Abstract: In order to tackle the problem relating to the inability of the S7-200 programmable controller to directly read the data of the SCL-61 ultrasonic water meter via the communication protocol preset by the programming software STEP 7-Microwin4, it is proposed to first write a program in S7-200 controller in accordance with the definition and characteristics of the controller′s free port communication protocol and the SCL-61 D water meter′s communication protocol specifications, and then use the free port mode to set up the communication link. As the meter′s data can be read out by using this method real-time communication between the meter and controller can be made in a secure and reliable manner.