My apologies for this late reply to your post.
The endings ~tara and ~ba can be a bit tricky. Many times, it can be interchanged.
However there are some characteristics that ~tara has which cannot be replaced by ~ba. These are:
1. To express a certain future.
For example, your friend is due to arrive at Tokyo-station. You tell him "Please call me WHEN you arrive to the station." And in Japanese, this would be "Eki ni tsuiTARA denwa shite kudasai." 駅に着いたら電話してください。
Here, these is an assurance that your friend is definetely going to arrive at the station at some point in the future, however you don't know WHEN.
2. To express a certain result.
For example, you were sick, but after sleeping, you feel better. "After I slept, I got well." And in Japanese "NeTARA naotta." 寝たらなおった。
3. To express unpredictable events
For example, you thought the store would be open, but when you went, it was closed. "When I went to the store, it was closed." And in Japanese "Mise ni itTARA shimatte ita." 店に行ったら閉まっていた。
-Hope that helps!