Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 111)

Publication 1785-UM024B-EN-P - January 2003
Program Synchronization D-3
Because every program scan is pushed out to the Maximum Program
Scan Time, it is advantageous to configure the Maximum Program
Scan Time parameter to be as close to the largest program scan time
as possible. The only way to determine the largest program scan time
for a particular application is to run the application in Asynchronous
mode under all conditions and then determine what the largest
program scan time was. You can determine this by viewing the Status
File word S:9. Be sure to consider all the events that may cause the
program scan time to temporarily increase such as PII’s, STI’s, On-line
edits, etc.
Adjusted Maximum Program Scan Time
The Maximum Program Scan Time parameter you enter for
Synchronous mode gets adjusted before being used by the ControlNet
PLC-5 Backup system. The Adjusted Maximum Program Scan Time is
equal to the smallest integer number of NUT’s whose time value is
greater than or equal to the Maximum Program Scan Time parameter.
For example, if the configured Maximum Program Scan Time
parameter is 65ms and the ControlNet channel NUT value is 10ms, the
Adjusted Maximum Program Scan Time would be 70ms (i.e. 10ms x
7). In this example, each program scan time would be fixed at 70ms.
Program Scan Overrun
For Synchronous mode operation, it is possible for the actual program
scan time to exceed the Adjusted Maximum Program Scan Time.
When this overrun condition occurs, the Sync LED turns solid red and
the Maximum Program Scan Time Exceed Counter (word 48 in the
ControlNet Backup Integer File) is incremented. You must zero out
the Maximum Program Scan Time Exceed Counter for the Sync LED to
turn from solid red to solid green. The Maximum Program Scan
Counter is cleared by writing a non-zero value into the Reset
Diagnostic Counters word (word 32 of the ControlNet Hot Backup
Integer file).
When a program scan overrun occurs, the two processors temporarily
go out of sync, however they do not stay out of sync. They go back
into sync at the earliest time possible. They should be back into sync
at the next housekeeping. Even when the processor are
resynchronized, the Sync LED remains solid red until you clear the
Maximum Program Scan Time Exceed Counter.