So, you may see where the problem lies in interfacing these two signals. This timing diagram shows both a TTL (bottom) and RS-232 signal sending 0b01010101 This means that an RS-232 signal can generally travel longer physical distances than their TTL counterparts, while still providing a reliable data transmission. The more extreme voltages of an RS-232 signal help to make it less susceptible to noise, interference, and degradation. On most PCs these signals swing from -13 to +13V. By the RS-232 standard a logic high ('1') is represented by a negative voltage – anywhere from -3 to -25V – while a logic low ('0') transmits a positive voltage that can be anywhere from +3 to +25V. The two differ solely at a hardware level. RS-232 signals are similar to your microcontroller's serial signals in that they transmit one bit at a time, at a specific baud rate, with or without parity and/or stop bits. The serial port on your computer (if it's lucky enough to have one, they're quickly becoming a relic) complies with the RS-232 ( Recommended Standard 232) telecommunications standard. A logic high ('1') is represented by Vcc, while a logic low ('0') is 0V. Serial communication at a TTL level will always remain between the limits of 0V and Vcc, which is often 5V or 3.3V. This method of serial communication is sometimes referred to as TTL serial (transistor-transistor logic).

UARTs transmit one bit at a time at a specified data rate (i.e. Most microcontrollers these days have built in UARTs (universally asynchronous receiver/transmitter) that can be used to receive and transmit data serially. These two devices are compatible from a software perspective, however you can't just hook a microcontroller up to a computer because the hardware interfaces are not compatible. Serial is very easy to implement, and it allows you to send/receive any data you need from your microcontroller to a computer's serial port so it can be viewed using a terminal emulator. One of the tools we use most when debugging our projects is serial input/output.

How does serial communication work