Original PC Hardware Reference

An Introduction to the XT (5160) CD: 21
On April 2, 1985, IBM finally introduced new models of the XT without the standard hard disk.
Designed for expansion and configuration flexibility, the new models enabled you to buy the system
initially at a lower cost and add your own hard disk later. The XT therefore could be considered
in configurations that previously only the original PC could fill. The primary difference
between the PC and the XT is the XT’s expansion capability, provided by the larger power supply,
eight slots, and better memory layout. These models cost only $300 more than equivalent PCs,
rendering the original PC no longer a viable option.
The extra expense of the XT can be justified with the first power-supply replacement you make
with an overworked PC. The IBM PC XT is available in two floppy disk models:
■ 5160068 XT with one full-height 360KB disk drive
■ 5160078 XT with two full-height 360KB disk drives
Both these models have 256KB of memory and use the IBM PC XT motherboard, power supply,
frame, and cover. The serial (asynchronous communications) port adapter isn’t included as a
standard feature with these models.
IBM introduced several more models of the PC XT on April 2, 1986. These models were significantly
different from previous models. The most obvious difference, the 101-key Enhanced keyboard,
was standard with these newer computers. A 20MB (rather than 10M) hard disk and one
or two half-height floppy disk drives were included. The new half-height floppy disk drives
allowed for two drives in the space that previously held only one floppy drive. With two drives,
backing up floppy disks became easy. A new 3 1/2-inch floppy disk drive, storing 720KB for compatibility
with the PC Convertible laptop computer, was released also. These newer XT system
units were configured with a slightly different memory layout, allowing for 640KB of RAM on the
motherboard without an expansion slot. This feature conserves power, improves reliability, and
lowers the cost of the system.
One 5 1/4-inch, half-height, 360KB floppy disk drive and 256KB of system-board memory was
standard with the XT Models 267 and 268. Models 277 and 278 have a second 5 1/4-inch floppy
disk drive. Models 088 and 089 were expanded PC XTs with all the standard features of Models
267 and 268, a 20MB hard disk, a 20MB fixed disk drive adapter, a serial port adapter, and an
additional 256KB of system-board memory—a total of 512KB.
The following list shows the highlights of these new models:
■ Enhanced keyboard standard on Models 268, 278, and 089; 101 keys, and no status LEDs
(XT interface cannot drive LEDs)
■ Standard PC XT keyboard on Models 267, 277, and 088
■ More disk capacity (20MB)
■ Standard 5 1/4-inch, half-height, 360KB floppy drive
■ Available 3 1/2-inch, half-height, 720KB floppy drive
■ Capacity for four half-height storage devices within the system unit
■ Capacity to expand to 640KB memory on system board without using expansion slots