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Published byDawson Sibert Modified over 2 years ago

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Text (and ) Plain text and formatted text. Plain text is usually coded in “ASCII” (American Standard Code for Information Interchange). A 7 bit code which allows 128 characters. Computers usually deal with 8 bits so ASCII appears to “waste” one bit.

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Text “ASCII” coded text was originally designed to connect terminals (keyboard and text monitors) to remote computers. Errors could occur in the connection. Bit 8 used for parity checks.

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ASCII Full list of ASCII codes will appear on my website and will be given as a handout. But common letters and numbers are easy to remember. Upper case letters –Add 64 (decimal) (40 (hex)) to position in alphabet. Eg Code for B is = 66 Or = 42 in hexadecimal.

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ASCII Lower case letters –Add 96 (decimal) (60 (hex)) to position in alphabet. Eg Code for a is = 97 Or = 61 in hexadecimal. Numbers –Add 48 (decimal) (30 (hex)) to number. Eg Code for 5 is = 53 Or = 35 in hexadecimal. Working in hex may be easier.

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Parity Since we mentioned it. Error checking mechanism. Odd or even, (but we decide first). In 7 bit code (like ASCII) we use the 8 th (MSB) for parity. We set the bit to one or zero to make the total number of 1’s odd (for odd parity) or even (for even parity).

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Odd Parity Example 1 –Say our seven bit number is There are 4 ones. –We add an 8 th bit of value 1 to make the total number of ones odd, giving (1) Example 2 –Say our seven bit number is There are 3 ones. –We add an 8 th bit of value 0 to keep the total number of ones odd, giving (0)

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Even Parity Example 1 –Say our seven bit number is There are 3 ones. –We add an 8 th bit of value 1 to make the total number of ones even, giving (1) Example 2 –Say our seven bit number is There are 4 ones. –We add an 8 th bit of value 0 to keep the total number of ones even, giving (0)

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Parity Checked by receiving computer to see if there is an error. Can you see a problem with this? Clue - 2 errors. Midi code (for sound synthesiser communication) very similar to ASCII, but no parity.

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