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4.4.6
Transition Contacts
PACSystems controllers provide four transition contacts: POSCON, NEGCON, PTCON and NTCON.
The power flow out of the POSCON and NEGCON transition contacts is determined by the last
■
write to the BOOL variable associated with the contact. The associated transition bit is updated
every time the function is called.
The power flow out of the PTCON and NTCON transition contacts is determined by the value that
■
the associated BOOL variable had the last time the contact was executed. The associated
transition bit is updated once per scan.
For an example showing the differences in the operation of the two types of transition contacts, see
Examples Comparing PTCON and POSCON.
POSCON and NEGCON
GFK-2950C
These transition contact instructions should not be
■
used in a parameterized block or user-defined function
block (UDFB) with a parameter or member. In these
cases, an R_TRIG or F_TRIG should be used instead.
Do not use POSCON or NEGCON transition contacts for
■
references used with transition coils (also called one-
shot coils) or with SET and RESET coils.
If a SETCOIL or RESETCOIL receives positive power flow
■
and its associated variable is not overridden, the
SETCOIL or RESETCOIL writes the expected result to
the transition bit for the associated variable (that is,
the transition bit is set if the variable's value is set from
ON to OFF or is set from OFF to ON, and is cleared when
its value remains the same). However, if the SETCOIL or
RESETCOIL receives positive power flow and its
associated variable is overridden, the SETCOIL or
RESETCOIL causes the transition bit to be cleared.
Do not use a transition contact with the same
■
reference address used on a transition coil because the
value of the transition bit, which stores the power flow
value into the coil, will be affected.
February 2018
Chapter 4. Ladder Diagram (LD) Programming
Warning
97
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