When is the last bus from Beijing Sanyuanqiao to Chengde, and how often it runs ?

Hi,
Thank you very much for the information. When is the last bus from Sanyuanqiao to Chengde, and how often it runs ? Would we be able to get tickets at station when we arrive ? Is there train / bus from Chengde to either Datong or Mutianyu great wall ? How long would it take and how often it runs ?