Avoiding confusion between no input and a zero in binary?
If you feed something to a UART port of a microprocessor you must follow the UART communication protocol if you want the microprocessor to understand what you are feeding it. You need to embed each ASCI character into a UART package that contains a start bit, a stop bit and possibly a parity bit, a lot more information is available on the UART Wikipedia page.

When you're not sending the bus is idle and it is kept in a logically high state. When you're starting the transmission of a package the first bit is always logically low. This is the start bit. Then follows eight data bits and then a stop bit which is logically high. The microprocessor knows when the next bit is arriving since it knows the bus speed that you have configured. Therefore, it is possible to transmit for example two zeroes next to each other. Both the microprocessor and the unit transmitting to the microprocessor needs to be configured with the same transmission speed, parity and number of stop bits.