Delta DVP-ES2 - Manual (Page 11)

1 . P L C C o n c e p ts
1 - 3
Scan time exception
PLC can process certain items faster than the scan time. Some of these items interrupts and halt
the scan time to process the interrupt subroutine program. A direct I/O refresh instruction REF
allows the PLC to access I/O immediately during user program evaluation instead of waiting until
the next scan cycle.
1.2 Current Flow
Ladder logic follows a left to right principle. In the example below, the current flows through paths
started from either X0 or X3.
X0
Y0
X1 X2 Y0
X3 X4
Reverse Current
When a current flows from right to left, which makes a reverse current logic, an error will be
detected when compiling the program. The example below shows the reverse current flow.
X6
X0
Y0
X1 X2 Y0
X3 X4 X5
a
b
1.3 NO Contact, NC Contact
NO contact
Normally Open Contact, A contact
NC Contact
Normally Closed Contact, B contact