A universal asynchronous receiver-transmitter is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least to most significant, framed by start and stop bits so that precise timing is handled by the communication channel. The electric signaling levels are handled by a driver circuit external to the UART. Common signal levels are RS-232, RS-485, and raw TTL for short debugging links. Early teletypewriters used current loops.