I am using an Arduino Nano clone with the SoftwareSerial library to communicate via UART with a third party serial device. In the board, I have 5 V that power up both the Nano and the serial device. The serial device has a removable internal battery pack and it is prepared to work without the battery pack as long as it is connected to the the external 5 V power supply. I have been able to operate without the battery pack in the serial device as long as I have the 5V power supply present AND the Nano connected to a computer, through its USB. If I remove the USB cable to the computer the serial connection between the Nano and the serial device does not occur. I have tried different possible solutions such as powering up the USB connector without the computer connected, different Arduinos, different serial devices, etc. always with the same result. I get it to work when:
a) I have 5V, internal battery, USB connection
b) 5 V and internal battery
c) 5 V and USB connection to computer
but I can’t get it to work with the 5 V alone, as it is supposed to do. Any help will be greatly appreciated.