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3. Polarity reversal and interrupt handling

Fig. 3 Polarity reversal detecting circuit

 The circuit is used to monitor the polarity changes, which occur at the input terminals at the moment of both party call establishment. Therefore, it is important to request the polarity reversal facility and get the approval to fix ITDL in parallel to TP line from the respective regional branch of the Telecommunication Company.

The opto-isolator SFH610 ensures complete isolation between 48V and ITDL. With the system connected to TP line and during the time 48V appears at A, current 1mA flows through the LED in the opto-isolator. The associate transistor then conducts and logic 1 appears on the emitter. At the moment of call establishment, polarity reverses and opto-isolator LED ceases conducting thus appearing logic 0 at the emitter to interrupt Z80.



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Last modified: July 13, 2002