Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 62)

Publication 1785-UM024B-EN-P - January 2003
4-8 Using I/O in Your ControlNet Hot Backup System
The application must only gather data from the PLC-5 whose primary
bit was set. If both programmable controllers have their primary bit
off, then neither PLC-5 is the primary and it does not matter from
which PLC-5 the data is gathered.
If the data going to the HMI device must be sent periodically, it may
be easier to have the PLC-5 write the data to the HMI instead of
having the HMI read the data from the PLC-5. In this case, you would
program the PLC-5 to periodically write the data to the HMI via a CIO
or MSG instruction. The rung with the CIO or MSG instruction would
be conditioned so that the rung only goes true if the PLC-5 was the
primary PLC-5. This procedure assumes that the HMI device can
handle either CIO and/or MSG instructions sent to it.
Data Written by the HMI
As explained above, data written by HMI devices must go to both
programmable controllers in a Hot Backup System. You can do this in
a few different ways. With an HMI device that can access the network
via RSLinx, follow this procedure:
1. In RSLinx, use the Alias Topic feature (under the DDE menu).
2. Set up a Topic for each of the programmable controllers.
3. Make an Alias Topic using the two Topics set up in step 2.
4. Configure the Switch When Bit is (Zero or One) parameter.
Select zero.
5. Enter the address of the primary bit in the appropriate field. the
primary bit is word 3, bit 2 of the CBI file.
For example, if the CBI file were file 20, you would enter
N20:3/2 in the field. This procedure will only work if the CBI file
is identical for both programmable controllers.
6. Activate the Update All Topics selection. This will write to all
topics in the list when any single topic has data written to it.
For HMI devices that cannot use RSLinx, you must modify the
application to write to both programmable controllers whenever a
write to a PLC-5 occurs.
Another option that does not require changes to the HMI application
is to write to the primary PLC-5. The data is then crossloaded from the
primary PLC-5 to the secondary PLC-5. Do this with the On Demand
Data Table crossload feature.