The TXD and RXD of serial prot in MCS51 are TTL. The sign is lower and always interrupted,so it is comfortable for communication of short distance.

• MCS5 1单片机本身的串行接口TxD ；RxD为TTL电平 ；信号幅值较低 ；易受干扰只适合很近距离的通讯 .