Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems.
The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System were typical and sold for many decades. The 1A family of Western Electric Company (WECo) key telephone units (KTUs) were in use in the 1950s. 1A equipment was primitive and required at least two KTUs per line; one for line termination and one for station (telephone instrument) termination. The telephone instrument commonly used by 1A systems was the WECo 300-series telephone. In the 1960s, 1A1 key systems simplified wiring with a single KTU for both line and station termination, and increased the features available. As the 1A1 systems became commonplace, requirements for intercom features increased. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time. The WECo 6A dial intercom system provided two talk links and was often installed as the dial intercom in a 1A1 or 1A2 key system. Unfortunately, the 6A systems were complex, troublesome and expensive, and never became popular. The advent of 1A2 technology in the 1970s simplified key system set up and maintenance. These continued to be used throughout the 1980s, when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems.
Two obscure key systems were used at airports for air traffic control communications, the 102 and 302 key systems. These were uniquely designed for communications between the air traffic control tower and radar approach control (RAPCON) or ground control approach (GCA), and included radio line connections.
Automatic Electric Company also sold a family of key telephone equipment, but it never gained the widespread use enjoyed by Western Electric equipment.
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