By Christine Murray MEXICO CITY (Reuters) – For a generation, ATT and Mexican billionaire Carlos Slims telephone business stood side by side and grew to become giants of the industry north and south of the Rio Grande. Now, ATT Incs planned $48.5 billion purchase of U.S. satellite provider DirecTV is severing a connection with Slim that goes back to 1990, and turning them into competitors. ATT said on Sunday it would sell its 8.4 percent stake in America Movil, Slims main cash cow, and withdraw its board members from the company to avoid conflicts of interest with the Mexicans TV operations across Latin America. America Movils shares fell more than 5 percent early on Monday, wiping over $3.5 billion off the companys value, although they recovered some ground in late morning trade and were down about 3 percent.