Mechanism controlling longitudinal movements of guide bars in a warp knitting machine is still employed in many factories in China. But it is use a lot of time in order to change the type of fabrics. This sort of mechanism is gradually replaced by stepping motor that controlled by microcomputer. So, it is need to send data that guide bar need to move from computer to microcomputer. A circuit of transmission data is designed. The serial communication uses port of RS232C and potential convertor MAX232ACPE without model. There are use only three lines of RXD, TXD and GND of the serial communication port in the microcomputer (80C51). The protocol of transmission data is programmed. The serial communication baud rate at 2400 is set. The data that control movement of guide bars can be transmitted rightly.