What if a USB IN request comes in while firmware is loading the IN FIFO for the MAX3420E?
固件正在装载IN FIFO时，如果收到USB IN请求，会发生什么情况？
The MAX3420E takes care of this. It automatically answers an IN request to an "unarmed" IN FIFO with a NAK (Negative Acknowledge) handshake. This handshake instructs the USB host that the endpoint is busy, and that the host should try later with another IN request.
MAX3420E注意到了这一问题。器件采用一个NAK(非应答)握手信号，即以IN FIFO“未准备好”自动响应IN请求。该握手信号通知USB主机端点忙，主机应稍后以另一个IN请求进行尝试。