Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 40)

Installation
Chapter 2
2Ć28
module must have its own unique station number. If you want to send the
same message through both 1775– KA modules, you must program the
two separate message instructions.
The second configuration provides system backup for the PLC–3
controller.
If the 1775–KA modules are Rev. C or earlier, you:
1. Assign different station addresses to each communication adapter
module.
2. If the programs in the primary and backup processors are identical:
you must be sure that all information sent to the primary processor is
also sent to the backup processor.
you must examine the run/backup bit (data table status section, file 0,
word 3, bit 17) on every rung used to transmit data. This bit is set in
the primary processor and reset in the backup processor. Examining it
helps to prevent sending duplicate messages over the Data Highway.
If the 1775–KA modules are Rev. D or later, you can:
follow the two steps described above
or
select BACKUP OPERATION with the PLC–3 LIST function.
To implement backup operation, follow these steps:
1. Using the option switches on both 1775–KA modules, set switch 2 to
the OPEN position. Recall (section titled Option Switches) that this
causes the module to disable its Data Highway port if the PLC–3
becomes deactivated.
WARNING: If you do not set these switches OPEN on both
1775–KA modules, these modules will assume the same station
address when the primary PLC–3 becomes deactive. This may
shut down communication on the Data Highway, and
unexpected machine motion may result.