LVDS_TX no output

Are you using an internal PLL or external PLL? The being said, even if you use an external PLL and set it up incorrectly, you should see something toggling on the output. Is SignalTap using the same clock as the parallel clock for altlvds? (My guess is the clock isn't toggling, but if SignalTap is running off of it, then it is). Worst case, remove the altlvds block and just send some toggling data out to make sure that works.