Redundant acknowledgment between agents performing a loopback test over bidirectional communications bus is described. In one example the acknowledgment is performed by initiating loopback communications from a first agent to a second agent, sending a packet including a redundant acknowledgment sequence from the first agent to the second agent, receiving the packet including the redundant acknowledgement sequence looped back from the second agent at the first agent, sending a test sequence from the first agent to the second agent, and receiving the test sequence looped back from the first agent.