Plc 3 Physical Addressing; Order Of Transmission - Allen-Bradley 1779-KFMR User Manual

PLC 3 Physical Addressing

Order of Transmission

The physical addressing format applies to physical read and physical write
commands. Physical word addresses run in sequence, starting with zero
(0) for the first word of programmable controller memory. Physical
addresses occupy 2 words (4 bytes), in the following bit format:
In this format, A1 through A24 represent the 1 to 24 bits of the physical
address. For example, to address a command message to Physical Word
Address 12,200 decimal (002FA8 hex) you would use the following
binary code in the address field:
Programmable controllers store data in 16-bit (2-byte) words. The bits in
these words are numbered (addressed) Octal 0 (least-significant) through
17 (most-significant). Because values are conventionally shown with the
least-significant bit to the right, we illustrate a word by showing Bits 0
through 17 going from right to left within a word, as follows:
Programmable Controller Word
17 16 15 14 13 12 11 10 07 06 05 04 03 02 01 00
Bits
We illustrate programmable controller words as shown above. However,
when a 1779-KFL or -KFM interface transmits data to or receives data
from its host device, it transmits or receives one byte at a time. The
interface always transmits (and expects to receive) the low byte (Bits 00
through 07) of a word before the high byte (Bits 10 through 17) of the
same word. Also, it transmits (and expects to receive the low bit first
within a byte. Thus, when a programmable controller word is traveling
between the interface and the host device, it will look like this:
First Byte
Bits 00 01 02 03 04 05 06 07
Time
Appendix B
Second Byte
10 11 12 13 14 15 16 17