Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 132)

Message Packet Formats
Chapter 5
5-53
Data Type Size
If Bit 3 is a zero (0), the data type defined in the ID Value Field uses 7 or
less bytes for each piece of data. The actual number of bytes the data type
uses for each piece of data is then contained in the Size Value Field (Bits
0, 1, 2).
If Bit 3 is a one (1), the data type defined in the ID Value Field uses more
than 7 bytes for each piece of data. The Size Value Field (Bits 0, 1, 2)
contains the number of bytes which identify the number of bytes the data
type uses for each piece of data. These bytes follow the parameter byte
and any ID type bytes.
Examples
The order of the bytes is least significant to most significant. One or more
most significant bytes containing all zeros are permitted, but the fields
thus generated are not different from those that omit these bytes. Thus the
following descriptors each have an ID type of 4 (integer) and a size value
of 3 bytes:
Bit 76543210
01000011
Bit 76543210
01001001
Bit 76543210
01001010
00000011 00000011
00000000
Type/Data
Parameter
The following example shows type “I” (integer) type/data parameters:
Bit 76543210
01000001
11111110
ID = 3 (Integer)
Size = 1 Byte
Value = –2
Bit 76543210
01000010
11111110
00000000
ID = 3 (Integer)
Size = 2 Bytes
(MS)
(LS) Value = 254