I have an R38 Arduino PLC. I am trying to connect it to an I2C LCD. When I have the PLC powered only by the USB chord through the computer, the PLC can find the I2C address at 0x27 using the I2C scanner code. However, when I power the PLC with my 12 vdc power supply, no address is found. I have tested two different I2C LCD's with an arduino Uno and they work correctly. If I test voltage on the SDA and SCL when the power supply is on, SCL = 5V and SDA = 0V. I have the switches setup for I2C (SDA and SCL).
When I only power the PLC with the computer USB chord, the code runs properly and characters will appear on the LCD screen. When I use the power supply, nothing happens. All other inputs/outputs are working properly. What could cause this and how can I troubleshoot this issue?