Spectrum-controls 1769sc-HART Modules Manual de usuario descargar pdf (Pagina 120)

Compact IO™ Isolated HART Analog Input Module

User's Manual 0300215-03 Rev. A

8-4

The algorithm can be implemented in ladder by masking and shifting four 6-bit

characters into a double word register then moving the three bytes into the packed ASCII

string.

Reconstruction of ASCII characters:

Unpacking packed ASCII strings requires flipping some bits in addition to

uncompressing the string itself. To unpack a packed ASCII string:

Unpack the four, 6-bit ASCII characters.

For each character, place the complement of bit 5 into bit 6.

For each character, reset bit Bit 7 to zero.

Repeat until the entire string is processed.

This algorithm can be implemented by loading three bytes into a 24-bit register and

shifting the four 6-bit characters into the string. Parse the resulting character to flip bit 6

as needed.

The ladder sample starting on the next page demonstrates how to pack 4 unpacked ASCII

characters into 3 bytes.

Figure 8-4a (Packed ASCII)

1 2 ... 115 116 117 118 119 120 121 122 123 124 125 ... 185 186

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