将接收到的并行数据转换成串行数据来传输。消息帧从一个低位起始位开始，后面是5~8个数据位，一个可用的奇偶位和一个或几个高位停止位。接收器发现开始位时它就知道数据准备发送,Converting the received parallel data into serial data to transmit. The message fr a me from a low start bit is followed by 5 to 8 data bits, parity bit, and one of the available one or several of the high stop bit. Receiver when the start bit is found, it knows that the data is ready to send