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1. Introduction
2. Binary Block Read Mode and Structure
2.1. Compressed Binary Block Read Sequence
2.2. Compressed Binary Block Structure
2.2.1. Data Block Structure
2.2.2. Record String Structure
3. Compression Decompression Methods
3.1. Table Decompression
3.1.1. Nibble Positioning
3.1.2. Record String Format
3.1.2.1. Data Decompression Considerations
3.1.3. Table Decompression Example
3.2. Space Compression
4. DLE Stuffing
5. The Cyclic Redundancy Check Option
5.1. Calculation
6. Customer Service
PollCat Series

3.1.2.1. Data Decompression Considerations

 

The Character Count Byte gives the number of characters before either type of compression. It may be necessary for the polling device to do table and space decompression operations in a nested order in order to determine how many nibbles to decompress.

This nested order would be done by first table decompressing the data, one character at a time, until a space compression character is encountered. At that time, the space compression character would be decompressed and the number of spaces counted. Table decompression would then continue until the total number of characters specified by the character count byte have been decompressed.

Another method would be to table decompress all nibbles in the string using the "offset byte -3" as a byte count. The last nibble (MSN of the last byte) will always be the one in question and should be immediately discarded if it is an "F" nibble. The table decompression string should then be space decompressed in a separate operation.

Once both decompression methods have been employed, the character count byte could be used to determine the number of valid characters and the last stray character, if not valid, could be discarded.