Data Acquisition

6.8.1 Character redundancy checks Character redundancy checks rely on the transmitter and receiver agreeing to use even or oddparity to calculate the parity bit to append to each character. For example, if even parity isdefined for a link, the 7-bit data byte ASCII 0110001 becomes 01100011, that is, a 1 isappended to the preceding seven bits to ensure that there are even numbers of 1 in the byte.The receiver checks that the arriving 8-bit byte has even parity. If it does, it extracts the firstseven bits as data. If the received byte has odd parity, the receiver reports an error.6.8.2 Block redundancy checksIn this method, an additional character called the block check character is calculated andadded to the stream of characters transmitted down the communications channel.For example, transmission of the three characters A B Z would have a block checkcharacter (BCC) calculated, which is added to the end. The two different techniques for calculatingthis block check character are indicated in Table 6.7. The techniques are:Vertical longitudinal block redundancy checkThis method relies on the calculation of even or odd parity for each individual character andthen for all the characters in a block. The mechanism is indicated in the following table.Arithmetic checksumThis checksum is calculated by adding all the bits and then discarding the carry bits. A paritybit is also calculated for each individual character. Table 6.7Two techniques for block redundancy checks6.8.3 Cyclic redundancy checksA more effective way of checking for errors is the cyclic redundancy check scheme that has aworst-case error checking ability of 99.9969%. There is thus minimal likelihood of errorsslipping through undetected by the receiver.