Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 45)

Commands
Chapter 4
4-9
indicates a low-battery condition. Because of these special functions, care
must be exercised in controlling word 027 with a write or bit write
command.
The processor does not prevent data from being written to this data table
word.
When it executes a read, write, or bit write command, the communication
adapter module controls data table locations at a station processor. When
addressing a command to a Bulletin 1774 PLC processor, note that there
are certain data table areas with a special function. These data table areas
are not to be used for control by communication adapter module
commands:
Any input image table word that has a corresponding input module in a
Bulletin 1778 or 1771 I/O rack
Word 377
Word 000
Input Image Table
When an input image table word in the PLC processor data table is
unused, that is, has no corresponding input module in an I/O rack, that
word can be used for storage. However, should an input module
correspond to that word, the data in the word is updated from the input
module each I/O scan.
Note that this restriction does not prevent any word of the input image
table from being written to another station. This control restriction applies
only when attempting to write data into input image table locations.
Word 377
Word 377 has special status functions within the PLC processor. For this
reason, the communication adapter module must not execute write or bit
write commands into this word.
A read command from a local station can address word 377 at a remote
station PLC processor.
Accessible Data Table Locations
- PLC Processors