AVR / Arduino connection to device pins (step driver) - differential inputs / outputs, what is it and what do they eat with?

Question

Arduino

Good afternoon.
There is a diagram for connecting a stepper driver:

As I understand it, it has an optocoupler.
But why is the management going on, uh... by minus, but by the general plus?
I tried to control the ka usually with a plus, and the minus is common - it works.
Tried according to the scheme - the same works (inverted the pins).
I take it this is from a tip?
But the frequency there is not high (up to 1kHz), the minimum response voltage is 2.5 volts - what the hell should be pickups?
Or what is it for?
What if all the same, do not do it according to the scheme, but in a simple way - manage the plus, and the land is common. What could be the consequences and how to get rid or minimize?
Tell me please.

Answer 1

The diagram shows an option for a controller with open collector outputs. Such a controller will simply not pull control according to a circuit with a common minus - it can only issue a "plus" through a resistor, with a resistance of kiloohms.
AVR controllers have combo outputs. With the classic setting (DDRx=1), they operate in push-pull mode, so it doesn't matter to them whether to use a common plus or a common minus - they have both plus and minus fed through the transistor.
There is nothing "differential" in this circuit, there is no interference protection either.