Hi,

Can anyone tell me if there is a way of solving the following problem?

I have been asked to look at a "complete" re-write of a system that consists of 3 SLC 5/05 CPUs, all linked to each other by ethernet comms. Each of the PLCs is pretty well full of IO, including remote IO racks. There are also two In Touch SCADA PCs communicating with all 3 of the PLCs.

The system has become very unwieldy over the years, having been worked on by numerous PLC programmers and even more numerous SCADA programmers, to the point that the system is full of bugs and nobody truly understands it any more. Obviously it's a big job.

Each of the CPUs is SLC 5/05 with 64 kb memory, with remaining instruction words being 6,593 on (1), 15,621 on (2) and 42,586 on (3), reflecting the PLCs that do the most work.

One of the main problems with this system is that there are 3 software programmes running (one in each CPU), with a lot of "cross control", in other words, one cpu controlling IO physically attached to another CPU, leading to lots of confusion when attempting to debug problems.

It would be good to have just the one CPU running most of the serious code, with the others just running mundane tasks such as controlling IO, handling comms, and running fault checking routines.

Problem is, I have no way of knowing at this time whether 64 kb memory is enough on one CPU to hold all the code that will be needed.

So, the questions are : Can I increase the memory on SLC 5/05 CPUs above the 64 kb limit? If not, is there a Control Logix option (more power, more memory) that will enable one CPU to use all the installed IO on the SLC 500 racks?

I guess I could ask Allen Bradley, but you guys are at the "sharp end" and know the real score, from hard won experience.

Thanks in advance

You are tapped out on memory for a SLC family. You'll have to go to a ControlLogix processor for more but It makes a lot of sense considering the big task you are about to undertake.

Can I do that? Will a ControlLogix processor fit in an SLC rack and can it access SLC IO and comms?

I've done Control Logix standalone systems and SLC standalone systems, but have never looked into merging the two into one system. Anyone know if it can be done?

No the SLC I/O and chassis will not work with the Controllogix. But you may look at adding a Controllogix chassis, processor, and network card. You could then use you existing SLC I/O as networked I/O

But before you think about that, and I'm not trying to be rude by this statement, but have you looked at the efficiency of you program code. Might be able to find ways to reduce need memory there. Especially when you have a lot of programmers who have had their hands in a program, you will find many times that there is a lot of wasted and redundant code

64kb is a lot of code ! If some of it is data tables, it may be worth shifting it to another processor.

Will it fit - YES - it will fit!
If you don't know ControlLogix processor fits perfectly to the SLC rack , but WILL NOT work .

Fortunately SLC and CLX connectors are appart and will not short anything.

The only way to control SLC I/O with CLX processor is to use a remote I/O.
Keep in mind that ControlNet SLC I/O is not supported.

Absolutely, I totally agree ..... the code is incredibly overdone and complex for what is essentially a relatively simple system in "installed plant" terms.

Having worked more with Mitsubishis,where the memory usage is calculated fairly simply as "steps of programme", (eg. 6142 steps out of 30,000) and I immediately know where I am, how is the memory usage calculated in an SLC?

For example, the two CPUs which do most of the work show the following in RS Logix:

1) 20,442 instruction words used. 3,971 Data Table. 6,593 instruction words left.
2) 38,795 instruction words used. 9,311 Data Table. 15,621 instruction words left.

Doesn't seem to add up ..... (2) has used more instruction words and more data tables yet has more instruction words left, but both have the same 64 kb memory?

It is no accident that SLC and CLX cards are the same exact form factor.. The original plan, as told many years ago when the first functional prototypes were being demonstrated internally at A-B, was for the ControlLogix platform to actually use the 1746 I/O! The decision was abandoned before the official release though.. I heard the reason was because the SLC cards are **NOT** hot-swappable and that the average customer wouldn't figure it out that he can pull THIS card under power but not THAT card...

The three plc's are currently running the system, so we know that there is enough capacity.
You have assumed that one big program and two small ones are the best option, this may not be the case.
If I were you, I would sit down with plain old paper and pencil and map out the entire process being controlled.
Perhaps it would be better to try and group the process into three logical areas, with one plc controlling each area, and only passing permissives and alarms between the systems.

Just something to think about.


Ken

I'll second Ken. It might take a month to do this. Its still time well spent and it will pay big dividends in time savings farther on down the road.

Especially when it comes to trouble shooting and the next expansion of the system.