5V TTL voltage levels consider everything above 2V a logic high, and can thus also receive data from 3.3V UART devices. Voltage that ranges between 0 and power supply voltage (Vcc) is also called TTL voltage levels.
Connecting your 3.3V bridge to a 15V RS232 line will quickly destroy it. To avoid frying your device, it is important to use the correct voltage. For most other UART devices, 0V indicates a 0 bit and the higher voltage indicates a 1 bit.
The sending party flips the signal every 104µs, and the receiving party checks the voltage on the line every 104µs. In that case, one bit takes up 1∕ 9600 of a second, or 104µs.
A common baud rate is 9600 bits per second. The baud rate is a term for the number of bits per second that are transmitted over the wire. To correctly communicate, both devices must be configured beforehand to use the same speed of communication, called the baud rate. There is no clock signal and no negotiation between the two devices. UART uses a binary protocol, so there are only two voltage levels: high and low. Now, the devices can send data to each other by varying the voltage on the Tx lines, and read data by checking the voltage on the Rx line.