KX-NCP1174XJ (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 10

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Each port has its own DTMF receiver. The receiver detects the signal using IC1 for port A through D, IC2 for port E through H, IC4
for port I through L and IC6 for port M through P. The CODEC detects the DTMF data to be effective at EST terminal=H. This infor-
mation is detected in the MPR via SAMSON, then DTMF signal is detected by MPR reading the DTMF receiver data that came
through the SAMSON local bus.