Implementation of serial communication between host computer and PLC based on Host Link protocol

The host link communication protocol between host computer and PLC (Programmable Logic Controller) is introduced in this paper, and a method of using API (windows application program interface) to realize serial communication between host computer and PLC is proposed, the software design process and the corresponding procedures of serial communication between host computer and PLC are given. Through reading or writing operations, data exchange between host computer and PLC are realized, which can effectively carry out the industrial field device monitoring and management. Taking OMRON CJ1H Series PLC as the testing object, testing software is written for verifying the function of the system. When the communication between host computer and PLC start, there are the reading delay and the writing delay which may seriously affect communication rate. By testing, the important data about delay time are obtained. It also shows the relationship between communication delay and byte length. According to obtained data, methods of improving the communication delay are given. The testing results show that the method is effective and real time in communication, and has the engineering application value.