KX-NT543RU, KX-NT543RU-B, KX-NT546RU, KX-NT546RU-B (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 10

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KX-NT543RU/KX-NT543RU-B/KX-NT546RU/KX-NT546RU-B
4.2.5.
ASIC (IC205)
IC205 controls KEY, LED, EHS and Headset-detect.
An operating clock is provided by IC501 and a reset signal from the IC501 starts either reset or normal operating.(L(0V):RESET)
The IC205 and IC501 are connected by an I2C communication.
IC501 checks the presence of IC205 by getting the signal from IC205.
LED and KEY Timing chart
The LED control and the KEY control use the same strobe(STB) signal. 
Therefore, the LED control and the KEY control are alternately executed. 
The timing chart is as shown in the following .
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