Simplelogix,
The --| |-- and --|/|-- symbols in ladder do not indicate the type of contact in the device... at least, not directly.
If a contact is properly named then you should be able to deduce the type of contact that is in the device. If not properly named, then who knows.
The following was something I wrote in response to a slightly different question... but it applies to your question.
This description indicates the relationship between the bit-name, the actual contact type in the device and the TRUE/FALSE result.
In programming logic, we use one bit per condition.
If we use one-bit to indicate the "Up Condition" of a roll-up door, what can we ascertain from that bit?
We can only determine whether the door IS UP or IS NOT UP. We can not deduce, infer or determine in any other manner that the door is DOWN. We simply don't have enough information to make that determination. All we can say is, either, the door IS UP or, the door IS NOT UP.
Of the two possible states that a particular condition might exhibit, one is the "reference state". The "reference state" is "declared" in the "name" describing the "condition". This is not necessarily the condition being sought at some particular part of the program.
There can be only one name for any given bit. However, that name could be one of two possible names. Referring back to the UP bit, the name of the bit could be "Door IS UP". It could just as easily be "Door IS NOT UP".
You can look at the door and see that the door IS NOT up. Is that an indication of TRUE? Or, is that an indication of FALSE?
The presence of a signal is not necessarily required for a TRUE response. Sometimes we are looking for the absence of a signal.
If we declare the name of the bit to be "Door IS UP" then the signal comes from the Normally Open contact on a limit switch.
Door
IS UP
--| |-- < --- This signal comes from the
Normally Open contact on a limit switch.
.
This piece of logic is asking if the "Door is UP" signal is ON.
If the door is up, then the signal is ON and the response is TRUE.
If the door is not up, then the signal is OFF and the response is FALSE.
Door
IS UP
--|/|-- < --- This signal comes from the
Normally Open contact on a limit switch.
.
This piece of logic is asking if the "Door is UP" signal is OFF.
If the door IS up, then the signal is ON and the response is FALSE.
If the door is NOT up, then the signal is OFF and the response is TRUE.
If the name is "Door is NOT UP" then the signal comes from the Normally Closed contact on a limit switch.
Door IS
NOT UP (typical fail-safe method)
--| |-- < --- This signal comes from the
Normally Closed contact on a limit switch.
.
This piece of logic is asking if the "Door is NOT UP" signal is ON.
If the door IS up, then the signal is OFF and the response is FALSE.
If the door is NOT up, then the signal is ON and the response is TRUE.
Door IS
NOT UP (typical fail-safe method)
--|/|-- < --- This signal comes from the
Normally Closed contact on a limit switch.
.
This piece of logic is asking if the "Door is NOT UP" signal is OFF.
If the door IS up, then the signal is OFF and the response is TRUE.
If the door is NOT up, then the signal is ON and the response is FALSE.
In human terms, the double negation cancels and we are asking if the "Door is UP".
Another version of the explanation...
The --| |-- symbol asks the program to see if the signal is ON.
If the signal is ON then the RESULT is TRUE.
If the signal is OFF then the RESULT is FALSE.
.
.
The --|/|-- symbol asks the program to see if the signal is OFF.
If the signal is ON then the RESULT is FALSE.
If the signal is OFF then the RESULT is TRUE.