Brother P-touch BB4, Руководство по обслуживанию

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3.2.4 Key Contacts and Solder Points
On the main PCB are four carbon-printed contact patterns. Each contact pattern has a
pair of electrodes.
Keypad
The rubber keypad is made of high-impedance silicon rubber. As shown in Figure 3.2-3,
each key on the rubber keypad consists of a key top, rubber spring, and conductive paint
which functions as a switching element.
If a particular key is pressed, the conductive paint of the key short-circuits the paired
electrodes carbon-printed on the main PCB.
Figure 3.2-3 Detailed Diagram of Keypad
Key scanning
Figure 3.2-4 shows the key scan circuit and solder points.
The CPU scans four keys including the ON/OFF key. Ports P50 through P53 on the CPU
issue a group of key scanning pulses. Port P20 acts as an input port that receives key
status.
Ports P70 through P77 act as input ports that receive solder point status.
Figure 3.2-4 Key Scan Circuit and Solder Points