Small, medium, large PLC's

I'm studying PLC's at college and as always, things fit into nice categories; but not in the real world. I have the book definitions of the 3 sizes, but as I don't yet work with them have no idea on the size of the systems that they are able to control. Are there any good websites that you could recommend for future reading. Looked at the live programming site that was recommended, I think I shall become very familiar with that one.
Thanks in advance for any help.

What is the book definition out of interest ?

PLC size

Max I/O points

User Memory Size

PLC type

Small

40/40

1k

unitary

Medium

128/128

4k

modular

Large

>128/>128

>4k

rack mounted





Doesn't really mean a huge amount yet, I'm sure that will change. I think the learning curve is gonna be quite steep.

What book is that out of ?

In industry there are only two sizes:
- Too large
- Too small.

Always specify too large.

Doug.
From what i have seen so far, must agree with your comments.

Simon, sorry don't have a book ref as they were from tutors notes. This matches odd bits from different web sites i've read.

Still doesn't answer the origional question.
Oh the search goes on.

invincible said:

Are there any good websites that you could recommend for future reading.

Click to expand...

You are there.

Hi invincible,

You have probably discovered that your tutors notes contain terms that are only used within the PLC's module of the HNC course, not in the real world. That has been debated on numerous occasions in the past on this forum, so there is no need to go over old ground.

I will assume that you are doing your HNC, in that case this site might be of some help to you, it has been posted numerous times in the past, the first time I saw it was when it was posted by Ron (rsdoran) about 4 years ago.

Anyway, use the site above to 'pass' the PLC module, but if you want any advice, help, assistance, explanation of anything to do with PLC's then post your questions here. Just remember, if you are going to ask homework questions, please show some effort to answer them yourself first.

Good luck

Paul

Paul, thanks for the link, it'll give me something to look at in-between doing Gantt charts.
Your correct in the assumption that it's the HNC course, spent a lot of time fixing things in the middle of a field. Never had to worry if the PLC "thinggy" was doing it's bit, couldn't do much with it if it wasn't.
Your right about the homework though, seen lots of stuff last year that was straight off winkopedia with a quick explanation. Unfortunatelt I'm a bit "old school", don't write anything I don't understand incase I'm asked questions. I might be a bit slower taking it in, maybe it's my age; but at least once it is in it tends to stay there.

Al

invincible said:

Paul, thanks for the link, it'll give me something to look at in-between doing Gantt charts.
Your correct in the assumption that it's the HNC course, spent a lot of time fixing things in the middle of a field. Never had to worry if the PLC "thinggy" was doing it's bit, couldn't do much with it if it wasn't.
Your right about the homework though, seen lots of stuff last year that was straight off winkopedia with a quick explanation. Unfortunatelt I'm a bit "old school", don't write anything I don't understand incase I'm asked questions. I might be a bit slower taking it in, maybe it's my age; but at least once it is in it tends to stay there.

Al

Click to expand...

Looks like you have the right attitude to do well.
Also, I wish I learned about Gnatt charts while in school, and didn't have to pick it up on the way.
Back to your question on sizes, where I work the sizes are:
Small, Siemens Logo, S7-200 or Modicon Momentum CCC 98020
Medium, Siemens S7-315 2DP or 2PN (Our most common PLC)
Large, Siemens S7-414 2DP or 3DP or Modicon Quantum 412A.

These may not fit into other people's definitions of small, medium and large.

Hi Mate,

I've been there too, with all this unitary nonsense!

IIRC on my assignments i used the following:

Unitary - Small
Allen bradley micrologix,siemens s7200, but in reality they are quite expandable but fit the brick type criteria.

Modular - medium

Siemens s7300,modicon TSX, Allen bradley SLC (although i suppose you could call that range Rack mounted)

Rack Mounted - Large

Siemens S7400.

Unitary,modular and rack mounted are out of date terms, and i remember my lecturer saying the same!

Hope this helps

Its all kinda gray now anyway.

TO me size is not some much a matter of I/O's as even the so called bricks can be expanded on now as it is more an issue of functions a processor can do and processing speed.

But with more PC based event driven logic being used even that is getting confusing to me.
Also size is blurred by the ablity to easily link multiplie PLC,s together.

Things I wished I had learned in school is more non ladder logic programing and have a better understanding of networking.But like others have said if you ask the question here someone can help you, even if it is nothing more than pointing you in the right direction.

Just remember, the PLC manufacturers don't give us PLCs sized according to HNC courses. They are driven entirely by marketing constraints (and don't let anyone say technological constraints). Siemens has been mentioned already in this thread, so to continue with that could Siemens put an S7-400 CPU and all its capabilities in an S7-200 case? I don't doubt it for a second. And would anyone continue to pay S7-400 prices? Absolutely not, even for the same performance. Humans have this strange perception system of what constitutes value for money. Generally if it's big we'll pay more for it (houses, jewels, cars, PLCs - they're all the same. I'm just not sure where cellphones fit in to this theory)

Also if you've decided to launch three PLC families (S7-200, -300, -400) the last thing you want to do is leave a great performance/price hole between them for competitors to exploit. So the capabilities and costs are deliberately smeared across the range: a fully expanded S7-200 can do much more than an entry-level S7-300, and the same for S7-300 and S7-400. Absolutely the last thing any salesman wants is for you the customer to have the opportunity to say "I'm sorry, you don't appear to have anything here that fits me."

Treat the PLC world like a rainbow. There's a continuous spectrum and it's pretty damned difficult to say where red ends and orange begins.

Ken