Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 84)

Programming Techniques
Chapter 5
5-3
When using PLC-3 processors in a backup system configuration, program
scans are not synchronized; and, although switchover is fast
(approximately 100 milliseconds), it is not instantaneous. Therefore, it is
possible that the primary and backup processors could read different input
conditions during a particular scan.
If all input signals are present for at least the recommended length of time,
the primary and backup processors will see the same input condition. If
these conditions are not met, however, outputs may be assigned different
states in the two processors. This could cause a sudden change in operation
at switchover.
Although using a backup communication channel can reduce the
probability of such an event, it is better to avoid this condition by adjusting
the program accordingly. The best course of action is to consider the
various options and risks while designing the system, writing the program,
or both.
To illustrate this point, consider the rung shown in Figure 5.1. This rung
represents a “3-wire circuit” used to activate or deactivate a motor starter.
Figure 5.1
Example Rung for Motor Starter
| I0:2 I0:2 O0:3
|
+–+––] [––+––]\[––––––––––––––––––––––––––––––––––––––––––––––––––––––––––(
)––+
| | 00 | 01 00
|
| | I0:2 |
|
| +––] [––+
|
| 02
|
10981ĆI
Pressing the start pushbutton energizes input I2/0. This energizes output
O3/0 which supplies power to the motor starter. When the motor starter
receives power, an auxiliary contact closes that energizes input I2/2. I2/2
seals in output O3/0 (that is, it keeps output O3/0 energized) until the stop
pushbutton is pressed.
Timing Considerations at
Switchover