| 摘要 |
There is disclosed an improved hold circuit arrangement for controlling the hold function in a key telephone line circuit without introducing an undesired series impedance in the longitudinal path of the telephone line during any of the line circuit's normal operating states. The improved circuit arrangement employs a combination of an opto-coupled energizing circuit for detecting and responding to the presence of d.c. current on the telephone line during the hold state and two miniature relays which operate to establish a condition of electrical isolation between the opto-coupled energizing circuit and the longitudinal path of the telephone line in all operating states except during the momentary transition from the line busy to the line hold state. In another embodiment of this invention, logic circuitry is associated with the opto-coupled energizing circuit to provide lamp control indicative of the line state. The invention reduces the overall cost and profile of the line circuit while providing a means for eliminating a common cause of longitudinal imbalance during the hold state characteristic of conventional line circuit technology.
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