Spectrum Controls 1769sc-HART Modules - Manual (Page 119)

Chapter 8: Programming Examples
User's Manual 0300215-03 Rev. A
8-3
8.1.2 Swap Byte Order
This ladder sample demonstrates how to reverse the order of the bytes for a floating point
tag and then convert it to 4 consecutive SINT tags, so that it can be used in a HART
message.
!
Attention
If the HART message being sent or received using the pass-through
command contains floating point values, the order of the bytes must be
reversed.
Figure 8-3 (Change Byte Order)
8.1.3 Converting Unpacked ASCII to Packed ASCII
Packed ASCII is a HART-specific 6-bit character code representing a subset of the
ASCII character code set (see table below). Produced by compressing four packed
ASCII characters into three 8-bit bytes, packed ASCII strings must be a multiple of 4
characters (3 bytes) and must be padded out to the end of the data item with space
characters. For example, 4 space characters at the end of a string would appear as the 3
bytes: 0x82, 0x08 and 0x20.
Construction of Packed ASCII characters:
Constructing a packed ASCII string is a simple matter of discarding the most significant
two bits from each character and compressing the result:
1. Truncate Bits 6 and 7 of each ASCII character.
2. Pack four, 6 bit-ASCII characters into three bytes.
3. Repeat until the entire string is processed.