Why using the HMI program can not communicate with PLC?

There are many reasons why this may happen. The users can list some of the reasons that may contribute to the error and remove the possible ones which are not the case, and then locate the correct answer to this question. Some of the commonest reasons are listed below: 
1.The communication cable is not connected well, or the users use the wrong communication cable. 
2.The communication parameters are set incorrectly. 
3.The selected communication protocol is not correct. Most PLCs provide many interfaces for supporting many kinds of communication modes and protocols, therefore when performing the communication between HMI and PLC, the most important thing is that the users must understand what kinds of the communication modes and protocols that Delta DOP series of HMI supports and know the current communication modes and protocols that the PLC provides as well. For example, LG PLC provides a CPU port and a CNET communication module for communicating with HMI. When the communication mode is set as via CPU port in Delta DOP series of HMI, the communication between PLC and HMI will be failed if the users use a CNET communication module for communication connections. 
4.The settings of PLC are not set correctly. Now there are many kinds of PLC in the market and each of them has its unique configuration and communication settings. For example, when connecting to the Mitsubishi FX series PLC, if the Mitsubishi communication card is used for the communications, the users have to complete the parameter settings in PLC first; otherwise the communication will not work successfully. Therefore, to familiar with the connecting PLC and to understand the communication settings of Delta DOP series of HMI in accordance with the connecting PLC are both essential to successful communication. 

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