Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 72)

Using the 1775ĆMX
Memory Communication Module
Chapter 4
4-25
Transfer Data Command
The TRANSFER DATA command lets you transfer all sections of the data
table, except the status section, from the primary processor to the backup
processor one time only. The transfer includes all sections of the data table
except the status section (section 13).
The TRANSFER DATA command and the START DATA commands are
quite similar. The primary difference between them being that the
TRANSFER DATA command causes the transfer to occur only one time
whereas the START DATA command causes the transfer to occur
continuously.
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 data table transfer. If not enough
memory is available, the backup processor will shut down.
Important: If you previously selected continuous data table transfer, you
must stop it before you can use the TRANSFER DATA command. Use the
STOP command to terminate a continuous data table transfer.
Download Command
The DOWNLOAD command lets you transfer the entire contents of the
PLC-3 memory from the primary processor to the backup processor. This
transfer occurs once each time you request it, and it has a higher priority
than the data transfer function.
After the primary processor executes a message instruction containing the
DOWNLOAD command, the memory communication module stops any
active data transfer and immediately transfers the entire contents of the
PLC-3 memory in the primary processor to the backup processor. The
transfer occurs once each time this function executes.
The backup processor cannot take control of outputs if the primary
processor faults during a memory download. The memory communication
module resumes its previous operation at the conclusion of the transfer.