Initiating data transactions on a system bus is disclosed. In some implementations, a controller receives first information from a first peripheral requesting a first data transaction. The first information is received over a first communication link between the controller and the first peripheral. The controller receives second information from a second peripheral requesting a second data transaction. The second information received over a second communication link between the controller and the second peripheral. The controller determines first and second ranks for the first and second data transactions, respectively, based on the first and second information, and initiates based on the first and second ranks, the first and second data transactions on a system bus.