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How to Symbol Program with Allen Bradley RS Logix 500

If you haven't already noticed, I am still young and "wet behind the ears" as the older PLC gurus might say. However one thing I noticed that separates us younger programmers from older generation PLC programmers is how they don't use symbols. Well most of them don't that I've personally been around. Have you ever looked at a PLC program that was written a while ago, or maybe more recently written by the older programmers? What's missing? Better yet, how do they remember all that stuff?

I am not really sure how "they" remember all those bit numbers and what they mean. How can anyone remember that B3:1/1 means "system is running" or I:1.0/3 means "proximity transfer is extended"? Either I need to work on my memory skills or I think I feel more comfortable remembering a symbol name when I program.

Symbols are used in RS Logix to identify a bit or memory area of a PLC by a name that you can more easily remember. See if you can see the symbols in this screen shot:

If you haven't found them by now, they are the descriptions in GREEN. 


RS Logix makes it extremely easy to use symbols in your program. When you enter a new instruction with a question prompt, you can start typing your symbol name and a pop out window will start narrowing your search down to the symbol you have already created in the database. Notice I said "already created". That means it would be a good idea if you made up all your symbols ahead of time using the spread sheet template downloadable here.

That's ok that you can't do them all before you start. Just keep creating them on the fly with terms or "short hand" that you can remember. Here are some tips for common short hand naming that I use myself.

When I am programming a hardware device such as a proximity switch or a photoeye input, I use the NFPA's Table E.1 Device and Component Designations as guidance to name all my hardware I/O. Below is a list of codes at the end of this article. This bascially means if I have a proximity switch connected to I:1.0/14 I could give it the symbol name:


PRS comes from the NFPA table which means "Proximity Switch".

TRANSFER means that this prox is connected to a transfer device of some sort

EXT means that the direction the prox is detecting is "extended"

You aren't required to use the NFPA's component designation, but this will give you some bases to start if you need ideas. More recently I have tried to trim my own symbols to the first two letters so I can drill down my symbol list very quickly in RS Logix. I've changed my own personal preference to PX for Proximity Switch. So if I enter an instruction in a rung, I can just type two letters "PX" at the question mark and low an behold EVERY SINGLE proximity switch on my automation system will come up in my list to choose from.

Your symbols will start to pop up in a quick pick list. Arrow down to find your proximity switch if you can't remember it, or better yet keep typing it all out and press enter when done. Viola! You've just typed your first symbol program.

Other symbols programming tips I can offer is a grouping of symbol names such as STATION 1 could mean S1. If you are state logic programming, you might remember a station more so by its function. Let's say you have a station on some automation that presses a part assembly together. I would call all the internal bits by "PRESS" to indicate all these bits that begin with PRESS have the same functionally with my press station.


The fun doesn't stop there. Can't remember which control relay enables the fast speed index? Just type CR to bring up the list of all your control relays.

How about when you are creating bits that transfer information from your Panel View to your PLC? You could also group all those symbol names together with a PV.

I hope you can see based on the samples shown that trying to memorize your program by the address names is pretty tough. Giving each of your symbols a name and you should be able to program faster than before. Group your symbols names to speed up symbol searches. Come up with your own "short hand" that you can remember easily or use the NPFA chart for some ideas.

NFPA 79 2002 Edition reference

Table E.1 Device and Component Designations

ABE Alarm or Annunciator Bell

ABU Alarm or Annunciator Buzzer

AH Alarm or Annunciator Horn

AM Ammeter

AT Autotransformer

CAP Capacitor

CB Circuit Breaker

CI Circuit Interrupter

CNC Computerized Numerical Controller

CON Contractor

COs Cable-Operated (Emergency) Switch

CPU Central Processing Unit

CR Control Relay

CRA Control Relay, Automatic

CRH Control Relay, Manual

CRL Control Relay, Latch

CRM Control Relay, Master

CRT Cathode Ray Tube, Monitor or Video Display Unit

CRU Control Relay, Unlatch

CS Cam Switch

CT Current Transformer

CTR Counter

D Diode

DISC Disconnect Switch

DISP Display

DR Drive

EMO Emergency (Machine) Off Device

END Encoder

ESTOP Emergency Stop

FLD Field

FLS Flow Switch

FS Float Switch

FTS Foot Switch

FU Fuse

GEN Generator

GRD, GND Ground

GUI Graphical User Interface

HM Hour Meter

HTR Heating Element

IC Integrated Circuit

INST Instrument

IOL Instantaneous Overload

I/O Input/Output Device

L Inductor

LED Light Emitting Diode

LS Limit Switch

LT Pilot Light

LVDT Linear Variable Differential Transformer

M Motor Starter

MD Motion Detector

MF Motor Starter - Forward

MG Motor Generator

MR Motor Starter - Reverse

MTR Motor

OIT Operator Interface Terminal

OL Overload Relay

PB Pushbutton

PBL Pushbutton, Illuminated

PC Personal Computer

PCB Printed Circuit Board

PEC Photoelectric Device

PL Plug

PLC Programmable Logic Controller

POT Potentiometer

PRS Proximity Switch

PS Pressure Switch

PWS Power Supply

Q Transistor

QTM Thermistor

REC Rectifier

RECP Receptacle

RES Resistor

RH Rheostat

S Switch

SCR Silicon Controlled Rectifier

SOL Solenoid

SNSR Sensor

SS Selector Switch

SSL Selector Switch, Illuminated

SSR Solid State Relay

ST Saturable Transformer

SUP Suppressor

SYN Synchro or Resolver

T Transformer

TACH Tachometer Generator

TAS Temperature-Actuated Switch

TB Terminal Block

T/C Thermocouple

TR Timer Relay

TSDR Transducer

TWS Thumbwheel Switch

V Electronic Tube

VAR Varistor

VM Voltmeter

VR Voltage Regulator

VS Vacuum Switch

WLT Worklight

WM Wattmeter

X Reactor

ZSS Zero Speed Switch

Article courtesy of



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