# Help

K

#### Kannan Moudgalya

Hi,

We are in the process of establishing 10 experimental rigs for teachingpurposes. Each of these rigs will be multipurpose and will be completely automated. We are looking at the possibility of using one data acquisition / control system for each rig. Although in universities people generally use PC based cards, I want to explore the possibility of using PLCs. I would like to identify the best of these so that I can buy 10 units. One of the important requirements of this control system is that I should be able to drive it from linux. Reliability and the ease of use are more important than the cost. Finally, if successful, this experiment may be repeated in other colleges/universities.

I want to know if anyone can answer the following questions:

1. When does one use a PLC as opposed to PC based cards?

2. What are the shortcomings of a PLC?

3. Which PLC meets the requirement that I should be able to drive it from linux - in other words, which PLC already has linux based software written for it?

4. What will be the cost of a PLC if I am looking at something like 24 analog inputs, 24 digital I/O and 6 analog outputs?

Regards.

Kannan Moudgalya

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M

#### Mark Bayern

Kannan Moudgalya wrote:
>
> Hi,
>
> We are in the process of establishing 10 experimental rigs for teaching
> 1. When does one use a PLC as opposed to PC based cards?

The PLC is a standalone controller usually optimized for single point I/O. It is hardened to stand 'normal' plant environments ... more heat, vibration and dust than your desktop PC. The PLC doesn't have any moving parts (hard disk), quite often it doesn't even have a fan.

> 2. What are the shortcomings of a PLC?

You can't play video games on it, and its hard to use to balance your checkbook. On the other hand, lots of us think these are benefits and not shortcomings. <smile>

> 3. Which PLC meets the requirement that I should be able to drive it
> from linux - in other words, which PLC already has linux based
> software written for it?

...hmmm... you don't need to 'drive' a PLC. They run standalone. You do need to program it, and I don't know of any PLCs yet that you can program from Linux. You can certainly monitor a running PLC from Linux. The Serial protocol, MODBUS is the easiest (and possibly the slowest). There is also the new ABEL (Allen Bradley Ethernet Library) that runs under Linux.

> 4. What will be the cost of a PLC if I am looking at something like

Thats a lot of analog I/O for a small system. Best 'bang for the buck' would probably be to use the Koyo DL205 series from Automation Direct, but you'll have to program them with a Micky$oft Windoze program. -- If any of the above bothers you, don't worry ... others here will have other opinions! Mark _______________________________________________ LinuxPLC mailing list [email protected] http://linuxplc.org/mailman/listinfo/linuxplc C #### Christopher Di Biase 1. well a PLC's are generally used when you want to have a dedicated control system that almost never goes down. (PLC's don't crash like computers. I know this isn't so much a problem with a Linux box, but hard drives still fail 2. As Mark Bayern already said, no video games... That and PLC equipment is generally more expensive to buy than a PC, but then again any commercial software based control system would cost a pretty penny too. 3. Well if you wanted a hardware solution, Allen-Bradley MicroLogix 1200, 1500, or maybe even an SLC 500 platform would probably do the trick, but there's no Linux programing software yet. 4. For the MicroLogix 1200, your looking at an AB list of around$2000 to get the analog modules. An SLC 500 platform would be marginally more than
that.

I don't claim to have the answer, but just an option that might fit your application (we like options, that's why we like Linux so much

--
Christopher Di Biase <[email protected]>

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M

#### Mark Bayern

Christopher Di Biase wrote:
>
> 4. For the MicroLogix 1200, your looking at an AB list of around \$2000 to
> get the analog modules. An SLC 500 platform would be marginally more than
> that.

Possibly. But that is before he pays for a license for each copy of RSLogix500 -- which at least doubles the price. ...ouch!

Mark

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J

#### Jean-Pierre van der Zanden

A reaction for Kannan :

We are using PLC's for controlling a process plant, (high temperatures, explosive gasses etc.).
The most important isue is for us that it is dedicated for its purpose. We wil NOT try to use Bill's operating systems to run our plant.
That's why we are very interested in LinuxPLC, we can make the OS dedicated for our purpose...

Best regards,

Jean-Pierre van der Zanden
Research & Development Engineer.
Xycarb Ceramics B.V.
Helmond, The Netherlands.

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C

P

#### Phil Covington

A reaction for Jean-Pierre van der Zanden:

If your process is that critical or is that hazardous (ie. high temperatures, explosive gases), I would not consider using anything but a PLC! Doesn't sound like PC based control is anything you should be considering...

Regards,

Phil

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C

#### Christopher Di Biase

I would have to agree whole heartedly! In a hazardous environment, I would only trust an embedded, solid state, hard wired control system. A soft control system is nice, but not even Linux or a commercial UNIX based system can match the reliability of a hard PLC. Heck, with that type of system I would recommend a hot backup type system (I know AB has a ControlNet hot back-up system for their PLC5's but I don't know who else would have something similar)

now, I'm not basing Linux or saying its not reliable,.. but hard drives fail, and even Linux crashes on occasion (usually due to poor application code, but still, nothing's perfect)

Just my opinion, nothing more

Christopher Di Biase
[email protected]

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J

#### Jiri Baum

Hello,

> I'm an user of PLC for some years,but I have never communicated to an
> PLC(from Siemens).Could someone please tell me where to get the drivers
> to communicate to an PLC (e.g.Siemens S7-200)?

Hmm, sounds like nobody here knows the answer...

Currently, linuxPLC doesn't have a driver for any Siemens, or indeed for much of anything.

Siemens will probably have some software to communicate with its products, but whether that software will do what you want the way you want it is another question. Still, I guess all you can do is try asking them.

Jiri
--
Jiri Baum <[email protected]>
Windows is not popular. Windows is *widespread*. Linux is popular.

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H

#### Harald Albrecht

I somehow missed this one on the first read...
> > I'm an user of PLC for some years,but I have
never communicated to an
> > PLC(from Siemens).Could someone please tell
me where to get the drivers
> > to communicate to an PLC (e.g.Siemens S7-
200)?

Depends on which communication path you intend
to use with the S7. As with the famous S5 there
is always the 3964R protocol via the serial
cable. It is documented (don't know where) and
there are drivers available -- I think even for
Linux.

Then there are several communication cards
available called communication processors or
"cp" for short. They are available in several
flavours for 3964R, Profibus and TCP/IP. The
TCP/IP comes in two flavours: either FMS-alike
(FMS = fieldbus message specification, which is
a very light MMS) or ISO. The latter allows to
connect to the cp and send ISO-conformant PDUs
(protocol data units). If you're interested I
can give you some more background information
about the protocol. With this you can access
data blocks, etc. within the S7.

The third solution is to use a function module
(like FM456-x). These beasts are basically PCs
same form factor as all other S7 modules. You
can use any OS but for Siemens' own RMOS32
you'll get drivers with tight integration to the
i/os of the S7 master. Again, if you need more
with our own software ACPLT/OV and ACPLT/FB to
implement a hierarchical concept for the
operational aspects of process control. ACPLT/KS
is used to integrate the FM into our process
control engineering internet consisting of ABB
Advant and Maestro stations, Foxboro AW51 and
AW70's, E+H Commutec S remote I/O function
module, and some other industrial systems.

Harald

--
Harald Albrecht
Chair of Process Control Engineering
Aachen University of Technology
Turmstrasse 46, D-52064 Aachen, Germany
Tel.: +49 241 80-7703, Fax: +49 241 8888-238
email: [email protected]-aachen.de

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