Well R80 and D7 will increase the pull to ground (extra to what R81 does) if the right side of the capacitor goes negative.
So it seems to be protection so changing states on the capacitor C22 cannot try to pull the input of U9C below ground and damage it.
Seems kind of unlikely through with 220k in series but maybe it's just there to stop the cap charging up negative because that causes some other problem, like a slower response when it changes to high.
D5 appears to be changing how much current feeds into the RN7C filter network when the input voltage is above 0.7V.
Might be to prevent a LOW signal from discharging C20 since it will have to discharge through the 220K. But a HIGH can go through the diode (less 0.7V and charge C20 quicker.
Just guessing though, I dunno exactly what that thing does
The resistors and diodes around C20 and C22 are delay networks. Low to high transitions are delayed less than high to low. C22 with resistors and diode form a short pulse when the preceding inverter changes state.
The way I see it is that the whole circuit is set up to be edge sensitive. It depends on the rest of the circuit as to why they did it this way.
If you have the board this is on, in a working state, then sure you could probe it to see what is happening.
The RC filters in the first section cause a delay between one input of the NAND or NOR gates connected to the input signal. This can be used to create pulsed signals. The so called differentiator or high pass filter in the second stage does this to. On a low to high change on the output of U11B the signal after the capacitor will go high and then slowly decay to low again, based on the RC time given by C22 and R81. When the output of U11B changes from high to low the signal after the capacitor goes below ground level and will then increase more rapidly based on the RC time given by C22 and the combination of the series resistance of R80 and D7 parallel to R81.
The diodes in the first section bring different delays for low to high and high to low signal changes.
I'm no expert on this, but think it acts as some sort of positive feedback. It might change the charging and discharging rate of C21 in some way.
I'm no expert on this, but think it acts as some sort of positive feedback. It might change the charging and discharging rate of C21 in some way.
If this is the case, looking at the upper half, it appears to me the output on N1-4D-11 will always be LOW regardless of the input signal on N1-3D-4, because the two inputs to U18D will always be one HIGH the other LOW.
Start with the input N1-3D-4 being HIGH and the inverter output LOW. The NOR output will be LOW. Now make N1-3D-4 go LOW. The direct input to the NOR gate goes LOW imediately. But its other input remains LOW for some time because of the RC delay to the inverter input. So there is a period when both of the NOR inputs are LOW which makes its output HIGH. The variable resistor allows adjustment of that pulse width.
XOR
0 0 ==> 0
0 1 ==> 1
1 0 ==> 1
0 0 ==> 0
By the way, I'm no expert, but I think the R/C part is probably not working as a filter here?
By the way, I'm no expert, but I think the R/C part is probably not working as a filter here?Correct. Both R32&C and R34&C are to make delays, not to act as filters.