Good question! It can be tough to measure the total I/O latency with DAW applications because most of them (including Pro Tools) do some auto-time-adjustment to compensate for some of the buffering delays (mostly in the disk buffers).
Here's how we measure Total I/O Latency...
First of all, you need two physical inputs and two physical outputs. Take a small patch cable and connect output 1 to input 1. Take another cable and connect output 2 to input 2.
In the DAW software (like Pro Tools), create 3 audio tracks and one Aux track.
On disk track A, put a small audio file/region with a nice square wave (or impulse). Route the output of this disk track to output 1.
On disk track B, route the input from channel 1. Route the output to some unused bus (you don't need it).
On the Aux track, route the input from channel one, and the output to channel 2.
On disk track C, route the input from channel 2, and route the output to some unused bus (you don't need it).
Record enable tracks B and C.
Record the impulse.
Look at the results on tracks B and C. The distance between the peaks on these tracks is the Total I/O Latency in the system (including converter delay, hardware delay, driver delay, and audio engine delay).
The reason we use 2 separate disk tracks is that any "auto-time-correction" used by the software will be cancelled out when you subtract tracks B & C, leaving you with the total I/O latency.
Regards,
Kenn LeGault
Digidesign