Panasonic KX-HCM110A Service Manual page 57

13.8. OTHERS
13.8.1. I/O Terminal
- The Input terminal has two systems; both of them are connected to the
Input Port of the IC103 GPIO.
- Due to Internal Pull-up Resistance, the PNP Transistor (Q504, Q503) on
the following level is usually in the OFF state and the Input Port
connected to the collector is at L level.
- If the terminal is short-circuited with the GND or the signal of L level is
input, the PNP Transistor goes ON and the Input Port goes to H level.
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