Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 143)

Data Encoding and Addressing
Chapter 6
6-6
corresponding place value and add the results of the multiplications.
Figure 6.5 shows the octal representation of the decimal number 239.
Figure 6.5
Octal Numbers
1 1 1 0 1 1 1 1
11336
1 x 2 1 = 2
1 x 2 0 = 1
1 x 2 2 = 4
0 x 2 1 = 0
1 x 2 0 = 1
1 x 2 2 = 4
1 x 2 1 = 2
1 x 2 0 = 1
3
8
5
8
7
8
3 x 8 2 = 192
5 x 8 1 = 40
357
8
= 239
10
7 x 8 0 = 7
Order of Transmission
PCs store data in 16-bit (2-byte) words. The bits in these words are
numbered (addressed) 0 through 17 octal, going from right to left within a
word, as follows:
15 14 13 12 11 10 07 06 05 04 03 02 01 00 17 16 Bits
PC Word
In PC memory, the words are arranged as shown above. However, when
the KF2 module transmits data over its asynchronous link, it transmits
one byte at a time. The module always transmits the low byte (Bits 00
through 07) of a word before the high byte (Bits 10 through 17) of the
same word. Also, the UART transmits the low bit first within a byte.
Thus, when a PC word is traveling over the asynchronous link, it will look
like this:
00 01 02 03 04 05 06 07 10 11 12 13 14 15 16 17 Bits
Time
First Byte Second Byte