Ladder Logic and the Dialect of PLCs
Written by Neal Babcock
Ladder logic is a programming language, like Visual Basic or C. When you learn any language, it is always important to understand the basic terms.
If you are just getting started, here are a few terms you should know. These apply to any PLC.
Excerpted from “The Beginner’s Guide to PLC Programming”
BIT – an address within the PLC. It can be an input, output or internal coil, among others.
RUNG – A section of the PLC ladder program that terminates in an output function of some type.
HARDWIRED INPUT – a physical connection to the PLC from an input device (switch or sensor, etc.)
HARDWIRED OUTPUT – a physical connection from the PLC to an output device (relay or pilot light, etc.)
INTERNAL COIL
This is a programmable bit used to simulate a relay within the PLC. The internal coil has no connection to the outside world. It does not connect to an output card. Internal coils are used to store information. The “contacts” of this “relay” can then be used multiple times in other parts of the program.
–] [-- Normally Open Contact
When used with a hardwired input, this instruction is off until there is a voltage applied to the input. The bit address then goes high, or on, and the instruction becomes “true.” It works the same way when it has the same address as an internal coil, except that the coil must be turned on by logic in the program.
--]/[– Normally Closed Contact
This is an inverted normally open contact. When used with a hardwired input, this instruction is “true” until there is a voltage applied to the input. It then goes low, or off, and becomes “false.” It also can be used with an internal coil, becoming true when the coil is off and becoming false when the coil is on.
-( )- Output Coil
When used with a hardwired output, this function is off until the logic in the program allows it to turn on. It then becomes “true”, and will energize the device that is wired to the respective output. If it is used as an internal coil, it will toggle the instructions associated with it. That is, it will close a normally open instruction and open a normally closed instruction.
+———–+
TIMER |
+– SEC —+ Timer
This function is used to supply a programmable delay. It requires the use of its “timer finished” bit, like a time delay relay uses its contact.
+————+
COUNTER |
+— 000 —+ Counter
The counter function is used to count events. It could be used to keep track of machine cycles, count parts, etc. It can be programmed with a preset value that triggers another event when the count is reached.
