Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 85)

Programming Techniques
Chapter 5
5-4
If the start pushbutton is pressed for too short a time, the input change may
be seen by the primary processor but not by the backup processor. Should
this occur, bit O3/0 would be set in the backup processor one program scan
later than in the primary processor (because input I2/2 from the auxiliary
contact would be set and energize output O3/0).
WARNING: To guard against personal injury and damage to
equipment, examine output bit O3/0 in series with the auxiliary
contact (I2/2) (Figure 5.1) for the seal-in when using a stand
alone processor. If you force output O3/0 on, removing the
force will not return the motor to its initial state.
Now, suppose you used the output bit (O3/0) as the seal-in instead of the
input from the auxiliary contact (I2/2) as shown in Figure 5.2. At first
glance this appears to have the same effect as the previous example; but a
closer look reveals that a problem could occur by making this substitution.
Figure 5.2
Example for False SealĆin
| I0:2 I0:2 O0:3
|
+–+––] [––+––]\[––––––––––––––––––––––––––––––––––––––––––––––––––––––––––(
)––+
| | 00 | 01 00
|
| | O0:3 |
|
| +––] [––+
|
| 00
|
10981ĆI
If you use bit O3/0 as the seal-in instead of the auxiliary contact (I2/2), the
backup processor would not set bit O3/0 at all (unless it was transferred to
the backup processor via a backup communication channel or memory
communication module). Even worse, a similar problem could occur if the
backup processor reads an input change that the primary processor misses,
or if the stop pushbutton is pressed for too short a time.