Hi PBB,
The latency number is generated when you do a latency measurement. Here's how it works for me:
I hook a piece of outboard up to a set of outputs and inputs on my Aurora, say out 3/4 and in 3/4. Then insert EI in an fx bin and choose those outputs and inputs from the dropdown boxes. Once that's done I click in the EI box to have it measure round trip latency...I think it's in the center of the box near the numeric display. It'll send a "ping" to the hardware and measure the transit time. If that works successfully you'll see a non-zero number appear there. Once that's showing you should be good to go.
A couple caveats -
1. If you change latency you'll need to re-test the round trip latency
2. If you put an EI on a bus the latency check won't work unless a track is routed to that bus.
3. EI ins and outs are always paired, so even if you only choose input 3 and output 3 in the dropdown boxes, input 4 and output 4 will be unavailable from that point forward because (for some reason) Sonar assumes that you're always working with pairs.
Dean