FX Series Programmable Controllers Basic Program Instructions 2
2-2
2.3 How to Read Ladder Logic
Ladder lo
ic is ver
closel
associated to basic rela
lo
ic. There are both contacts and coils
that can be loaded and driven in different confi
urations. However, the basic principle remains
the same.
A coil drives direct outputs of the PLC (ex. a Y device) or drives internal timers, counters or
fla
s (ex. T, C, M and S devices). Each coil has associated contacts. These contacts are
available in both “normall
open” (NO) and “normall
closed” (NC) confi
urations.
The term “normal(l
)” refers to the status of the contacts when the coil is not ener
ized. Usin
a rela
analo
, when the coil is OFF, a NO contact would have no current flow, that is, a load
bein
supplied throu
h a NO contact would not operate. However, a NC contact would allow
current to flow, hence the connected load would be active.
Activatin
the coil reverses the contact status, that is, the current would flow in a NO contact
and a NC contact would inhibit the flow.
Ph
sical inputs to the PLC (X devices) have no pro
rammable coil. These devices ma
onl
be
used in a contact format (NO and NC t
pes are available).
Example:
Because of the close rela
association, ladder lo
ic pro
rams can be read as current flowin
from the left vertical line to the ri
ht vertical line. This current must pass throu
h a series of
contact representations such as X0 and X1 in order to switch the output coil Y0 ON. Therefore,
in the example shown, switchin
X0 ON causes the output Y0 to also switch ON. If however,
the limit switch X1 is activates, the output Y0 turns OFF. This is because the connection
between the left and the ri
ht vertical lines breaks so there is no current flow.
X0 X1
Y0
PC Program
I
N
P
U
T
O
U
T
P
U
T
Programmable Controller
DC Power Supply
X0
X1
Y0
AC
Power
Supply
COM
(Y0)
Toggle switch
Limit switch
Motor
