Table of Contents
Advertisement
Chapter 4. Ladder Diagram (LD) Programming
Positive Transition Contact POSCON
POSCON starts passing power flow and
continues passing power flow to the right only
when all of the following conditions are met:
the input power flow to POSCON is ON,
■
the value of the associated variable is ON,
■
and
the transition bit for the associated variable
■
is ON
The POSCON's transition bit is set to ON when
the variable associated with the POSCON
transitions from OFF to ON.
The transition bit is set to OFF when the associated variable is written to while the POSCON or
NEGCON contact is passing power flow, regardless of whether the value written is ON or OFF.
Power flow stops when the transition bit is set to OFF.
Depending on the logic flow, writes to the POSCON's or NEGCON's associated variable can occur at
different intervals within the Controller scan:
multiple times during a Controller scan, resulting in the transition bit being ON for only a portion
■
of the scan.
several Controller scans apart, resulting in the transition bit being ON for more than one scan.
■
once per scan, for example if the POSCON or NEGCON's associated variable is a %I input bit.
■
The source of the write is immaterial; it can be an output coil, a function block output, the input scan,
an input interrupt, a data change from the program, or external communications. When the variable
is written, the transition bit is immediately affected. The scan does not affect the transition bit. The
only way to clear the transition bit is to write to the associated variable.
Overrides
Overrides do not protect transition bits. If a write is attempted to an overridden point, the point's
transition bit is cleared. As a result, any associated POSCON or NEGCON contacts will stop passing
power flow.
Transition to RUN Mode
Variables that are non-retentive and not overridden will have values and transitions cleared to 0.
▪
▪
Variables that are non-retentive and overridden will retain their values and transition bits.
▪
Variables that are retentive will retain their values and transition bits.
Operands for POSCON and NEGCON
Parameter Description
BOOLV
The variable associated with
the transition contact
98
PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
Negative Transition Contact NEGCON
NEGCON starts passing power flow and continues
passing power flow to the right only when all of
the following conditions are met:
the input power flow to NEGCON is ON
■
the value of the associated variable is OFF, and
■
the transition bit for the associated variable is
■
ON
The NEGCON's transition bit is set to ON when the
variable associated with the NEGCON transitions
from ON to OFF.
Allowed Operands
BOOL variable: I, Q, M, T, S, SA, SB, SC, and G,
symbolic discrete variables, I/O variables
Bit reference in BOOL variable: I, Q, M, T, S, SA, SB,
SC.
Optional
No
GFK-2950C
Advertisement
Table of Contents
