KX-TDA100DUP - Panasonic PBX Service Manual (repair manual). Page 16

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For the protection against such as an over current owing to short in wiring, +40V, 20mA and 80mA constant-current circuits by
Q400, U402, R408, and R410 are used.
The following is the flow chart of telephone switching at boot-up.
Do the same for the 2nd to 4th ports (circuit numbers 500-599, 600-699, 700-799) 

PT_POW

PT_CUR

H

H

0V

H

L

0V

L

H

40V80mA

L

L

40V20mA