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R38 to I2C liquid Crystal Display no connection found when 12 vdc power supply is on

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Jason Martin
ArduinoPLCI2C

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?

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Support Team
Best Answer

Dear Jason, 


Here you can find a link to our blog post which shows how to work with I2C in our Arduino based PLCs, 


it seems that the Red Dip Switches have not been configured properly, when it does not receive power from the external Power Supply and only with the 5Vdc USB power, then it does not handle the red dip switch positions as the external circuitry is not powered, but once you connect the external power (12/24 Vdc) then it uses all the internal PCB circuitry. 


Keep us posted on this issue and have a nice day,

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