Circuit principle: SCK, miso and MoSi are the clock signal, master station input signal and master station output signal of SPI interface respectively. It is defined that the SPI interface of STC12 works in the slave mode, and stc11 acts as the SPI master station. Since stc11 single chip microcomputer has no SPI function, the SPI master station function must be simulated by software. Here, stc11 is only used as the transfer station for byte data transmission, and the verification, storage and conversion of information frames are not carried out. The instructions sent by the monitoring computer are received through the serial port of stc11 and sent to STC12 for processing through SPI. Stc11 receives the return information through SPI, and then sends it to the monitoring computer through serial port. In the design, STC12 passes the SPI_ The falling edge of the output of TX pin tells stc11 to start the analog SPI master station function and read a byte of data from the SPI interface register of STC12.