Basic PLC Symbols and Addresses in LogixPro

INTRODUCTION TO

PLC PROGRAMMING

Symbols and Addresses for the Rockwell/Allen-Bradley SLC500

using The Learning Pit’s LogixPro Simulator

When PLCs were first developed in the late 1960s the format for PLC programming was based on relay ladder logic line diagrams.

The format was designed that way so that technicians familiar with relay ladder logic could easily transition to PLC ladder logic.

This is the same circuit logic as a line diagram and as a PLC program.

The control devices are programmed as inputs. The symbols are the same as those for normally open and normally closed contacts:

Examine if Closed: XIC

Examine if Open: XIO

The loads are programmed as outputs. The symbol for a standard output is similar to a pair of parentheses with a space between them:

Output Energize - OTE

Variations will have a letter inside to indicate its function such as L for Latch or U for Unlatch.

The physical input and output (I/O) devices are connected to terminals on PLC I/O cards. The address indicates whether the connection is an input or an output, the card number and the number of the termination point:

In LogixPro the I/O simulator shows cards with switches connected to the inputs and lamps connected to the outputs.

The input cards are I:1 and I:3. The output cards are O:2 and O:4

In this example a pushbutton is connected to input card I:1 on terminal 00.

In the program, the address on the input symbol indicates the address of the terminal point.

In this example a lamp is connected to output card O:2 on terminal 00.

In the program, the address on the output symbol indicates the address of the terminal point.

An address does not have to represent a physical input or output. It can refer to a single bit in memory that can be used as a control as it changes state (1 or 0), or it can represent some other function such as a timer or counter, or it can specify a type of number like integer or floating-point.

A list of input and output types can be viewed in the data table:

S2 Status B3 Binary T4 Timer C5 Counter R6 Control N7 Integer F8 Float

Addresses are available from 0 to 15; a 16-bit Word. You can select from 100 Words; from 0 to 99.

Data Table showing Binary Table

Recap… PLC programs are similar to relay logic ladder

diagrams. Input symbols resemble normally open and

normally closed contacts and are called XIC and XIO respectively.

The basic output symbol is called OTE and looks like a pair of parentheses. –( )-

Inputs and outputs use addresses that show their location on an I/O card or in memory.

The address follows a format: Type:Word/bit e.g. I:3/15 O:2/4 S2:7/3 T4:99/13

The status of bits can be monitored in the Data Table.

References: Simpson, B. (2001). LogixPro Simulator. The

Learning Pit. Retrieved from http://thelearningpit.com