Preventing Blank based on another Cell

I would like to validate data but cant quite get the formula right. I
have a cell (say F7) that cannot be blank if (say E7) another cell
equals a text value (say "S"). Anyone have an idea? I am stumped at
this seemingly easy task.

I forgot to add the valid combinations:
E7 = P and F7 = Entry: OK
E7 = P and F7 = Blank: OK
E7 = S and F7 = Entry: OK
E7 = S and F7 = Blank: INVALID

bluegrassstateworker wrote:
> I would like to validate data but cant quite get the formula right. I
> have a cell (say F7) that cannot be blank if (say E7) another cell
> equals a text value (say "S"). Anyone have an idea? I am stumped at
> this seemingly easy task.