Signal filtering, what's wrong with the circuit?
This is my understanding of the circuit. We need to know the minimum and maximum pulse widths on the incoming serial data. I have just assumed a square wave to get started. We also need to know more about the load on IC9, I have just assumed two load states.

You need more signal conditioning, the transistor that was removed was probably useful. I added a diode ("new") to offset the signal above zero. You probably need more than this.







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