High-speed USB implements a PING-NAK mechanism for OUT transfers. When the host wishes to send OUT data to an endpoint, it first sends a PING token to see if the endpoint is ready (i.e. if it has an empty buffer). If a buffer is not available, the FX2LP returns a NAK handshake. PING-NAK transactions continue to occur until an OUT buffer is available, at which time the FX2LP answers a PING with an ACK handshake and the host sends the OUT data to the endpoint. The EPxPING interrupt is asserted when the host PINGs an endpoint and the FX2LP responds with a NAK because no endpoint buffer memory is available. These interrupts are active only during high speed (480 Mbits/sec) operations.