Allen-Bradley 1779-KFMR User Manual page 203

Appendix B
This does not present a problem at programmable controller nodes on the
link because programmable controllers always store and retrieve their data
in this same order of low byte first. It can, however, require you to do
some extra computer application programming to maintain the proper
byte and word order in programmable controller data stored in a
computer.
Three factors that can influence the ability of your computer to handle
programmable controller data are:
The size of words in your computer's memory.
The left-to-right or right-to-left ordering of bits within a word in your
computer's memory.
Whether the computer considers the low-order byte of a word to have
an even or an odd address.
If your computer uses something other than 2-byte, 16-bit words, you
should design your application programs to make the proper conversions
from programmable controller word addresses to computer word
addresses. When stored in a computer, each programmable controller
word should start on an even-byte boundary.
Figure B.4.A shows a 16-bit word in programmable controller memory.
Figure B.4.B shows a 16-bit computer word with right-to-left byte and bit
order (as in DEC PDP-11/34 or VAX 11/780). It also represents a 16-bit
word in a 8-bit processor (such as Zilog Z-80 or Intel 8086
microprocessor). If your computer has this type of word order, the
conversion is straightforward.
Figure B.4.C shows a 16-bit computer word with left-to-right byte and bit
order (as in IBM Series 1). If your computer has this type of word order,
the conversion is more complex. You will have to swap bytes into and out
of buffers.
Figure B.4.D shows a 16-bit computer word with left-to-right byte order
and right-to-left bit order (as in Zilog Z-8000 or Motorola 68000
microprocessors). If your computer has this type of word order, your
communication driver must handle the task of byte swapping as it loads
data into a buffer. Successive bytes received from the programmable
controller must be stored in the following order:
1,0,3,2,5,4,7,6,9,8, ...