Instructions & Information about the Checksum Encoder/Decoder

Checksum

The cheksum is primarily used to detect errors in data transmission on communication networks.  The idea is to add up all the block of data that is being transmitted and transmit the sum along with the data.  The receiver adds up its received data blocks and checks if the received checksum bits match the calculated checksum.

There are some basic methods to calculate the checksum bits.  The four methods illustrated in the Checksum Encoder/Decoder are Single-Precision, Double-Precision, Residue Checksum & Honeywell Checksum methods.

Single-Precision method calculates checksum as the modulo-2d addition of the data words in the block.  Double-Precision calculates checksum as modulo-22d addition of the data words in the block.  Residue Checksum takes into account the carry out of the dth bit as an end-around carry.  Honeywell Checksum concatenates data words together into pairs for the checksum calculation. It mainly guards against errors happening in the same position.  The four methods are illustrated in Fig.1.



Fig.1 Checksum Encoding Methods

Help with the Checksum Encoder

The checksum encoder calculates the checksum bits for the data words supplied by the user. The user can make a choice of one  among the four methods for calculating the checksum.  The data words should be binary bits or hex digits of equal length. The maximum allowable length of the datawords is 32 for binary and 8 for hex. The number of datawords input can be from 0-10 in any order. However, there needs to be an even number of datawords to compute the Honeywell Checksum.  On clicking "Encode -->", the checksum bits are calculated according to the chosen method.

Help with the Checksum Decoder

The Received Data Words and the checksum are also shown. The user can modify them to simulate error conditions during transmission.  The received data bits and the checksum bits should be in binary/hex form. The maximum length is 32 for binary and 8 for hex. The number of bits must correspond to the checksum encoding method that was chosen. For example, single-precision and residue encoding methods use the same number of bits for the checksum as the number of bits for the data word while double-precision and Honeywell encoding methods use twice the number of data word bits for the checksum.

On clicking "Decode -->", the new checksum bits are computed according to the same method and are compared with the received checksum bits.  If they do not match, the fault is detected.