From what I understand you can have an octave leap from one downbeat to the next in first species as long as it rebounds with a step in the opposite direction.
However, in programming second species, I can find no rule on what acceptable half notes I can have:
An escape tone (2nd-7th) would cause a leap of a 7th which is note allowed...
A change of register (Up a 9th, down a 2nd) would require a leap of greater than an octave which isn't allowed.
I don't see anything about leaps of a 6th and 3rd or a 5th and a 4th. Are those allowed?
Or are octave leaps between downbeats just not allowed in second species?