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_TRANSFER_EXT
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.
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Symbol Addressing