Look at the actions in positive terms...
Starting the system...
System ON => Run Conveyor
Conveyor Aux ON => Run 3-sec Timer
System ON & 3-sec Timeout => Run Blower
- Process runs -
Stopping the system...
System OFF => Stop Blower
Blower Aux OFF => Run 2-sec Timer
System OFF & 2-sec Timeout => Stop Conveyor
Don't consider code yet...
You have two devices: the conveyor and the blower.
Under what condition(s) will each device run?
The Conveyor begins running whenever the System goes ON, and continues running until the System goes OFF -AND- the Blower has been OFF for 2-sec.
The Blower begins running after the Conveyor has been ON for 3-sec, and continues running until the System goes OFF.
Let's get on the merry-go-round...
"System ON" is a "positive-form" indication.
If "true" then the system is on.
If false then the system is off... NOT ON.
"System OFF" is a "positive-form" indication.
If "true" then the system is off.
If false then the system is on... NOT OFF.
NOT "System ON" is the "negative-form" indication of "System ON".
If NOT "System ON" is "true" then the system is off.
If NOT "System ON" is "false" then the system is on.
If it is true that the system is off... then, the system is NOT on.
System is NOT on.. => NOT "System ON".
If NOT "System ON" is "true" then the system is NOT on... the system is off.
OK... off the merry-go-round...
The point being... "System OFF" is exactly equivalent to NOT "System ON"
The Conveyor begins running whenever System ON, and continues running until the System goes OFF NOT "System ON" -AND- the Blower has been OFF for 2-sec.
The Blower begins running when System ON -AND- after the Conveyor has been ON for 3-sec, and continues running until the System goes OFF NOT "System ON".
Now, let's try some preliminary code...
System
ON
---| |-------------+-----( ) Conveyor
|
Blower |
OFF |
Conveyor 2-sec |
---| |-------|/|---+
Conveyor
System ON
ON 3-sec
---| |-------| |---------( ) Blower
.
Now, when turned on, System ON goes TRUE... the Conveyor runs immediately.
Since the system was off, the Blower has been off for some time(**)...
..."Blower OFF 2-sec" signal is ON...
...however, the Conveyor code is looking for the "Blower OFF 2-sec" to be OFF!
So... when the system is started, the "Blower OFF 2-sec" element in the Conveyor rung is FALSE. This means that, even though the Conveyor is on, it is NOT latched on... at least, not yet. At this point, the Conveyor is ON simply because the System is ON.
(**Note: If the PLC was just turned on... "Blower OFF 2-sec" will be off, thus latching the Conveyor immediately when the system goes on... however, because the Blower is not running at this point, the "Blower OFF 2-sec" signal will go on in 2-seconds. Since the Blower can not run until the Conveyor has been on for 3-seconds... the "Blower OFF 2-sec" signal will be on before the Conveyor starts.)
As soon as the Conveyor begins running, a "Conveyor ON 3-sec" Timer begins running.
When the system first goes on, the Conveyor is off...
...So, the "Conveyor ON 3-sec" signal is OFF...
...so, the Blower can not run until the Conveyor has been on for at least 3-sec...
As soon as the Conveyor has been on for 3-sec, the "Conveyor ON 3-sec" signal goes ON...
... and so, the Blower goes ON.
Now, since the Blower is ON... the "Blower OFF 2-sec" signal goes OFF.
Now the Conveyor is self-latched through the "Blower OFF 2-sec" element.
When the system is turned off... the Blower goes OFF immediately because there is no latching mechanism in place for the Blower.
As soon as the Blower goes off, a "Blower OFF 2-sec" Timer begins running...
At this point, System ON is off, but the Conveyor is self-latched through the "Blower OFF 2-sec" element. The Conveyor continues running while the "Blower OFF 2-sec" signal is off. At the end of 2-seconds, the "Blower OFF 2-sec" signal goes ON, the Conveyor stops.
Let's complete the code... by the way... both timers are On-Delay.
System
ON
---| |-------------+-----( ) Conveyor
|
Blower |
OFF |
Conveyor 2-sec |
---| |-------|/|---+
Conveyor
Aux +-------+
---| |-----+ TIMER +-----( ) Conveyor ON 3-sec
| 3-sec |
+-------+
Conveyor
System ON
ON 3-sec
---| |-------| |---------( ) Blower
Blower
Aux +-------+
---|/|-----+ TIMER +-----( ) Blower OFF 2-sec
| 2-sec |
+-------+
.