Rockwell Automation 1747-SDN SLC 500 DeviceNet Scanner Module User Manual - Manual (Page 111)

Publication 1747-UM655B-EN-P - June 2007
AutoScan 111
For even I/O allocation sizes, the formula for calculating the Input or
Output data offset since there are two bytes in each word is:
M file Input (Output) [Word] Offset = ((Node Address) x (I/O
Allocation Size / 2))
When using the default fixed mapping size of 4 bytes or 2 words, the
input data for devices are allocated in the 1747-SDN module's input
table. Note that M files are used because discrete mapping is not used
with AutoScan. In this case node 14 has an offset of 14*2=28 word
therefore node 14 data is located in the data map at word 28 of the M
file.
EXAMPLE
When using five bytes for each device, the input data for
devices are allocated in the 1747-SDN module's input table.
Note that M files are used because discrete mapping is not
used with the AutoScan feature. In this case node 15 has an
offset of 15*5=75 byte, therefore node 15 data is located in the
data map at word 75/2 = 37.5 of the M file with the non-integer
result the offset starts at the upper byte of the word.