We have a PLC Siemens S7 300 CPU 315 DP, using NT631C-V2 Omron HMI via Siemens HMI Adapter. Now having problems with its HMI that can not connect to the PLC, already I check the serial cable between the HMI and HMI Adapter and no problems. Please help from the experts what causes the HMI and PLC can not connect, thanks.