This RS-232 type line receiver to drive CMOS logic uses a Schmitt-trigger feedback network to ~give about 1-V input hysteresis for added noise immunity. A possible problem in an interface which connects two pieces of equipment, each plugged into a different ac receptacle, is that the power line voltage might appear at the receiver input when the interface connection is made or broken.
Rs232-to-cmos-line-receiver - schematic

The two diodes and a 3-W input resistor will protect the inputs under these conditions.

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