Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 71)

Using the 1775ĆMX
Memory Communication Module
Chapter 4
4-24
Start Data Command
The START DATA command lets you transfer the entire contents of the
data table, except the status section, from the primary processor to the
backup processor. The START DATA function is equivalent to the DATA
TABLE XFER function in LIST.
After the primary processor executes a message instruction containing the
START DATA command, the memory communication module
continuously transfers the entire data table, less the status section, from the
primary to the backup processor.
For the transfer to occur, the primary processor must be in run mode or test
mode; however, the backup processor may be in any mode. Even if the
backup processor is in program mode, the data table will be transferred
successfully. If you switch the primary processor to program mode, the
transfer will stop and the memory communication module reverts to LIST
control. If you switch the processor to program mode then back to run
mode or test mode, the message instruction must be executed for the
transfer to become active again.
The memory communication module forces the data table in the backup
processor to be identical to the one in the primary processor. Thus,
deletions as well as creations may take place in the backup processor.
Further, if a section of the data table exists in the backup processor but not
in the primary processor, that section will be deleted in the backup
processor.
CAUTION: To guard against damage to equipment, make sure
there is sufficient memory available in the backup processor to
accommodate the data table when the memory communication
module (1775-MX) executes a START DATA command. If not
enough memory is available, the backup processor will shut
down.
Important: Use of the commands STOP, TRANSFER $<file list>, and
START DATA is mutually exclusive. You can use one, and only one, at a
time.