became one of the most popular upgrades for the AT systems because many newer systems came with that type of drive as standard equipment. Earlier AT systems still can use the 720K and 1.44M drives, but they need to either upgrade the ROM to support it (recommended) or possibly use software drivers to make them work.

AT BIOS Versions

There have been three different BIOS versions used in the IBM AT. They can be identified by their date and summarized as follows:

• January 1, 1984: The first AT BIOS version supported only 1.2M and 360K floppy drives directly. Only 14 hard disk types were supported. It came on the Model 068 and 099 systems with the Type 1 motherboard.
  
  
• June 10, 1985: The second AT BIOS added support for 720K 3 1/2-inch drives directly (no drivers required). Also more hard disk drive types were added, for a total of 22 usable types. A new test was added to the POST which causes the POST to fail if the clock speed is altered from 6MHz. This BIOS was used on the Model 239 with a Type 2 motherboard.
  
  
• November 15, 1985: The third and final AT BIOS added support for a 1.44M 3 1/2-inch drives (no drivers required). Enhanced 101-key keyboard support was added. The POST test checks for 8MHz operation, and fails if the system is running at any other speed. This BIOS was used on Model 319 and 339 systems, and came on a Type 2 motherboard.

Table 23.1 Lists the different IBM Family/1 (PC, XT, and AT) BIOS versions. It also shows the ID, Submodel, and Revision bytes which can be determined by the software function call

Int 15h, C0 = Return System Configuration Parameters

Some of the systems such as the PC and earlier XT and AT systems only support the ID byte; the submodel and revision bytes had not been established when those systems were developed. The table also shows the number of drive types supported in the AT and XT-286 systems BIOS.

The BIOS date is stored in all PC-compatible systems at memory address FFFF5h. To display the date of your BIOS, a simple DEBUG command can be used to view this address. DEBUG is a command program supplied with MS-DOS. At the DOS prompt, execute the following commands to run DEBUG, display the date stored in your BIOS, and then exit back to DOS:

C:\>DEBUG
-D FFFF:5 L 8
FFFF:0000                 30 31 2F-32 32 2F 39 37                 01/22/97
-Q

In this example, the system queried shows a BIOS date of 01/22/97.

AT Motherboard BIOS Hard Drive Tables

The AT BIOS contains a special table that is used by the hard disk controller driver to determine the hard drive parameters. When a hard disk is installed into this type of system, the "type" of drive is entered into the CMOS RAM by whoever has installed the drive. Then every time the system boots, it looks up the parameters by consulting the CMOS RAM for the particular type selected. Older systems were, therefore, limited to what different drives they could support or recognize by the entries burned into their BIOS table. The table used in IBM AT and PS/2 systems is shown here in this section.

The various IBM AT and PS/2 systems that use a BIOS drive table do not necessarily have all of the entries shown here. The number of table entries contained in a particular system BIOS can vary from one version to the next. For example, the original AT BIOS (01/10/84) only had Types 1-14 usable, while the later AT BIOS versions (06/10/85 and 11/15/85) had 1-14 and 16-23 usable. The XT-286 had 1-14 and 16-24 as usable types. Some of the PS/1 and PS/2 systems had the table filled as far as Type 44.

Non-IBM systems quickly adopted special "User Definable" or even "Auto-Detect" types where you could either manually enter the complete table entry (rather than selecting a predetermined "type"), or the system would automatically read the type information directly from the drive.

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NOTE: If you have a non-IBM PC-compatible system, the IBM table may be inaccurate for many of the entries past Type 15. Instead, you should consult your CMOS Setup program; most will show the available types as you scroll through them. Another option is to consult your system, motherboard, or BIOS documentation to see if it shows what the correct table entries are. A final alternative is a program such as the Seagate FINDTYPE program which will scan your BIOS, locate the table, and display or print it for viewing. This program can be downloaded from the Seagate Web site or BBS. Most compatibles follow the IBM table for at least the first 15 entries.

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Most PS/2 systems have the drive's defect map written as data on the drive one cylinder beyond the highest reported cylinder. This special data is read by the IBM PS/2 Advanced Diagnostics low-level format program. This process automates the entry of the defect list and eliminates the chance of human error, as long as you use only the IBM PS/2 Advanced Diagnostics for hard disk low-level formatting on those systems.

This type of table does not apply to IBM Enhanced Small Device Interface (ESDI) or SCSI hard disk controllers, host adapters, and drives. Because the ESDI and SCSI controllers or host adapters query the drive directly for the required parameters, no table-entry selection is necessary. Note, however, that the table for the ST-506/412 drives can still be found currently in the ROM BIOS of most of the PS/2 systems, even if the model came standard with an ESDI or SCSI disk subsystem.

Table 23.9 shows the IBM motherboard ROM BIOS hard disk parameters for AT or PS/2 systems using ST-506/412 (standard or IDE) controllers.

Table 23.9  IBM AT and PS/2 BIOS Hard Disk Table

Type	Cylinders	Heads	WPC	Ctrl	LZ	S/T	Meg	M
1	306	4	128	00h	305	17	10.16	10.65
2	615	4	300	00h	615	17	20.42	21.41
3	615	6	300	00h	615	17	30.63	32.12
4	940	8	512	00h	940	17	62.42	65.45
5	940	6	512	00h	940	17	46.82	49.09
6	615	4	65535	00h	615	17	20.42	21.41
7	462	8	256	00h	511	17	30.68	32.17
8	733	5	65535	00h	733	17	30.42	31.90
9	900	15	65535	08h	901	17	112.06	117.50
10	820	3	65535	00h	820	17	20.42	21.41
11	855	5	65535	00h	855	17	35.49	37.21
12	855	7	65535	00h	855	17	49.68	52.09
13	306	8	128	00h	319	17	20.32	21.31
14	733	7	65535	00h	733	17	42.59	44.66
15*	0	0	0	00h	0	0	0.00	0.00
16	612	4	0	00h	663	17	20.32	21.31
17	977	5	300	00h	977	17	40.55	42.52
18	977	7	65535	00h	977	17	56.77	59.53
19	1024	7	512	00h	1023	17	59.50	62.39
20	733	5	300	00h	732	17	30.42	31.90
21	733	7	300	00h	732	17	42.59	44.66
22	733	5	300	00h	733	17	30.42	31.90
23	306	4	0	00h	336	17	10.16	10.65
24	612	4	305	00h	663	17	20.32	21.31
25	306	4	65535	00h	340	17	10.16	10.65
26	612	4	65535	00h	670	17	20.32	21.31
27	698	7	300	20h	732	17	40.56	42.53
28	976	5	488	20h	977	17	40.51	42.48
29	306	4	0	00h	340	17	10.16	10.65
30	611	4	306	20h	663	17	20.29	21.27
31	732	7	300	20h	732	17	42.53	44.60
32	1023	5	65535	20h	1023	17	42.46	44.52
33	614	4	65535	20h	663	25	29.98	31.44
34	775	2	65535	20h	900	27	20.43	21.43
35	921	2	65535	20h	1000	33	29.68	31.12
36	402	4	65535	20h	460	26	20.41	21.41
37	580	6	65535	20h	640	26	44.18	46.33
38	845	2	65535	20h	1023	36	29.71	31.15
39	769	3	65535	20h	1023	36	40.55	42.52
40	531	4	65535	20h	532	39	40.45	42.41
41	577	2	65535	20h	1023	36	20.29	21.27
42	654	2	65535	20h	674	32	20.44	21.43
43	923	5	65535	20h	1023	36	81.12	85.06
44	531	8	65535	20h	532	39	80.89	84.82
45	0	0	0	00h	0	0	0.00	0.00
46	0	0	0	00h	0	0	0.00	0.00
47	0	0	0	00h	0	0	0.00	0.00

*Table entry 15 is reserved to act as a pointer to indicate that the type is greater than 15.
Type = Table entry number
Cylinders = Total number of cylinders
Heads = Total number of heads
WPC = Write Pre-Compensation starting cylinder
65535 = No Write Pre-Compensation (also shown as -1)
0 = Write Pre-Compensation on all cylinders
Ctrl = Control byte, with values according to the following table:

Bit Number	Hex	Meaning
Bit 0	01h	Not used (XT = drive step rate)
Bit 1	02h	Not used (XT = drive step rate)
Bit 2	04h	Not used (XT = drive step rate)
Bit 3	08h	More than eight heads
Bit 4	10h	Not used (XT = imbedded servo drive)
Bit 5	20h	OEM defect map at (cylinders + 1)
Bit 6	40h	Disable ECC retries
Bit 7	80h	Disable disk access retries

LZ = Landing-Zone cylinder used head parking
S/T = Number of Sectors per Track
Meg = Drive capacity in Megabytes
M = Drive capacity in Millions of bytes

Modifying ROM BIOS Hard Disk Drive Parameter Tables

Because the IBM tables in the AT and XT-286 systems (as well as many of the compatibles of that day) were fixed, technicians often found it necessary to modify the BIOS in those systems to add drive types for new drives I wanted to install. For example, I added two new drive types to one of my old AT systems. Those types--25 and 26--have these parameters:

Type	Cylinders	Heads	WPC	Ctrl	LZ	S/T	Meg	M
25	918	15	65535	08h	918	17	114.30	119.85
26	918	15	65535	08h	918	26	174.81	183.31

WPC = Write Pre-Compensation start cylinder
Ctrl = Control Byte, 08h = More than 8 heads, else 00h
LZ = Landing Zone or head parking cylinder
S/T = Sectors per Track
Meg = Megabytes
M = Millions of bytes

In my old AT system, these table entries originally were unused (zeros), as are the remainder of types from 27-47. By burning a new set of ROMs with these two new completed entries, I was able to use a Maxtor XT-1140 drive to maximum capacity with an MFM 17-sector per track controller (as Type 25) or an RLL 26-sector per track controller (as Type 26). This method precluded the need for a controller with its own separate on-board BIOS to override the motherboard table values and therefore also saved memory in the C000 or D000 UMA segments, where such a hard disk controller ROM normally would reside.

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TIP: If you are interested in performing this modification, get the IBM AT Technical Reference Manual (sold by IBM or Annabooks), which documents the position and format of the drive tables in the BIOS.

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Changing the Hard Disk Controller Head Step Rate

Another more complicated modification that you can perform to the AT BIOS is to increase the stepping rate of the hard disk controller. The first edition of this book briefly mentioned this modification, and a reader wrote to me to express interest in it. Details of the modification can be found on the CD; however, the performance is relatively slight.

AT Technical Specifications

Technical information for the AT system is described in this section. You will find information about the system architecture, memory configurations and capacities, standard system features, disk storage, expansion slots, and keyboard specifications, as well as physical and environmental specifications. This type of information can be useful in determining what types of parts are needed when you are upgrading or repairing these systems. Figures 23.11 and 23.12 show the layout and components on the two different AT motherboards.

FIG. 23.11  The IBM AT Type 1 motherboard.

System Architecture
Microprocessor	80286
Clock speed	6 or 8MHz
Bus type	ISA (Industry Standard Architecture)
Bus width	16-bit
Interrupt levels	16 (11 usable)
Type	Edge-triggered
Shareable	No
DMA channels	8 (7 usable)
Bus masters supported	Yes
Upgradable processor complex	No

FIG. 23.12  The IBM AT Type 2 motherboard.

Memory
Standard on system board	512K
Maximum on system board	512K
Maximum total memory	16M
Memory speed (ns) and type	150ns dynamic RAM chips
System board memory-socket type	16-pin DIP
Number of memory-module sockets	18 or 36 (2 or 4 banks of 18)
Memory used on system board	36 128Kx1-bit DRAM chips in 2 banks of 18, or 18 256Kx1-bit chips in one bank
Memory cache controller	No
Wait states:
System board	1
Adapter	1
Standard Features
ROM size	64K
ROM shadowing	No
Optional math coprocessor	80287
Coprocessor speed	4 or 5.33MHz
Standard graphics	None standard
RS232C serial ports	1 (some models)
UART chip used	NS16450
Maximum speed (bits per second)	9,600 bps
Maximum number of ports supported	2
Pointing device (mouse) ports	None standard
Parallel printer ports	1 (some models)
Bi-directional	Yes
Maximum number of ports supported	3
CMOS real-time clock (RTC)	Yes
CMOS RAM	64 bytes
Battery life	5 years
Disk Storage
Internal disk and tape drive bays	1 full-height and 2 half-height
Number of 3 1/2-, 5 1/4-inch bays	0/3
Standard floppy drives	1x1.2M
Optional floppy drives:
5 1/4-inch 360K	Optional
5 1/4-inch 1.2M	Standard
3 1/2-inch 720K	Optional
3 1/2-inch 1.44M	Optional (8MHz models3 1/2-inch 2.88M	No
Hard disk controller included:	ST-506/412 (Western Digital WD1002-WA2 or WD1003 WA2)
ST-506/412 hard disks available	20/30M
Drive form factor	5 1/4-inch
Drive interface	ST-506/412
Drive capacity:	20M	30M
Average access rate (ms)	40	40
Encoding scheme	MFM	MFM
BIOS drive type number	2	20
Cylinders	615	733
Heads	4	5
Sectors per track	17	17
Rotational speed (RPMs)	3600	3600
Disk storage
Interleave factor	3:1	3:1
Data transfer rate (K/sec)	170	170
Automatic head parking	Yes	Yes
Expansion Slots
Total adapter slots	8
Number of long and short slots	8/0
Number of 8-/16-/32-bit slots	2/6/0
Available slots (with video)	5
Keyboard Specifications
101-key Enhanced keyboard	Yes (8MHz models)
Fast keyboard speed setting	Yes
Keyboard cable length	6 feet
Physical Specifications
Footprint type	Desktop
Dimensions:
Height	6.4 inches
Width	21.3 inches
Depth	17.3 inches
Weight	43 pounds
Environmental Specifications
Power-supply output	192 watts
Worldwide (110/60,220/50)	Yes
Auto-sensing/switching	No
Maximum current:
90-137 VAC	5.0 amps
Operating range:
Temperature	60-90° F
Relative humidity	8-80 percent
Maximum operating altitude	7,000 feet
Heat (BTUs/hour)	1229
Noise (Average db, operating, 1m)	42
FCC classification	Class B

Table 23.10 shows the AT system-unit part-number information.

Table 23.10  IBM AT Model Part Numbers

Description	Number
AT 6MHz/84-key keyboard, 256K
one 1.2M floppy drive	5170068
AT 6MHz/84-key keyboard, 512K, serial/parallel
one 1.2M floppy drive, 20M hard disk	5170099
one 1.2M floppy drive, 30M hard disk	5170239
AT 8MHz/84-key keyboard, 512K, serial/parallel
one 1.2M floppy drive, 30M hard disk	5170319
AT 8MHz/101-keyboard, 512K, serial/parallel
one 1.2M floppy drive, 30M hard disk	5170339
System Options
20M fixed disk drive	6450205
30M fixed disk drive	6450210
30M fixed disk drive upgrade kit	6450468
360K half-height floppy disk drive (AT)	6450207
1.2M high-density drive	6450206
3 1/2-inch, half-height, 720K external drive (AT)	2683191
Serial/parallel adapter	6450215
80287 math coprocessor option	6450211
Floor-standing enclosure	6450218
Enhanced Keyboard Accessories
Clear keycaps (60) with paper inserts	6341707
Blank light keycaps	1351710
Blank dark keycaps	1351728
Paper keycap inserts (300)	6341704
Keycap-removal tools (6)	1351717

An Introduction to the XT Model 286

On September 9, 1986, IBM introduced a new AT-type system disguised inside the chassis and case of an XT. This XT Model 286 system featured increased memory, an Intel 80286 microprocessor, and as many as three internal drives standard. The computer combined an XT's cost-effectiveness, flexibility, and appearance with the high-speed, high- performance technology of the Intel 80286 microprocessor. This model looked like an XT, but underneath the cover, it was all AT.

The IBM XT Model 286 can operate as much as three times faster than earlier models of the XT in most applications. It has a standard 640K of memory. Various memory-expansion options enable users to increase its memory to 16M.

Standard features in this system include a half-height, 1.2M, 5 1/4-inch, high-density floppy disk drive; a 20M hard disk drive; a serial/parallel adapter card; and the IBM Enhanced keyboard. You can select an optional, internal, second floppy disk drive from the following list:

• Half-height, 3 1/2-inch, 720K floppy drive
  
  
• Half-height, 3 1/2-inch, 1.44M floppy drive
  
  
• Half-height, 5 1/4-inch, 1.2M floppy drive
  
  
• Half-height, 5 1/4-inch, 360K floppy drive

The IBM XT Model 286's performance stems primarily from the AT motherboard design, with 16-bit I/O slots and an Intel 80286 processor running at 6MHz. In addition to the type of processor used, clock speed and memory architecture are the primary factors in determining system performance. Depending on the model, the IBM AT's clock speed is 6 or 8MHz, with one wait state; and the XT Model 286 processes data at 6MHz, with zero wait states. The elimination of a wait state improves performance by increasing processing speed for system memory access. The zero-wait-state design makes the XT Model 286 definitely faster than the original AT models that ran at 6MHz and about equal in speed to the 8MHz AT systems. Based on tests, the XT Model 286 also is about three times faster than an actual XT.

Because the XT Model 286 is an AT-class system, the processor supports both real and protected modes. Operating in real address mode, the 80286 is 8088-compatible; therefore, you can use most software that runs on the standard PC systems. In real address mode, the system can address as much as 1M of RAM. Protected mode provides a number of advanced features to facilitate multitasking operations. Protected mode provides separation and protection of programs and data in multitasking environments. In protected mode, the 80286 can address as much as 16M of real memory and 1G of virtual memory. In this mode, the XT Model 286 can run advanced operating systems such as OS/2 and UNIX. When the XT Model 286 was introduced, it was the least-expensive IBM system capable of running a true multitasking operating system.

The IBM XT Model 286 has a standard 640K of RAM. Memory options enable the system to grow to 15 1/2M, much higher than the 640K limit in other PC XTs. If you add an operating system such as OS/2 or Windows, you can take advantage of the larger memory capacities that the XT Model 286 provides.

A 20M hard disk drive is a standard feature in the XT Model 286, as is a 5 1/4-inch, 1.2M, high-density floppy disk drive. A similar floppy disk drive is standard on all models of the AT. Floppy disks formatted on a 1.2M floppy disk drive therefore can be read by an AT or an XT Model 286. The 1.2M floppy disk drive also can read floppy disks formatted with PC-family members that use a 360K floppy disk drive. Figure 23.13 shows the interior of an XT 286 system unit.

The XT Model 286 features the IBM Enhanced keyboard with indicator lights. Many IBM personal computers use the Enhanced keyboard, but the XT Model 286 was the first PC XT to feature keyboard indicator lights.

Five slots support the advanced 16-bit cards or 8-bit cards; three support only 8-bit cards. Two of the three 8-bit slots support only short cards.

A hard disk and floppy drive adapter card are standard features in the XT Model 286. This multifunction card takes only one 16-bit slot and supports as many as four disk drives (two floppy disk drives and two hard disk drives).

The serial/parallel adapter, another standard feature, is a combination card that requires only one slot (either type) and provides a serial and a parallel port. The parallel portion of the adapter has the capacity to attach devices, such as a parallel printer, that accept 8 bits of parallel data. The fully programmable serial portion supports asynchronous communications from 50 bps to 9,600 bps, although even higher speeds are possible with the right software. The serial portion requires an optional serial-adapter cable or a serial-adapter connector. When one of these options is connected to the adapter, all the signals in a standard EIA RS-232C interface are available. You can use the serial port for interfacing a modem, a remote display terminal, a mouse, or other serial device. The XT Model 286 supports as many as two serial/parallel adapters.

FIG. 23.13  The IBM XT-286 interior.

A standard IBM XT Model 286 offers these features:

• 80286 processor at 6 MHz with 0 wait states
  
  
• 640K of motherboard memory
  
  
• 1.2M floppy drive
  
  
• 20M hard disk
  
  
• Five 16-bit and three 8-bit expansion slots
  
  
• Fixed disk/floppy disk drive adapter (occupies one 16-bit expansion slot)
  
  
• Serial/parallel adapter (occupies one 16-bit expansion slot)
  
  
• Enhanced 101-key Keyboard with indicator lights
  
  
• CMOS Time-and-date clock with battery backup

XT Model 286 Models and Features

The XT Model 286 processor is as much as three times faster internally than the preceding XT family and as much as 25 percent faster than the AT Model 239, depending on specific applications. A 20M fixed disk and a 1.2M, 5 1/4-inch floppy disk drive were standard on the XT Model 286. One additional floppy disk drive can be installed internally as drive B. Any type of floppy drive can be added as a second half-height floppy drive, including both the double and high-density versions of the 5 1/4- and 3 1/2-inch drives.

If you want to be able to read standard 5 1/4-inch data or program floppy disks created by the XT Model 286 on other PC systems, you might want to add a 5 1/4-inch 360K floppy disk drive, which provides full read/write compatibility with those systems. This is due to the fact that the 1.2M drives write a narrower track than the 360K drives, and are unable to properly overwrite a floppy disk written on first by a 360K drive. If full read/write compatibility with 360K drives is not important, you can add a second 1.2M high-density floppy disk drive.

You can add any 3 1/2-inch drive, including the 720K and 1.44M versions. Because the 1.44M does not have any read/write compatibility problems with the 720K drives, however, and the 1.44M drives always can operate in 720K mode, I suggest adding only the 1.44M 3 1/2-inch drives rather than the 720K versions. The higher-density drive is only a small extra expense compared to the double-density version. Most people do not know that full ROM BIOS support for these 1.44M drives is provided in the XT Model 286. Unfortunately, because IBM never offered the 1.44M drive as an option, the supplied Setup program does not offer the 1.44M drive as a choice in the Setup routine. Instead, you have to use one of the many available public domain AT type setup programs, or "borrow" such a program from an AT-compatible system.

XT Model 286 Technical Specifications

The technical information for the XT 286 system described in this section covers the system architecture, memory configurations and capacities, standard system features, disk storage, expansion slots, keyboard specifications, and also physical and environmental specifications. You can use this information to determine the parts you need when you are upgrading or repairing these systems. Figure 23.14 shows the layout and components on the XT 286 motherboard.

System Architecture

Microprocessor	80286
Clock speed	6MHz
Bus type	ISA (Industry Standard Architecture)
Bus width	16-bit
Interrupt levels	16 (11 usable)
Type	Edge-triggered
Shareable	No
DMA channels	8 (7 usable)
Bus masters supported	Yes
Upgradable processor complex	No
Memory
Standard on system board	640K
Maximum on system board	640K
Maximum total memory	16M
Memory speed (ns) and type	150ns dynamic RAM chips/SIMMs
System board memory-socket type	30-pin (9-bit) SIMM
Number of memory-module sockets	2
Memory used on system board	One bank of 4 64Kx4-bit and 2 64Kx1-bit DRAM parity chips, and one bank of 2 9-bit SIMMs
Memory cache controller	No
Wait states:
System board	0
Adapter	1

FIG. 23.14  The IBM XT-286 motherboard.

Standard Features
ROM size	64K
ROM shadowing	No
Optional math coprocessor	80287
Coprocessor speed	4.77MHz
Standard graphics	None standard
RS232C serial ports	1
UART chip used	NS16450
Maximum speed (bits per second)	9,600 bps
Maximum number of ports supported	2
Pointing device (mouse) ports	None standard
Parallel printer ports	1
Bi-directional	Yes
Maximum number of ports supported	3
CMOS real-time clock (RTC)	Yes
CMOS RAM	64 bytes
Battery life	5 years
Disk Storage
Internal disk and tape drive bays	1 full-height and 2 half-height
Number of 3 1/2-/5 1/4-inch bays	0/3
Standard floppy drives	1x1.2M
Optional floppy drives:
5 1/4-inch 360K	Optional
5 1/4-inch 1.2M	Standard
3 1/2-inch 720K	Optional
3 1/2-inch 1.44M	Optional
3 1/2-inch 2.88M	No
Hard disk controller included:	ST-506/412 (Western Digital WD1003-WA2)
ST-506/412 hard disks available	20M
Drive form factor	5 1/4-inch
Drive interface	ST-506/412
Drive capacity	20M
Average access rate (ms)	65
Encoding scheme	MFM
BIOS drive type number	2
Cylinders	615
Heads	4
Sectors per track	17
Rotational speed (RPMs)	3600
Interleave factor	3:1
Data transfer rate (K/sec)	170
Automatic head parking	No
Expansion Slots
Total adapter slots	8
Number of long and short slots	6/2
Number of 8-/16-/32-bit slots	3/5/0
Available slots (with video)	5
Keyboard Specifications
101-key Enhanced keyboard	Yes
Fast keyboard speed setting	Yes
Keyboard cable length	6 feet
Physical Specifications
Footprint type	Desktop
Dimensions:
Height	5.5 inches
Width	19.5 inches
Depth	16.0 inches
Weight	28 pounds
Environmental Specifications
Power-supply output	157 watts
Worldwide (110v/60Hz, 220v/50Hz)	Yes
Auto-sensing/switching	Yes
Maximum current:
90-137 VAC	4.5 amps
Operating range:
Temperature	60-90° F
Relative humidity	8-80 percent
Maximum operating altitude	7,000 feet
Heat (BTUs/hour)	824
Noise (Average db, operating, 1m)	42
FCC classification	Class B

Table 23.11 lists the XT Model 286 system-unit part numbers.

Table 23.11  IBM XT-286 Model Part Numbers

Description	Number
XT Model 286 system unit, 6 MHz 0 wait state, 640K, serial/parallel, 1.2M floppy drive, one 20M hard disk	5162286
Optional Accessories
5 1/4-inch, half-height 360K drive	6450325
3 1/4-inch, half-height 720K internal drive	6450258
3 1/2-inch, half-height 720K external drive	2683190
80287 math coprocessor option	6450211
Enhanced Keyboard Accessories
Clear keycaps (60) with paper inserts	6341707
Blank light keycaps	1351710
Blank dark keycaps	1351728
Paper keycap inserts (300)	6341704
Keycap removal tools (6)	1351717

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