Data exchange over 5 meter cable
I'll also vote for RS232, you should be careful about one thing that no-one else has mentioned though. You don't say what microcontrollers you are using, but the following is true for most that I've used...

Your microcontroller will have at least one data transmit line (TX) and at least one data receive line (RX). These lines will toggle between low &amp; high as you transmit/recieve between voltage levels of 0V &amp; ~5V or 0V &amp; 3.3V depending on the device. Assuming both microcontrollers use the same voltage levels then, in principle, you could just connect TX from one micro to RX of the other and vise a versa. This works for short distances &amp; low data rates, but I think 5m is way too long for this and I also think it is bad practice to do this between systems. 

Better to use a level converter (such as the MAX232 family for 0-5V systems or the MAX3232 for 0-3V systems) on each end. These convert the logic 0 to ~+12V and a logic 1 to ~-12V so that your comms line conforms to the RS232 standard electrically.  

Hope this helps,

Joe