We recently added a built-in probe function like this to MediaLab. Check this post out for some samples and let us know how it works for you:

Probe Samples:
www.empirisoft.com/support/showthread.php?t=375

And here a couple Biopac posts that involve TTL signals:
www.empirisoft.com/support/showthread.php?t=518
www.empirisoft.com/support/showthread.php?t=571

-Blair