Detecting and evaluating earthing mode required - using microprocessor-controlled dialling pulse switch in bilingual telephone subscribers appts.

摘要

In a procedure for recognition and evaluating the earthing mode needed for dialling, where the telephone station hasa reverse-polarity protecting bridge and a microprocessor-controlled number-dial pulse switch in front of the speech circuit, a keyboard for dialling and a third line for earthing are provided. A controller/evaluator (SA) is provided, with at one input, the potential to be earthed via the third line. The potential criteria are produced via two control points (P1,P2) of the microprocessor (Mp) and are so evaluated that after a dial pulse character the switch interrupts the subscriber line with positive supply potential for a set time. For negative or positive earth demands the third line is coupled to the positive line before the switch or to earth. Zener diodes (D1,D2) provide the supply voltage for the microprocessor during key depression. For zero potential at a switch point (E) the microprocessor can read out criteria to open the switch for a set time.