Explain how the even parity system works when transmitting ASCII codes. Include a description of the roles of the sending device and the receiving device during transmission.

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The sender's role is to add the parity bit. ASCII characters are represented using a 7-bit binary string. A parity bit is added to the beginning of the binary string (i.e. at the most significant bit, or MSB). Since it's even parity, the number of 1's in the 8-bit binary string must be even.

For example, if the sender wanted to transmit the ASCII code for the letter 'A' (ASCII code 65), which is represented in binary as 1000001, a 0 would be added as the parity bit at the MSB. Therefore, 01000001 would be transmitted.

Similarly, for ASCII code 'C', which is represented in binary by 1000011, a 1 would need to be added to make the number of 1's even. In this case, 11000011 would be transmitted.

The receiver's role is to check that the number of 1's in the received binary string is even. If the binary string fails this parity check, the received will ask the sender to send the data agin.

For example, if 01000101 was received, this would fail the parity check; the number of 1’s in the binary string is not even, so the data would have to be resent. However, if 11000101 was received, then this would pass the parity check, meaning that it’s likely (though not necessarily guaranteed) that the data has been received as it was sent, and has not been corrupted.

Henry B. A Level Computing tutor

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