Panasonic KX-FT21RS - Manual (Page 135)

6.11. OPERATION PANEL
The unit consists of an LCD (Liquid crystal display), KEYs, photo interrupters (sensor), and analog switches (for key scan).
The key scan (analog switch control, reading), sensor detection (sensor control, reading) and LCD control are all controlled by I/O
of ASIC (IC1) on the digital board.
Fig. a
6.11.1. KEY SCAN
When CN1(8) (OPSEL) is "H", the analog switch (IC1) is available. The "KLATCH", "KSCLK" and "KTXD" control the analog switch
(IC1) to read at KIN0~3.
Analog Switch (IC1) Control
0 1 2 3 4 5 6 7
KLATCH (A) L H L H L H L H
KSCLK (B) L L H H L L H H
ktxd (C) L L L L H H H H
Key Matrix
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KX-FT21RS