Problem in Simple modbus in Nano

Dear all,

I have tried to use SimpleModbus Slave and successes in Arduino UNO and computer(use Mtester.exe). But when changed the MCU to Arduino Nano with same program, No respone.

Please help


More information I want to add. When I download program, the RS485 module need to disconnect, both Nano and Uno. Also, external DC7.5V is applied, instead of using USB power.