Feynman's XOR in LTspice

[Electronaut]

Hello,
Just for fun I tried to use LTspice to simulate the "Non-Workable" XOR with feedback from Feynman's Lectures on Computation (p.43, Fig. 2.27). What surprises me about this is that LTspice shows stable dependence when viewing V(In) and V(Out), so the timing problems that Feynman talks about do not show up. Also, the feedback seems to result in a stable self-contradictory steady state when the input pulses are 1. I've attached the LTspice schematic file.
Any tips would be appreciated. --Electronaut

 

[alec_t]

IMO what you show is a NOR gate, not an XOR. Also, the output cannot reach a logic 1 level.

 

[Electronaut]

Thank you. Correct, I used a NOR instead of an XOR. Back to the LTspice drawing board...

 

[alec_t]

In case you haven't found it, there's an XOR gate in the LTS 'Digital' folder.

 

[Electronaut]

Thanks again. I tried working with the generic xor from LTspice, but I suspect that this is a "mathematical XOR" that would not have the timing characteristics of a real transistor circuit, and this is what I wanted to experiment with. Indeed, if I did this correctly the LTspice simulation would probably fail due to the timing problems that Feynman talks about.

 

[alec_t]

I suspect the 'fail' would actually be a high frequency oscillation, as the circuit attempts to be in two states at the same time.

Edit:
Suspicions confirmed :-

 

[Electronaut]

By zooming in on those high-frequency oscillations it appears that they are very regular, like a slightly distorted sine wave. This is probably an artifact of the model used. I suspect that if implemented in hardware the resulting oscillations would be more chaotic, sensitive to temperature, etc.

 

[alec_t]

I'm sure you're right. Even if you build a simulated XOR with simulated BJTs and other discrete components, you may still be some way from reality.