Connecting A PS2 Keyboard To A 3v3 Device
From This TBS Post by @mixtel90
I've just found the (working) board that I used resistor/diode voltage clamping on. The series resistors were 270R and the diodes were BAT85, although most small diodes will do, Schottky is preferable.
Current through the diodes and into the 3V3 supply is restricted by the pullup resistors in the keyboard and the 270R series resistors. The pullup resistors are usually 2K minimum , so it's only about 1.5mA in total over both signal lines. The diodes ensure that the ESD protection diodes in the RP2350 don't pass any appreciable current.
This is a neat way to connect a PS2 keyboard to a 3V3 device. The reference I originally found to it referred to a HP data sheet for a FPGE or something similar but I've lost both references now.