to the design of the T100/102, the line "RP" is read
as bit 5 of port DO (D0-D7),
and it is normally kept high through R157. To change this level
to low, it is necessary to connecte it to ground; FC1 does this
when its LED is activated during each Ring. The network composed
of C1,R1,DZ1,DZ2 and D1 allows a flow of current into the LED
only during the ringing cycle.
It is to be noted that the pulse sequence on RP will mirror the actual ringing sequence; it is therefore possible to count not only the number of rings, but the actual number of ringing pulses. If it is desirable to receive a single pulse for each ring, it is sufficient to connect a capacitor between Rp and Ground (in parallel to FC1) so that the Time constant R157*C allows a complete discharge during the time interval between rings, but not between ringing pulses. The 4N26 can be replaced with a variety of compatible opto-isolators.