The efficiency of manual switching is too low to meet the needs of large-scale deployment of phones. With the development of semiconductor technology and the maturity of switching circuit technology, it has been found that electronic technology can be used instead of manual switching. As long as the telephone terminal user sends a series of electrical signals to the electronic device, the electronic device can connect the circuit of the requesting party and the requested party according to a preset program, and monopolize the circuit, and will not share with the third party (of course, Due to design flaws, there may be situations in which multiple people share circuits, also known as "strings". This type of exchange is called "programmed exchange." And this kind of equipment is also a "programmed switch."