PLC Student

henryhughes

Member
Join Date
Sep 2004
Location
England
Posts
6
I currently work for an engineering company that deals solely with the mechanical side of things, and we hire other companies in to do the electrics/control panels etc.

Obviously it would be better if we did everything in house, and I'm therefore doing a National certificate in Engineering Electronic/electrical. Now its not the best college in the world, but its ok for the maths, science, digital electronics, analogue etc, but the PLCs they possess are over 30 years old, which is fine for explaining the basics of how PLC's work, but I'm not going to stick a 30 year old PLC in a Conveyor control Panel. So I'm thinking of buying a new PLC (not too expensive i here, when I've asked around for advice in the real world, I'm just getting Names thrown at me "siemans are good, mitsubusi are awful, Alan Bradleys are great", which meens nothing to me at the moment. People employed at the companies that make the control panels are reluctant to tell me too much for obvious reasons. Other People see it as a little club where they dont want new members, so they just say its "too hard, you'll never understand it".

So My question is.... lol are you still reading this?. What PLC unit should i buy, what extra bits do i need to go with it etc, and I'm only going to be playing with it, so it doesn't need to be a huge thing that'll be controlling 30 robots, but it would be good if it can be used later on for a small conveyor control panel, when I've finished with me playing. The companies we provide our Machines to use siemans plc's, so maybe it would be better having one of them? even though Alan Bradleys are so Fantastic, apparently.

Now if you've managed to read all this and not got bored, and even manage to give some advice, I'll be extremely greatful.
 
What model? seriously at the minute i havnt a clue. I'd buy an expensive one thinkin its got to be good, where a cheaper model will do the job just as good.
 
The problem with Siemens is that although the small inexpensive PLCS, like an S7-200, would be adequate for a small conveyor control, it uses different programming software from the larger S7-300 that large automation projects would require. Siemens is also a little difficult to learn on.

If your company uses Siemens, find out what model. If it is the 200 you are in luck, and entry isn't too bad. If they use larger models then you may want to look at a different brand or model, and resign yourself to learning new softwre for real world projects.
 
You need more application info.
How many I/O?
Do you need analog?
How large of programs do you anticipate?
Will scan time be an issue?
Will you need external communication?
If you want to get your feet wet and have basic off / on logic , go for one of the inexpensive "brick" units. All makers have them and they are a fraction of the cost but they usually have fixed I/O.
 
<nods to Tom Jenkins>

That's what I'd recommend. In fact, I'd just suggest outright that he buy an S7-300 series and play with that.

I object to the postulation that Allen Bradley PLC's are "better" than Siemens'. I can't find a spit's worth of difference between their capabilities.

If you can do it with the one, you can do it with the other. The software interface isn't any more difficult on the one than on the other as well. If there is an application or two that "can't" be done on one, but can be done on the other, then I'll bet you my bottom dollar that the one that "can't" do it will develop a new card, CPU, or whatever, until it can do "it" as well.

Neither AB nor Siemens is going to lose a sale because their PLC "can't" do it.
 
I would also like to add that Siemens programming software, as well as the S7-300/400 hardware is expensive.
If your company is serious, they should purchase this for you.
Find out what PLCs were used on your last few jobs, and start there.

Hope this helps,

Doug
 
Henry,

You didn't identify the "30 years old" PLC.

As far as which PLC to go with...

BIAS ALERT ! ! !

Stay away from AB... too expensive, not intuitive and very restrictive (even the latest & greatest is restrictive... despite all of the "almost TI hoop-la").

Stay away from Siemens S7-300, -400... too expensive, not intuitive, and way too much like their S5 (the least intuitive for English thinkers).

My best recommendation is TI... it still exists, it's still supported, but Siemens (the current owner) has made it "Legacy".

I've gone through a dozen different PLC's... TI is the BEST ALL AROUND PLC! I really wish that popular demand would bring it back to the world of the current-living.

The next best thing is Siemens S7-2XX. It's cheap and easily expanded with Digital and Analog I/O. I ain't crazy about the Siemens Interface but the capabilities are substantial.

I really wish there was a Ti-Soft interface for S7-2XX.

Automation Direct? Good product... however, math is normally BCD based, not Integer or Real based... what a pain.
 
Well, since your company uses Siemens, go get an S7-300 on Ebay and get to work. Siemens gets a lot of flack for being "different", but like most things, it's only difficult "if you don't know how to do it very well". The main thing is to just sit down with it, connect a few lights and switches, and start challenging yourself with little tasks.

By the way, your mechanical background is a good fit. I have taught dozens of Siemens S5 classes, and my mechanical engineering students always blew away the electrical and computer science students (I think it had something to do with their inate ability to better understand the overall process). I didn't even bother with ladder, and went straight to STL, and they did just fine.
 
Hello Henry,

Stay away from the S7-200, the software for programming is quite different. An S7-200 is a very small and limited PLC for small projects (originaly a TI design). Try to go for an S7-300. Siemens has got a version which you can buy at very low cost expecialy designed for people who wanted to start with PLC's.
It has DI,DO,AI and AOutputs. Contact your local rep.
I bought one's for our electricien to play around with it and it costed at that stage 2 years ago about 400€.

Rudi
 
Henry,
you are about to make an incredibly important decision for your company. It will affect the course it will steer for at least 20 years. I am not kidding.
You are also very lucky that you are in a position where you can actually make an intelligent review of your needs and requirements and then choose from that. In most other companies the PLC, networking types, programming style, HMI etc. is almost set in stone.
5 years from now you will find that you have vested so many resources in the selected PLC, HMI etc that you cannot change.

Write down on paper exactly what your needs are now and for anything you might do with a PLC in many years from now. Then make your decision.

Examples on questions you should ask yourself:
How many i/o ?
Central or distributed i/o ?
What kind of HMI ?
- Roll your own (not recommended)
- Fixed operator panel
- Shrink wrapped PC HMI (the way things goes)
Do you export worldwide ? Then you need world wide support.
Is lowest price or highest reliability your first priority ?
Do you need to integrate tightly with drives or motion ?

Lastly:
The cost of the programming software is irrelevant.
I would stay with one of the major automation players. Maybe having another one as an option.
I would try not to mix vendors, having only one to blame is gold.
And I would NOT recommend a legacy product as the TI PLC (even if it is brilliant).

Post here for more advice :)
 
The usual supsects

henryhughes said:
I'm only going to be playing with it, so it doesn't need to be a huge thing that'll be controlling 30 robots, but it would be good if it can be used later on for a small conveyor control panel, when I've finished with me playing.

If you are buying this thing out of YOUR pocket, and it's to experience programming a REAL PLC (instead of simulation), then there are two PLCs that usually get recommended on this site:

1) DL05 from Automationdirect.com
2) Micrologix 1000 from Allen-Bradley ("Starter Kit").

Either of these will only set you back about US$200 (once you purchase the PLC, software and cables), and would have enough capability to put in a (small) conveyor panel (thus allowing you to recoupe your investment when you're done playing.)

If a simulated PLC would be enough, then check out TheLearningPit.com. They have a free trial download, and the full thing is only US$35.

I would also recommend buying the book and/or videos shown in the banner of this site. They do a great job of teaching you the basics



People employed at the companies that make the control panels are reluctant to tell me too much for obvious reasons. Other People see it as a little club where they dont want new members, so they just say its "too hard, you'll never understand it".

You won't find that attitude here. We're here to help everyone understand, and as long as you are willing to expose your ignorance, we'll try to explain it. Keep in mind that the more specific the question, the more focused the answer you'll get.

Many of us work for systems integrators / panel shops, and carry this desire-to-help attitude in our regular professional life. If you aren't getting that kind of service from your regular SI, you might consider finding a new SI. You pay for what you get, and it may be that you need more than just box with a PLC in it.
 
Last edited:
So if someone asks Henry 10 years from now "Why did you pick this particular PLC for our automation needs ?", should he say "because it was the cheapest I could get at the time, and I could reuse it right away".
All the major automation players have starter kits, some even for the HMIs. But is this really the sole requirement for such a decision ? :rolleyes:

I would look as far into the future as it is possible, and I would not let the immediate expense or convenience decide such a strategic question.
 

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