Question

Explain with the aid of a diagram how the start, data, parity and stop bits are arranged in a serial word. Should the stop bit be pulled high or low?

Answer 1

Solution:- In general, distinct packets or frames are used by the serial links for transmitting data. The transmitter transmits the frames which is set of bits. There are fours types of bits that a frame consists of:

1. Start Bit- It signals the beginning of a frame.
2. Data Bits- It comprises of data.
3. Parity Bits- It is an extra bit that is used to detect transmission errors.
4. Stop Bit- It signals the end of frame.

Lets take an example. We want to send two bytes. Each byte consists of one start bit, followed by 8 data bits and one stop bit, forming a 10-bit character frame. The last bit is used as parity bit sometimes. The communicating parities or the sender and the receiver must agree on some pints before the start of transmission-

• Number of data and formatting bits
• Order of data bits
• Presence or absence of parity bit
• Form of parity(even or odd)
• Transmission speed

As the stop bit is actually a stop period. The stop period for the transmitter may be irrationally long. It cannot be shorter than 1 or 2 bit times. The stop period of receiver is less than the transmitter. So, at the end of each period the receiver stops to check for the next start bit, This gap keeps the synchronization between the receiver and the transmitter.