The case history started in the 1940s of the last century when the European and American PA/GA and intercom paradigms originated. At the time, industrial intercom and control communication became duplex and simplex. Military application and high requirements to speech illegibility led to the creation of standards for communication in high ambient noise environment. At half-duplex (variation of duplex connection) connection, the built-in echo suppression system automatically controls the direction of the voice transmitting; in this case, communication delays happen as well as loss of speech and parts of important information. These peculiarities caused the development of simplex communication systems and technologies of simplex calls management correspondingly.
Shipboard, military, air service dispatch and railway communication stimulated the development and became the basis of modern simplex communication.
The choice between a key (or a lever mechanism) and a button became one of the matters of principle. On the one hand, establishing of simplex connection with a direct call key is easier and more convenient. However, it was quite problematic to use a key at that moment. It was connected with severe conditions of communication equipment operation in the industrial sector (dust, industrial overall and mittens) and, mainly, with ice covering. If there was some condensate or moisture near the button, then at low temperatures the button got covered with icing which prevented pressing the button.
The mechanism of key pressing allowed releasing the key from icing; at the same time it provided the ease of call making while wearing working clothes due to the optimal size of the lever. Along with this, light signaling was developed which allowed the operator not only to make a call mechanically, but also to control the connection state visually.
Besides, the key operation is provided by the optic galvanic separation which is used in the mechanism. It allowed avoiding risks connected with the possibility of the operator’s injury caused by high voltage in case of cable failure and breakdown. The modern key is a mechanical double key with two LEDs and a “rocking lever” which is an optic disconnection switch.
It is worthwhile noting that the keys were single. Only recently the modern double construction (which is rather difficult, despite its apparent simplicity) has been developed. Material, mounting seat, clearance and flexibility of the revolving spring – all these components passed several stages before the key has become a reliable, explosion proof and resistible to the temperature drop mechanism of the call control.
Nowadays, industrial communication systems consist of different devices of calls management: function keys, direct call keys, handsets and dial pads as well as touch screens. In this case, direct call keys and function keys can be used for management of simplex and duplex communication.
The criteria of choosing this or that tool are the following: the equipment operation conditions, number of subscribers which can be called from the device, presence of external systems integrated with industrial communication system, communication network architecture (distributed IP, centralized ISDN) and others. However, as a rule, a handset, a button and a touch screen are used for duplex connections at which timely conversation management is less important.
Speaking about the button, it is necessary to note that its mechanism in contrast with the key requires only one action (pressing) which does not permit speaking about convenient one-side management of communication. In case of sensor control of calls, pressing is not always obvious especially taking into account severe operation conditions (at industrial sites), large traffic volume and high intensity (at transport and aviation sites). That is why a double key (and a lever switch in general) is a usual attribute of simplex communication providing the reliable and timely management of connection in any conditions.