The AT keyboard, named for the AT series of Personal Computers (PCs) it was included with, was a type of computer keyboard used by IBM® and its competitors in the 1980s. It had 84 keys arranged in three distinct groups, with letters and commonly used keys like the Space bar occupying the center, a set of function keys to the left, and a number pad on the right. The AT keyboard used a round 5-pin connector and an electrical signaling scheme that outlived the keyboards themselves. Today, these types of keyboards are generally only used in aging legacy hardware setups and historical displays.
PC keyboards can be divided into three distinct generations based on the layout of their keys. Early keyboards included with the first models of the IBM® PC had awkward key placements and little or no separation between different types of keys. In addition, important keys like Shift and Enter were small and labeled only with symbols. IBM® addressed many of these issues with the AT keyboard, named for the AT series of PCs with which they were originally included.
Unlike its predecessor, the AT Keyboard had three distinct groups of keys. On the left were ten function keys, on the right was a number pad, and in the center were the most frequently used keys, including Shift, Enter, and the Space bar. The overall layout of the AT keyboard is fairly similar to more modern computer keyboards, with a few notable exceptions. Below the tab key was a Control key, with the Caps Lock key placed toward the bottom right under the Shift key. The Escape key was located on the right with the number pad. There were no dedicated arrow keys; the two, four, six, and eight keys on the numeric keypad doubled as down, left, up, and right arrow keys.
In addition to refining the layout of the keyboard, IBM® made changes to both the physical connector and electrical signaling used in the AT keyboard. The physical connector was a round plug about a half an inch (1.27 cm) across, with five metal pins across and a small indentation to prevent the connector from being plugged in upside down. The electrical signaling was bidirectional, meaning a PC could send data to the keyboard as well as the keyboard sending data to the PC.
Just a few years after the debut of the AT keyboard, IBM® released a new “Enhanced Keyboard” with 101 keys. The layout of this keyboard became an industry standard, supplanting the earlier layouts and enduring mostly unchanged for over twenty years. Some aspects of the AT keyboard were carried over into the new design, including the 5-pin connector and the signaling protocol, which was used until the introduction of Universal Serial Bus (USB) keyboards.