Rockwell Automation 1771-KA2 Communication Adapter Module - Manual (Page 43)

Commands
Chapter 4
4-7
The second restriction listed above applies when the destination station is
a PLC-2 family PC. For these processors, when the station
communication adapter module receives a bit write command it
manipulates the 8-bit byte of the 16-bit memory word in which the
addressed bit is located. (This may be the low byte, containing bits 00-07,
or the high byte, containing bits 10-17.) Should program instructions
control other bits within the same byte, there is a slight possibility that the
communication adapter module may write over programmed status for
these program-controlled bits. This would occur only if the program
caused a bit to be altered during the time the communication adapter
module was executing a received bit write command.
Therefore, when using the bit write command, address only bits within a
byte that is set aside exclusively for control by these commands.
The unprotected read command transfers word data from a remote station
processor to the local station data table. A single unprotected read
command can access from 1 to 122 consecutive data table words.
The unprotected read command is not restricted by user programming.
This command can read data table words from any remote station
processor, regardless of either memory access rung programming or
module switch settings.
Because this command controls data table words at its local station
processor, the rules for data table control apply when using this command.
(Data table control is the subject of sections titled “Accessible Data Table
Locations-PLC-2 Processors” and “Accessible Data Table Locations-PLC
Processor”).
When it executes a read, write, or bit write command, the communication
adapter module controls data table locations at a station processor. This
section outlines the recommendations for control of data table locations in
PLC-2 family processors.
A KA2 module executes read, write, or bit write commands to control any
accessible data table words in any of these processors. It also controls
user-selected status words in the data table of these processors. (Status
words are described in chapter 6.) A KA2 can access most data table
words. However, certain memory areas in these processors have special
functions that prevent control of these areas by the module. The
following are data table areas with a special function in these processors:
Read Command
Accessible Data Table Locations
- PLC-2 Processors