Tag Based Programming PLC

[BobB]

The main contenders these days are Rockwell's ControlLogix, Siemens S7 and Schneider's Concept/Unity.

Just finished a job with a Siemens S7. It is as tagged based as Omron is - you can allocate specific symbol names to specific I/O or registers. You can also just type in the I/O number as you do with Omron except you need I, Q etc as a prefix, or you can type in the symbol as you do with Omron. Absolutely no difference there. The software is definately not as friendly although not too bad when you get around the German way of thinking a bit. The processor was a 317 if I remember correctly which I understand is a 400 processor in a 300 package. I think the latest generation is the 318 processor which I believe is also a 400 processor in a 300 package.

The major differences I found between the 2 brands is that Omron have a lot of error checking built in to the firmware and Siemens use OBs for doing it. If you do not put in all the right OBs the processor is likely to stop or not reset. Thank heavens I was working with an experienced (15 years) Siemens man on the job or I would still be doing it. I was using Step 7 Pro 2006 - it was the latest at the time but I hear there is a new version.

The other major difference I found is the unfriendly nature of the Siemens PLC for someone who has not used one for a number of years. it works much more like a straight micro processor than most PLCs. It would probably be chicken feed for someone who regularly uses micro processors - I do not and never have.

I cannot comment on Control Logix as I have not used one but that may be about to change. I have just quoted a job with one but have to win it first. I hope I do as I have wanted to have a good look at these babies for a while but have not had the time. Been too busy. I have had a quick look at the software and it appears to be reasonable but have not had a good look yet. I did not like working with the SLC very much. Too much mucking about but the new one looks to be a whole different kettle of fish.

I had a look at Concept when it first came out and hated it. I have had a look at Unity as well and disliked what I saw. Apparently it is also quite buggy from what a friend of mine tells me. He uses it all the time and was, in fact, one of the big chiefs at Schneider here in Ozz. He uses Schneider purely on price. From what I have seen I do not want to go that path but would like to have a good go at Control Logix.

In recent months I have also done projects with Mitsubishi (prefer not to but is OK - do not like the software), Hitachi (prefer not too - the software looks and feels like it did from the first Windows version and similar in many ways to Siemens). May soon have one with Toshiba which I have never used before.

Could be a whole new learning curve if I win the Toshiba and Control Logix jobs.

Also looks like I have won a job with the latest Citec SCADA and Plant To Business. Have not used P2B before either - or Crystal Reports for that matter. A friend of mine knows Reports back the front thank heavens.

Have also won a couple of jobs with some new generator and ATS control equipment. Have to learn that in a hurry as well. Few interesting learning months coming up maybe.

If I do win the Control Logix job I am sure I will have plenty of stupid questions for you Philip. Do you know Toshiba too? Hope someone here does if I win that one. I do not know anyone who has used one before.

By the way Paulus, good questions.

 

[RMA]

The 318 is quite old and was the top of the 300 Series but it is actually a 400 Series CPU inside it. The 317 is a genuine 300 Series CPU. The new top of the line 300 is the 319 and it's typically about 10x faster than the 317, which is going some! I've not used one yet and I've no idea what CPU is inside it. As far as I'm aware the 318 either is being phased out, or already has been.

 

[PhilipW]

Bob

If I do win the Control Logix job I am sure I will have plenty of stupid questions for you Philip.

By all means. In fact if it does go ahead get in touch by email.

 

[geniusintraining]

Bob


By all means. In fact if it does go ahead get in touch by email.

Philip and Bob,

If you do the e-mail thing, keep us posted...the stupid questions that you will ask, will be over my head im sure, but I may get to learn a few things and Bob if you get it you should make it like a class project, I would love to see it from start to finish

 

[danw]

[font=&quot]Step7 lets you use names most of the time.[/font] BTW, who came up with the name 'tag'. Back in the dark ages there were symbol tables. Debuggers that used symbols were called symbollic debuggers.QUOTE]

I thought tags were an inheritance from the process DCS where tags have been used for decades, coming directly from the use of stainless steel ID tags wired/chained to the field instrumentation or glued to the process controllers in the control panels.

Dan

 

[BobB]

Thanks Philip. If I win the job I will certainly send you some stupid questions. Will also post here as it is interesting always when someone goes from using on product to another. It is usually a bit of a struggle until you get to understand the bits and pieces.

I tend to like to learn another brand on the smaller PLCs in the range but it is becomning more difficult as all manufacturers are upping the ante. The smaller PLCs are getting left behind.

 

[Paul T]

Paulus wrote:

Hello PhillipW,
Isn't the idea of 'Tag' based programming (ala ControlLogix etc.) just that. I.e. the user is simply unaware of the addresses being used / allocated for the tag names.
Each and every tag must occupy a section of PLC memory. That piece of memory surely must be at a specific address within the CPU.
Tag / Symbol / address, PLC / PAC etc, don't believe the hype.
I Could be wrong!

Actually, the AB Control Logix processors do in fact hide the "specific address" from the programmer. Gone are N's, B's, F's, etc. You type the variable when you define it in the program, same as with a higher level language. Once it's typed, it has attributes appropriate to it's type and you use them in programming. You can also create your own types if you have need to.

The architechture is more flexible in that it's structured to more easily support subroutines and the use of local variables in the subroutines. You can even pass arguments into and out of subs (limited ability, but it beats the zero ability in Logix500!) It takes a bit of getting used to, but for large systems, or systems with identical components running in parallel it has advantages over Logix 500. It is also supposed to run faster than Logix500 in a SLC or PLC, but I've never used it on a system big enough to tell.

BobB, don't sweat it- the actual ladder is very similar; it's the data tables and overall architecture that are really different. Once you get used to that, the programming is not a problem. What is sometimes a problem is interfacing it with an HMI; some of the wonderous new tools don't link to HMI/SCADA very well, even their own. If you're doing an HMI as well, you want to take that into account when setting up the data tables.

 

[BobB]

Lost the job with the CLX - got belted - someone else was 2/3rds of my price. Must really need the work. I priced it competatively and did not put in any learning time or the cost of the VERY expensive software. Stupid!!!

Belted the heck out of the job with the Toshiba and won it. Good money in it for sure - I did not want the job. It will be an absolute pain. Priced it and added 50% and still won it. Looks like the others wanted the job less than I did.

Oh well!!!! Might make a bob for a change.

 

[punder30]

I used to hate tag based addressing, well still do when someone else writes it with no organization. At the company I work for we came up with our on tag protocol if you will which organizes to find stuff easier.

i.e. all timers start off with TIMER_xx_xxxx
i.e. all bits or registers that that a HMI writes to the PLC : HW_xxx_xxx

So when you search for a timer they will all be together.

Also, I do not know about other tag based products like Omron,etc., but AB Controllogix you can still put descriptions in addition to the tag.

 

[Coachman]

It has been a while since I used Siemens S7, that is tag based also. Is
that true.

 

[BobB]

It has been a while since I used Siemens S7, that is tag based also. Is
that true.

Have a look at my post at the top of this page.

Yes and no. You can program by tags, or bits, or bytes, or words. Whichever you prefer. I still prefer using bits and words mostly but tags can be handy.

For example, I always use "PFR" for power failure relay, "ESTOP" for the obvious, "LTEST" also is obvious.

 

[Peter Nachtwey]

It has been a while since I used Siemens S7, that is tag based also. Is
that true.

For the most part. What I don't like is that I can't use a name for a pointer offset. Pointer offsets must be hard coded so if a structure changes one must go through the whole program adjusting pointer offset. One gets around this by using subroutines or functions to access data within a data block. This is a technique used by many object oriented languages. It is sometime inefficient but it makes it so all the access it done with 'accessors' methods. A method is a function that is associated with a class or in the case of a PLC, a structure. It is too bad the concept of classes has not made it to the PLC world, yet.

There are also cases where accessing data in other DBs where the addresses must be hard coded. Again this can be avoided by using the 'accessor' technique which can be very inefficient when implemented on a PLC. I think there is also a limitation on the number of FCs and FBs which means you cant have a bunch or small accessor functions any way.

In high level languages one can have as many small subroutines as you want. Many will compile to in-line code so the subroutines are more like a macro than a subroutine as the over head of passing parameter and return results is eliminated.

I have Step7 V5.3 correct me if these things have been updated in V5.4

 

[swhite65]

Peter:
Could you give an example of the pointer offset with respect to your dislike of its implementation or its shortcoming?

Is the limitation on FC's and FB's a function of memory or some other limitation imposed by the particular version of software?

I'm trying to better understand the points you are making. Are you a Siemens guy for the most part or do you have other preferences for PLC software packages?

I am not very familiar with the Siemens software and am trying to draw a comparison of capabilities with other software I'm familiar with.

I am going to start another thread which has probably been discussed from different angles but I'll try it anyway.

 

[jimbo3123]

I'm surprised no one has said this yet, but GE Fanuc's PAC Systems PLCs RX7i and RX3i have tag based addresses.
GE's setup is kind of nice in that you can assign traditional addresses to the tags. That lets you do cool things like use the new PAC PLCs as direct replacements for old 90-30 or 90-70 PLCs without having to re-write the program or HMI programs that referance the data addresses.

 

[BobB]

I'm surprised no one has said this yet, but GE Fanuc's PAC Systems PLCs RX7i and RX3i have tag based addresses.

Have had this available for about 4-5 years.

GE's setup is kind of nice in that you can assign traditional addresses to the tags.

This is exactly what I have been doing for the last 4-5 years as well as having tag based available. Nothing is hidden from sight. I like that particularly when optimising blocks for SCADA systems. The more contigious the blocks the faster and more efficiently the reads take place. Ideally one should head for one block which means only 1 header for the SCADA to send out.

That lets you do cool things like use the new PAC PLCs as direct replacements for old 90-30 or 90-70 PLCs without having to re-write the program or HMI programs that referance the data addresses.

That too for the last 4-5 years.