From FB post about Arduino driving a ‘HC595 chip.
You don’t need an oscilloscope to see digital signals. If you slow down the speed, you can see what’s happening by looking at LEDs. Everything is digital, just on or off.
Plug two red LEDs, transistors, and current limiting resistors for LEDs (330 ohms), and for the bases (4.7k) and use those as probes to show what the signal levels are.