Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 115)

Publication 1785-UM024B-EN-P - January 2003
Application Guidelines E-3
What Information Should Be Transferred?
The secondary PLC-5 transfers selected PLC-5 memory files on
qualification. Timers, counters, and PID structures contain past state
information, so these critical state variables are automatically
transferred. You can also select a range of files to transfer by entering
the beginning file number and ending file number in the configuration
parameters. In addition to the file range, you can specify a limited
number of selected files for transfer. By setting the appropriate bit in
the transfer register, the secondary PLC-5 transfers timers, counters,
PD structures, and/or the files you have selected. When the secondary
transfers these files, the transfer bits are cleared by the secondary, and
no further transfers take place until the transfer bits are once again set
by the secondary PLC application program.
While the transfer register can, in most cases, function as the sole
mechanism for transferring state files, other information may still need
to be transferred. For instance, an integer file may be used as a past
event holder in some applications. If this file can cause the state of the
primary and secondary to be different, you may wish to transfer the
integer file just shortly after entering Run mode. The additional
transfer may be necessary even though all of the primary PLC-5’s data
table is transferred on qualification. This transfer may be necessary
because there can be a time delay between the time that the entire
data table is transferred and the time the secondary actually goes into
Run mode. It is possible for one of the state variables to change on
the primary while the secondary acquires the past state information.
Other state variables include the control registers used in FAL,
Diagnostic, Sequencing, FSC, FIFO, and Fill instructions. The control
registers are stored in file type R (e.g., R6:0). All of these variables are
transferred upon qualification of the secondary PLC. Many of these
instructions operate in All mode. In other words, the instruction
begins and completes within a program scan, and the instruction
control register is armed for reset on the next enable condition. All
mode execution of instructions typically does not raise different state
issues between primary and secondary because both programs begin
and execute the instruction to completion upon the enabling event.
Therefore, the instructions are synchronized to produce the same
outcome.
Many of the above instructions can also operate in incremental mode.
In other words, each time the instruction is enabled, the control
register counter is incremented. Instruction completion depends either
on the instruction reaching the end-of-file condition or upon satisfying
the instruction’s purpose (e.g., a File Search and Compare finds a
matching word and stops). Any instruction that takes several program
scans for completion may be a candidate for transfer to the secondary
because past event information is recorded in the control register.