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.
AUTO_MODE, RUNNING, POWER_ON, ALL_HOME, INDEX_ON_STAT, etc...
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
ABU Alarm or Annunciator Buzzer
AH Alarm or Annunciator Horn
CB Circuit Breaker
CI Circuit Interrupter
CNC Computerized Numerical Controller
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
DISC Disconnect Switch
EMO Emergency (Machine) Off Device
ESTOP Emergency Stop
FLS Flow Switch
FS Float Switch
FTS Foot Switch
GRD, GND Ground
GUI Graphical User Interface
HM Hour Meter
HTR Heating Element
IC Integrated Circuit
IOL Instantaneous Overload
I/O Input/Output Device
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
OIT Operator Interface Terminal
OL Overload Relay
PBL Pushbutton, Illuminated
PC Personal Computer
PCB Printed Circuit Board
PEC Photoelectric Device
PLC Programmable Logic Controller
PRS Proximity Switch
PS Pressure Switch
PWS Power Supply
SCR Silicon Controlled Rectifier
SS Selector Switch
SSL Selector Switch, Illuminated
ST Saturable Transformer
SYN Synchro or Resolver
TACH Tachometer Generator
TAS Temperature-Actuated Switch
TB Terminal Block
TR Timer Relay
TWS Thumbwheel Switch
V Electronic Tube
VR Voltage Regulator
VS Vacuum Switch
ZSS Zero Speed Switch