Modbus RTU

Hello,
I need your help for Modbus RTU

  1. I installed Modbus RTU library
    2)open the Modbus slave example as my project
    3)upload the project to Arduino mega2560.
    4)Install Modbus Master simulator software in my PC
    5)connect my PC to Arduino board
    6)Launch Modbus Master simulator

---works perfect---

Then I connect TX0, RX0 to my serial cable , serial cable to USB adapter, lauch the Modbus Master simulator, however, it does NOT work.

OK, now.
The issue is solved.

Add a TTL to RS232 adapter, RS232---USB adapter, then works.