The two most commonly used serial asynchronous communication protocols are 8bit data mode and 9bit data mode.This paper makes a detailed analysis of these two communication protocols.It also analyzes whether the different protocols can communicate normally,especially the feasibility of sending data in 9bit mode and receiving data in 8bit mode.This method has been analyzed from the protocol and verified by experiments.This paper also gives the method of how to analyze the communication protocol from the source of the communication waveform when communication errors occur in practical applications,and how to find and confirm whether it is the problem of the communication protocol setting by using the method of receiving data transfer.