What is the lowest latency communication method between a computer and a microcontroller?

Question

I have a project in which I need to have the lowest latency possible (in the 1-100 microseconds range at best) for a communication between a computer (Windows + Linux + MacOSX)

Answer 1

For info, I just ran some tests on a Windows 10 PC fitted with two dedicated PCIe parallel port cards.

Sending TTL (square wave) pulses out using Python code (actually using Psychopy Builder and Psychopy coder) the 2 channel osciloscope showed very consistant offsets between the two pulses of 4us to 8us.

This was when the python code was run at 'above normal' priority. When run at normal priority it was mostly the same apart from a very occassional 30us gap, presumably when task switching took place)