To conserve bandwidth and to enable inactive links to transition to low-power states, USB 3.0 hosts stop requesting to send or receive data from SuperSpeed endpoints that are in the flow control condition. This condition indicates that the endpoint temporarily can’t send or receive data. To request to resume communications, the endpoint sends an ERDY Transaction Packet. A device can send the ERDY at any time without waiting for the host to request a packet.

On receiving the ERDY, the host resumes communications with the endpoint. An IN endpoint is in the flow control condition after responding to an ACK Transaction Packet with either of the following A NRDY Transaction Packet. A Data Packet with the End of Burst (EOB) field set to 1, indicating that the packet is the last in a burst. The device sets EOB if the data payload is equal to the endpoint’s maximum packet size and the endpoint is returning fewer than the number of packets requested in the previous ACK Transaction Packet.

An OUT endpoint is in the flow control condition on responding to a Data Packet with either of the following A NRDY Transaction Packet. An ACK Transaction Packet with the NumP field set to zero, indicating that the endpoint can’t accept any Data Packets. Hosts retain the option to attempt communications with bulk endpoints in the flow-control condition before receiving ERDY.