Start bit and end bits in Serial Data Transmission Confusion

Question

I am bit confused how start and stop bit are differentiated from the actual data bits. For example say "data" whose binary is 01100100 01100001 01110100 01100001 is being set from System A to System B as a single packet (because it's less than 64 Kibibytes) bit by bit. Please let me know how start bit and stop bits are added to these data bits. There were two related thread on Stacloverflow with only one answer this was not accepted but is very confusing. Can someone explain it in simple terms please. Thank you

Answer 1

When you want to send data over serial line, you need to synchronize transmitter and receiver. The start bit simply marks the beginning of the data chunk (typically one byte with or without parity bit), and the stop bit marks the end of data chunk.

In the beginning, there’s no data being transmitted - let´s say there is ‘0’ on the line for some time. The receiver is waiting for the start bit (both start and stop bits are always ‘1’). When the start bit arrives, it starts an internal timer and on every tick it reads the value from the line, until all data and parity bits are read. Then it waits for the stop bit and then it begins to start waiting for a new start bit.

Without the start bit, the receiver would not now when to start reading data bits. Imagine sending zero byte without parity: The line would just stay in 0 state all the time.

The stop bit is not necessary, it’s there just for enhancing the reliability (both receiver and transmitter must use the same frequency).

So, the start and stop bits don’t need to be distinguished from data bits. Quite the oposite: They allow the receiver to properly identify data bits.

When sending your data, you would take them byte by byte and for each of them you would send the start bit (‘1’) first, then individual bits, then maybe parity bit and then a ‘1’ - the stop bit, everything at a given frequency. Then you would wait at least for one timer tick.

Usually you don’t need to do all of this, because there are specialized chips for this on the board. You just provide your data using a buffer and wait until they’re sent, or you wait for data being received.