Trying to understand the following circuits

[shemo]

1) 1.png, the ESD diodes looks like two regular diodes in reverse. Then how can it conduct any current? one of them would block it?

2)2.png, IR module, there is photo diode there but the plus side is tied to end. so when will it conducting?

3)3.png, there are no caps in LINEINL, LINEINR, are the caps purpose are to block DC?
only AC signals coming in?

4)4.png, what's 600R-100M means for the Ferrite bead? R133, R134, R135 with cap are the bypassing path to filter any AC noise for the DC power source?
Is the cap is to filter noise then would the ferrite beads redundant?

5)5.png, when pressed, the path brining the PWRON signal to GND. what's the purpose of that?

6)6.png, when NMI# active low is on, D4 is on then quickly bringing NMI# back to high , is that purpose is to make sure NMI# stays low only for a short time?

 

[BigBoss]

1-Diodes are TVS so Transient Voltage Supression diode type and they are used to protect transient voltage spikes that generally come from external sources.
2-Photodiodes are generally biased with a small currents but we don't know what there is in Modulator.There may be current sink ??
3-Yes, they are DC blocking caps.
4-Ferrite Beads are used to filter noises coming from external/internal noise sources like here.
5-I think it's a reset or restart circuit
6-Diode is a Schottky Barrier diode so it's very fast.This diode can be used-I'm not sure- to create a time delay using with internal cap. and 47K resistor.So, it's a kind of delay circuit.

 

[D.A.(Tony)Stewart]

1) TVS are zeners diodes not just silicon diodes
4) 600 Ohm @ 100MHz ferrite
5) just a momentary active low switch with cap debounce. Needs a pullup for logic levels. Also performs as power on low reset.
6) If there is dual power ( battery and external ) then diode may serve some isolation purpose. Otherwise?