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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 deter-
mined!).  






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.

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

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






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).

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".

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