ESP32-EVB hardware serial does not work when USB is not connected to a computer

[smcd22]

Hello,

I've been using the ESP32-EVB, with the MOD-RS232 to connect to another device over RS232. This works well when the ESP32-EVB is connected to the computer with the USB connection, but when only using the external power supply (5V,2A) and disconnecting the USB, the serial communication over RS232 no longer works. The other functionality (wifi, relays) work fine when the board is powered solely by the power supply. I'm using the HardwareSerial Serial1, with the standard pins (RX1=36, TX1=4) for the connection. It still doesn't work when I don't include `Serial.begin` and any references to the usb serial monitor.

Thanks for your help

[LubOlimex]

It should be problematic due to two or more GNDs applied to the same board. Probably there is voltage difference between the GNDs and the board hangs.

Try the other way around - disconnect ESP32-EVB from the MOD-RS232 and the power supply, then first apply the external power supply, after that connect the MOD-RS232. Does it behave differently?

[smcd22]

I've now tried that, but still have the same issue. Any other suggestions?

Thanks

[LubOlimex]

Measure the voltage between the two GNDs. One of the GNDs has to be disconnected, e.g. measure the voltage when one of the supplies is attached to the board and the other is disconnected.