XMODEM协议是一种串口通信中广泛用到的异步文件传输协议。分为标准Xmodem和1k-Xmodem两种，前者以128字节块的形式传输数据，后者字节块为1k即1024字节，并且每个块都使用一个校验和过程来进行错误检测。在校验过程中如果接收方关于一个块的校验和与它在发送方的校验和相同时，接收方就向发送方发送一个确认字节(ACK)。由于Xmodem需要对每个块都进行认可，这将导致性能有所下降，特别是延时比较长的场合，这种协议显得效率更低。(XMODEM protocol is a widely used serial communication in the asynchronous file transfer protocol. Divided into standard Xmodem and 1k-Xmodem two, the data of 128 byte blocks, the blocks of bytes for 1K is 1024 bytes, and each block with a checksum and the process for error detection in the verification process. If the receiver is about a block and check with it at the sender's check and at the same time, the receiver to the sender sends a confirmation byte (ACK). Because the Xmodem needs are recognized for each block, which will lead to the decline in performance, especially the long delay situations, this agreement is less efficient.
In addition to Xmodem, and Ymodem, Zmodem protocol. Their agreement is similar to Xmodem, the difference is that Ymodem allows batch file transmission, high efficiency; Zmodem is improved by Xmodem, it only needs to damage other block retransmission, the correct block does not need to send a confirmation byte. Reduces the amount of communication.)