Electronics > Repair

The art of logic signal manipulation with analogue components (D/R/C)

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max.wwwang:
[Original title:  Please help figure out what's going on here]

Can someone experienced in digital circuits please help me understand what the tricks here are. It's part of a digital circuit; its interaction with the world outside it is all digital (LOWs and HIGHs). This is fine. But in this little part, I find it very difficult to understand the function of the resistors and diodes in the areas [1] and [2].

This is drawn up by myself based on a circuit board of my repair project, which I carefully checked many times so it should be correct. Simply because I don't understand what's going on here, chances are the components are arranged in a very bad way which might otherwise be much more revealing and intuitive!

[Edit] Have added a tidier (I think) version. Both are the same thing.

Psi:
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

max.wwwang:

--- Quote from: Psi on February 27, 2023, 07:49:24 am ---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.

--- End quote ---
It is also what I thought is possibly the only use of D7. But as you said, can this be possible (and it requires the negative voltage to be less than -0.6V or so for D7 to be useful)? Possibly yes ...


--- Quote from: Psi on February 27, 2023, 07:49:24 am ---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

--- End quote ---
Find it hard to get my head around here.

At first I think we can ignore C20 and C21 since they appear to be just 'standard' capacitors that exist everywhere near any chip (I understand they are for noise filtering, for example). It's just C22 that seems a real deal because it's in serial with the line, not bypassing. But physically they three are all the same puppy (same look, same physical size), though I'm not sure if they have the identical capacitance.

wasedadoc:
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 U11B changes state. The input to U9C will return to low even if the output of U11B remains high.

max.wwwang:

--- Quote from: wasedadoc on February 27, 2023, 09:16:33 am ---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.

--- End quote ---

That's very specific and interesting! I may possibly set up a minimal circuit to demonstrate this. (Or perhaps the easiest way is simply to probe these pins!)

For the second bit, I presume it applies only on one side of the pulse on the output pin of U11B? Is it like an overshoot on the falling edge?

Anyway, I'm convinced that these passive components are essentially all for the tweaking of the signal, be it delay, push, sharpen, strengthen, dampen, or whatever.

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