Data Acquisition

The calculation of the block checksum is performed simply by adding the hexadecimalvalues of all the ASCII characters in the message and converting the resulting hexadecimalnumber into its ASCII equivalent digits. This is shown in Figure 6.15 below. Note that wherethe summation is greater than 0 × FF, the most significant digit is discarded.Figure 6.15Block checksum calculation * 2A1 31R 52D 44+ 2B0 300 300 307 372 32. 2E1 310 30SUM2A4ErrorsIf the remote device receives a message with an error it will respond with the ? character.Alternatively, there may be no response at all if an incorrect address or a command prompthas been used. Typical error response messages are indicated in Figure 6.16.Figure 6.16Typical error response message for an ASCII based protocolThere are three popular forms of error checking used in many protocols. These are, in orderof increasing error-detecting capability:• Character redundancy checks• Block redundancy checks• Cyclic redundancy checks