Modicon Memory

I am selecting a PLC for my application. Modicon "COMPACT" PLC is having I/O capacity of 256 I/Os and Memory of 512 KB, while SLCs have I/O capacity of 4096 and memory of only 16 KB. Can someone polease help me in deciding which is better?

PLC memory sizing has historically been done using a different method than computer memory, so there is no easy comparison.

The Modicon memory of 512K is the total size of their SRAM (like computers). This SRAM needs to be allocated into logic memory, I/O state memory (Modicon calls this state RAM), data storage, reserved memory, and loadable (add-on executables) memory.

The typical allocation results in the Compact 512K memory size being equivalent to 16K words of 984 ladder logic or 204K bytes (8 bits) of IEC languages.

I do not know about the SLC memory but I believe it is the size of the logic area only.

Also the Modicon Compact is maximum of 256 Input and 256 Output words not I/O points. There are trade off between the maximum count of digital points and analog words (the fewer analog the more digital allowed).

With Modicon's processors you should also look at what editor you are going to use (which Jim mentioned above). The Concept package will give you a lot a flexibility in symbols, data arrays, multiple IEC programming languges, etc. Note that the Concept package, depending on the language used will carry a higher memory overhead than the ProWorx32 package.

If you are planning on just using the ProWorx32 package as a comparision then I would recommend the A-B, since depending upon the complexity of your ladder progamming you are stuck with a "flat" memory space in the Modicon, where in the A-B you have the concept of "Files" which makes recipes in the PLC easier to adapt and expand over time.

Note that the capabilities/memory usage of the Modicon are very much defined by which programming package you choose.

Just my two cents & every one here will have a differing opinion. You are the only one that knows your requirements.

Darren

Its actually a bit misleading as well...the specs claim 512 KB user memory for the Compact. I was impressed..as some of the controllers nowadays do have that kinda memory (CompactLogix of Allen Bradley for eg: has 256 KB)

But again, the I/O capacity for Compact is limited to 256. But yes, the temperature rating at 70 Deg. Centigrade is good. How come siemens and AB and the bunch are struck at 60 Deg. Centigrade?

<<CompactLogix of Allen Bradley for eg: has 256 KB>>

Actually they have a new one that has 1.5MB of memory. Although, due to the memory overhead added by the tag based memory structure I would compare this to a 64K SLC/Micrologix processor. But, that additional overhead gives you capabilities you could only dream of having in a PLC a few years ago. But if you are doing simple On/Off, timers, basic sequence and you do not need some of the additional network/programming capabilities of the ControlLogix platform then I would stick with the SLC/Micrologix line (more people are familiar with it's functionallity)

As far as the temp specs go, who knows. It was most likely marketing/customer driven choice. I have seen a lot of Modicon stuff used for RTU applications in water/wastewater, which means outdoor panels, etc. That could be the reason (market driven).

Darren

Lark, yes it is confusing.

Manufacturers have never been consistent in their memory sizing. In the old days some quoted total memory while others quoted only the usable ladder logic memory. Now we have to deal with "ladder logic" versus "IEC" sizing also.

They also messed around with I/O capability, for example: total number of digital Inputs + Outputs versus the number of Inputs seperate from the number of outputs.

The Schneider Electric/Modicon TSX Premium offering is more similar to ControlLogix than is their TSX Compact. Also, the TSX Compact is 256 words of inputs and 256 words of outputs - not 256 digital I/O. An extreme case for the E258 processor (using remote I/O) would be 65504 digital inputs and 16 digital ouputs.