Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 61)

Publication 1785-UM024B-EN-P - January 2003
Using I/O in Your ControlNet Hot Backup System 4-7
There are two areas of concern with HMI devices in a PLC-5
ControlNet Hot Backup System:
• the data gathered by the HMI device must only gather it from
the primary PLC-5. There are special requirements that allow the
HMI device to determine which of the programmable controllers
is the primary.
• data that is written to the PLC-5 by the HMI device must be sent
by the HMI device to both programmable controllers. With
single programmable controller systems, the HMI sends the
inputs to the single PLC-5. In a hot backup system, the inputs
must also be sent to the secondary PLC-5. There are different
procedures to follow to make sure that the input data is sent to
the secondary PLC-5.
In each of these situations, different procedures are required,
depending on whether the HMI uses RSLinx. If the HMI can use
RSLinx, the HMI can remain unchanged. If the HMI cannot use
RSLinx, the HMI will require additional programming so the HMI can
operate properly in a hot backup system.
Data Gathered by the HMI
Data gathered by the HMI must come from the primary PLC-5. This
can be challenging, because both the primary and secondary PLC-5
are both on the channel, and there is no predetermined way to know
which of the two is the primary PLC-5. If the HMI can access the
network with RSLinx, the following procedures can help:
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 the same file number for both programmable controllers.
With HMI devices that cannot use RSLinx, you must modify the
application so it only gathers the data from the primary PLC-5. The
application must be configured so it frequently polls the primary bit
for both programmable controllers.