1.Please remove the HDD and check the DIP switch setting, we can refer to the information in the following picture.
The default setting of COM2 is RS-485/422.
2.After setting the DIP switch to RS485 mode, please enable "RS485 Auto Flow" in BIOS.
3.HMIPSPS752D1X01 is a product of Easy Line Harmony Panel iPC. The port definition of HMIPSPS752D1X01 is different with HMIPSP S-Panel PCs, so user can’t refer to the manual of HMIPSP S-Panel PCs.
Here is the Pin connection of HMIPSPS752D1X01 COM2, user can connect to Pin 3 and Pin7 for RS485 Modbus RTU communication.