Dennis0Bauer
Member
I want to do an If/Else like structure in ladder programming, does anyone know a link to a good tutorial? I tried to google it but all i get is C(++) in my search results.
If/Else in Ladder programming
- Thread starter Dennis0Bauer
- Start date
I am not sure this is exactly what you are looking for. What PLC are you using?
A very basic IF/ELSE would look something like this:
I:1 O:1
------------[ ]-------------------------( )
I:1 O:2
------------(/)-------------------------( )
Basicly IF Input 1 is true set Output 1 coil.
Else set Output 2 coil.
A very basic IF/ELSE would look something like this:
I:1 O:1
------------[ ]-------------------------( )
I:1 O:2
------------(/)-------------------------( )
Basicly IF Input 1 is true set Output 1 coil.
Else set Output 2 coil.
Same mistake!!!
in the case of normal IF/THEN, part of code (IF or THEN ) should not be executed at all
in your case, all of the code executed all time and output 1 or 2 asigned FALSE or TRUE!!!
LadderLogic
Member
Yes, but in the PLC world most of the time, who cares? Unless each section of the code is huge and heavy with math, the differences in scan times would be negligeable.
TConnolly
Lifetime Supporting Member
That is not the whole story. In the old original programming languages, the IF-THEN-ELSE function was a two-branched condition, "IF [first expression true] THEN [do this or jump here] ELSE instead [do this or jump there]".
Instead, the "IF-THEN" function only looks at ONE condition and does ONE thing if the condition is true. If not true, then it does nothing. Wait! That is exactly what the typical PLC rung logic does.
"IF-THEN" function in PLC relay logic:
| IF TRUE DO THIS |
|----| |---------( )----|
| |
|Otherwise, do nothing! |
| |
"IF-THEN-ELSE" function in PLC relay logic:
| IF TRUE DO THIS |
|----| |---------( )----|
| |
| ELSE DO INSTEAD |
|----|/|---------( )----|
The "DO" output coils can instead be JUMPS to a PLC subroutine where anything possible can be done.
Last edited:
TConnolly
Lifetime Supporting Member
This varies by PLC brand/model.
An AB PLC/5 has the INV instructon which inverts the rung state, very hand for IF/THEN/ELSE rungs. However, it is the only AB PLC that has it.
An AB PLC/5 has the INV instructon which inverts the rung state, very hand for IF/THEN/ELSE rungs. However, it is the only AB PLC that has it.
Yeah, that's a pretty non-standard instruction in the AB world. Especially that the PLC5 is a waning platform.
balash
Member
siemens also have a NOT function which inverts the RLO
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