Im planning on connecting my Arduino to an air con which i want to communicate via the rx and tx terminals the air con uses a 12v serial communication line. that sends binary code to carry out the certain functions over the 12v line and from what i've gathered the Arduino shouldn't be connected to anything over 5v ? can anyone shed some light on this problem ?
im very new with Arduino btw
You can buy (or build) level converters based on the Maxim MAX232 device.
my concern around this would be if i drop the voltage down will that interfere with the communication that travels in that 12v line ?
As said from @anon73444976, a RS232 level converter will do the job (it's not just a simple level converter)
@josh6899, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with your project 
See About the Installation & Troubleshooting category.
But you need way more information than that!
Just what is this "12v serial communication line"? What are the logic levels? How many wires? What is the protocol?
Where is the documentation? Do you have the documentation, in which case provide it here as a link? If you do not have the documentation, how do you know how it works?
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