Steve Bailey
Lifetime Supporting Member + Moderator
Can't get any more ambiguous than verbal specs. They come with implied deniability. "I never said that".
Ladder Logic diagram help
- Thread starter coldplay2010
- Start date
Then we ask the operator how its suppose to run.
Steve Bailey said:
Ronnie Sullivan said:
Rob... said:
Steve Bailey said:
If anyone ever asks me to sum up my job as a small OEM automation engineer in one paragraph, I'm just going to show them this thread. I don't know whether to laugh or cry at the accuracy of itgclshortt said:
Firejo
Member
There are a couple of ways to to do this but at your level I'd study up on "examine if open" instructions (XIO). Also, just remember that a ladder diagram is nothing more than a positive rail connected to a negative rail through whatever is on the rung (inputs and outputs). To make an output go high (on) the circuit needs to be complete (keep that in mind when looking at XIO's).
Post what you come up with even if it doesn't work.
Dhitzfelder
Member
converting a more traditional programming statement into ladder logic is fairly straightforward.
all rungs are "if/then" statements, with the XIC and XIO contacts being arguments, arguments in place sequentially are ANDed
would be read as (Input1 AND NOT Input2)
arguments in parallel are ORed:
would be read as (Input1 OR NOT Input2)
the coil at the end is the "then" statement:
so, for "if(Input1 AND Input2) then Output1;"
Be careful, "implied" NOTs don't exist. As a programmer, I understood what you meant, but the PLC won't.
Edit: well, the formatting of line spacing didn't come out the way I expected...
all rungs are "if/then" statements, with the XIC and XIO contacts being arguments, arguments in place sequentially are ANDed
Input1 Input2
|------| |-------|\|---------
would be read as (Input1 AND NOT Input2)
arguments in parallel are ORed:
Input1
|-----| |-----|------
|
Input2 |
|-----|\|-----|
would be read as (Input1 OR NOT Input2)
the coil at the end is the "then" statement:
Output1
-----------------()--------|
so, for "if(Input1 AND Input2) then Output1;"
Input1 Input2 Output1
|-------| |-------------| |--------------------------------------()-------|
Be careful, "implied" NOTs don't exist. As a programmer, I understood what you meant, but the PLC won't.
Edit: well, the formatting of line spacing didn't come out the way I expected...
Last edited:
Steve Bailey
Lifetime Supporting Member + Moderator
Not quite. Typical ladder logic rungs are really "if/then/else".
This rung:
X Y
--] [----------------------( )-
Is really:
If X = True
Then Y = True
Else Y = False
Dhitzfelder
Member
Steve Bailey
Lifetime Supporting Member + Moderator
Absolutely. When expressing Boolean logic in a textual language execution speed will be significantly faster when using Boolean operators rather than if/then/else.
However,
If
You choose to use the if/then construction
Then
You really need to use if/then/else
Else
You are liable to get incorrect results
End If
Else
Dhitzfelder
Member
PLCs execute left to right. sure, boolean equivalent statements can be written, and are functionally identical.
Also, the problem originally stated was already in the form of "if/then".
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