Normally open and normally closed contacts used in PLC ladder logic instructions. What do they stand for and how to use them in writing PLC ladder logic.

 Normally open  Contacts such as ------||------ 

PLC logic is often read as follows

• XIC- Examine a bit for an ON condition
• XIO- Examine a bit for an OFF condition

These contacts are infinitly available in PLC programming. They can be placed in any and as many rungs as you wish.

Inputs as physical devices

These PLC imputs are typically labeled by an X addresses. This is one physical wire that is attached to the PLC typically providing 24VDC, 100VAC when a switch or other device is actuated.

Example labeling of PLC inputs depending on the manufacturer of the PLC being used.

• Mitsubishi PLC's use labeling of X1, X2, X3
• Allen bradley use I:1/0, I:1/1, I:1/2 to I:/7 then starts at I:2/0
• Fanuc PLC- use X1.0-X1.7, then 2.0-2.7 ect. F addresses are coming internally from drive inputs.

Inputs as an internal input/relay address

These contacts will look identicle to Physical devices on most PLC's. Some examples of typical PLC addressing shown below.

• Allen Bradley uses B3:0/0, B3:0/1 to B3:0/7 and jumps to the next 8 bits which is B3:1/0.
• Mitsubishi PLC's use Fanuc PLC's- Use R addressing such as R1002.0-R1002.7, R1232.0
• Fanuc PLC's Use R addressing such as R1002.0-R1002.7, R1232.0