KX-TDA6178X, KX-TDA6178XJ - Panasonic PBX Service Manual (repair manual). Page 12

12

The off-hook status is detected when the telephone is off-hook. When a telephone is off-hook, change in current is detected in SLIC,
and this information is transmitted to the ITA/B pin of SLIC (27, 36pin) as OFF-HOOK=L/ON HOOK=H signal. This digital signal is
received by CODEC IC and recognized as off-hook signal. The off-hook status is analyzed by software. 

During ringing, the ring current is permanently sensed by the SLIC(IC200A-X). Signal of the ring current is supplied to the
CODEC(IC201A-X) by means of the ITA/B signal.This current signal is continuously sampled by the CODEC.
When the handset of a ringing telephone is picked up, the current is compared against the programmed ring trip threshold. If it is
above the programmed ring trip threshold, the ringing signal is automatically removed from the Tip-Ring interface during zero cross-
ing.