Basic Ladder Logic Symbols

PLC ladder logic consists of a commonly used set of symbols that represent instructions. Understanding these basic symbols is essential to understanding PLC operation.

Picture 1: NO/NC Contacts, Coil & Box

Contacts

One of the most confusing aspects of PLC programming for first-time users is the relationship between the device that controls a status bit and the programming function that uses a status bit. Two of the most common programming functions are the normally open (NO) contact and the normally closed (NC) contact (Picture 1). Symbolically, power flows through these contacts when they are closed. The normally open contact (NO) is closed when the input or output status bit controlling the contact is 1. The normally closed contact (NC) is closed when the input or output status bit controlling the contact is 0.

Coils

Coils represent relays that are energized when power flows to them (Picture 1). When a coil is energized, it causes a corresponding output to turn on by changing the state of the status bit controlling that output to 1. That same output status bit may be used to control normally open and normally closed contacts elsewhere in the program.

Boxes

Boxes represent various instructions or functions that are executed when power flows to the box (Picture 1). Typical box functions include timers, counters, and math operations.

Entering Elements

Control elements are entered in the ladder diagram by positioning the cursor and selecting the element from a list. In the following example shown on Picture 2 the cursor has been placed in the position to the right of I0.2. A coil was selected from a pulldown list and inserted in this position.

Picture 2: Entering Element in Ladder diagram

AND Operation

Each rung or network on a ladder represents a logic operation. The following programming example shown on Picture 3 demonstrates an AND operation. Two contact closures and one output coil are placed on network 1. They are assigned addresses I0.0, I0.1, and Q0.0. Note that in the statement list a new logic operation always begins with a load instruction (LD). In this example I0.0 (input 1) and (A in the statement list) I0.1 (input 2) must be true in order for output Q0.0 (output 1) to be true. This same logic is also shown in a function block diagram. The following truth table on Picture 3 represents the state of the output for each combination of input states.

Picture 3: AND Operation Ladder diagram

OR Operation

In this example (Picture 4), an OR operation is used in network 1. In the following example, if either input I0.2 (input 3) or (O in the statement list) input I0.3 (input 4), or both are true, then output Q0.1 (output 2) is true. The following truth table on Picture 4 represents the state of the output for each combination of input states.

Picture 4: OR Operation Ladder diagram