










                     Ŀ
                           
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                             Ĵ        Ĵ
                                       





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Binary  data is  made up  1's and  0's. PARITY  is a  term used to describe the
number of 1's in a binary number. If the total number of 1's is an EVEN number,
then the PARITY of that binary number is EVEN. If the total number of 1's is an
ODD number  then the PARITY  of that  binary  number is ODD.  That seems pretty
straight forward!

A PARITY  Bit  is  used  is  ASYNCH  (NON-synchronous) communications to detect
errors.  The PARITY  bit is  created automatically  by the  Modem and is placed
right  after  the  Stop  bit  (which  is  not  counted  when  PARITY  is  being
determined!).




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This generated  PARITY bit can be  either a 1 or  a 0 depending on  a couple of
considerations:

    1.   The Type of PARITY selected

     2.   The actual PARITY of the binary number that is being transmitted.





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                    Ŀ
                     Let's look at a specific binary number 
                    Ĵ
                             Ŀ         
                                                        
                              1    1111001   0          
                                                        
                                      
                                                        
                                              
                             STOP            START         
                              BIT             BIT          
                                       7                    
                                   DATA BITS                
                    


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Now  then, let's  say that  we have  set our  protocol to  7-E-1 which you will
recall means, 7 Data Bits, Even PARITY, and 1 Stop Bit. Count the number of 1's
in the Data Bit. If you came up with 5, go to the head of the class! Since 5 is
an ODD number, the PARITY bit will need to be a 1 to keep the PARITY EVEN.







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                    Ŀ
                     Let's look at a specific binary number 
                    Ĵ
                                                            
                           Ŀ       
                                                       
                            1  1   1111001    0        
                                                       
                                  
                                                       
                                             
                           STOP               START       
                            BIT         7       BIT        
                                    DATA BITS              
                             PARITY BIT                     
                    

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The Data Bit was ODD, so a 1 PARITY bit is added to make the number EVEN (5 + 1
= 6). Incidentally, the original Data Bit  could be sent without the PARITY bit
if we were set for 7-N-1 (7 Data bits, NO PARITY, and 1 Stop Bit).

PARITY and ERROR DETECTION

We have  already discussed the  fact that binary  data is made  up 1's and 0's.
PARITY is a term that is used to describe the number of 1's in a binary number.
If the total  number of 1's is an  EVEN number, then the Parity  of that binary
number is EVEN. If the total number of  1's is an ODD number then the Parity of
that binary number is ODD. That seems pretty straight forward!



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PgDn

Remember, a Parity  Bit is used is ASYNCH communications  to detect errors. The
Parity bit is created automatically by the  Modem and is placed right after the
Stop  bit  (which  is  not  counted  when  Parity  is  being determined!). This
generated  Parity bit  can be  either a  1  or  a 0  depending on  a couple  of
considerations: the Type of Parity selected and the actual Parity of the binary
number that is being transmitted. Let's look at a specific binary number:

                     Ŀ
                              Ŀ         
                                                         
                               1    1111001   0          
                                                         
                                       
                                               
                                                          
                              STOP            START         
                               BIT             BIT          
                                        7                    
                                    DATA BITS                
                     

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PgDn



Now  then, let's  say that  we have  set our  protocol to  7-E-1 which you will
recall means, 7 Data Bits, Even Parity, and 1 Stop Bit. Count the number of 1's
in the Data Bit. If you came up with 5, go to the head of the class! Since 5 is
an ODD number, the Parity bit will need to be a 1 to keep the Parity EVEN.

                  Ŀ
                           Ŀ         
                                                         
                            1  1   1111001    0          
                                                         
                                    
                                               
                                                          
                           STOP         7      START         
                            BIT     DATA BITS   BIT          
                                                             
                             PARITY BIT                       
                  

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The Data Bit was ODD, so a 1 Parity bit is added to make the number EVEN (5 + 1
= 6). Incidentally, the original Data Bit  could be sent without the PARITY bit
if we were set for 7-N-1 (7 Data bits, NO Parity, and 1 Stop Bit).

So how does  this all come together? Both ends  of the COMMunications link must
use the  same Parity (translate  that to SAME  PROTOCOL). If you  are calling a
Bulletin Board and DOWNloading a file, your  computer checks each Byte as it is
received.  If a  Byte is  received which   has the  wrong Parity,  an Error  is
detected and  your Computer sends a  message back to the  BBS demanding that it
re-send that messed up Byte.

You've probably  seen your D/L Pause  briefly while error correction  is taking
place.  Using today's  sophisticated File  transfer protocols,  if an  error is
generated it is usually due to line noise.
