plc

dear sir,
i have doubt about in ladder logic, what is mean by 1K ladder logic and how we calculate the size of the ladder logic.can anyone help.my e-mail ID is [email protected]

thanks

regards
S Shanmugam.

Since you didn't specify what brand of PLC and your question is severely lacking in details, Your best bet is to look in the manual for the PLC you are using. This information is usually in there, in fact there might even be a "method of calculating" memory usage chapter.

1k = 1 kilobyte ..ie 1024 when refering to binary. Now here comes the unique part...for PLC's in a lot of cases when they say 1k they mean 1024 WORDS ..a word being 16 bits.

This is not exact though because I am not sure ALL brands do this, 1k can be 1024 bytes.

As an example:The DL05 Micro PLC which has 6K words of memory comprised of 2.0K of ladder
memory and 4K words of V-memory (data registers). Program storage is in the FLASH memory which is a part of the CPU board in the PLC. In addition, there is RAM with the CPU which will store system parameters, V-memory, and other data which is not in the application program.

Checkout the specifics for the brand you have.

Memory size is a pretty esoteric topic. Some guys measure it in words, some in bytes. Usage for various instructions also varies considerably. different manufacturers and even different models from the same manufacturer require different amounts of memory for he same instruction, particularly things like math functions. The only way is to follow the instructions in the manual for that manufacturer, usually back in an appendix.

Trying to pre-determine memory requirement is usually an exercise in futility. It is time consuming, tedious, and not worth the effort. Generally if the PLC you are using has the I/O capacity you require it will have sufficient memory for your logic. Many models have different memory capacities, and it isn't too expensive to uprade. However, in my experience the tendency is to buy way more memory than is really needed, at least in recent years.

As stated, depending on brands, the amount of available "k" is a very fluctuating parameter, and relates to various memories and units : processor internal memory, user data storage memory, including or not the program memory, and can be measured in bits, bytes, words, steps...

Example : 120k CPU, where you choose by "cursors" which amounts of memory are assigned to progam (steps), data files (words), machine langage, sampling trace, ...

Your post seems to refer to program length ("ladder logic") often measured in k-steps. If your PLC offers a "10k-steps program memory", you only have to understand what means a step / what can be written in a step.

In "ladder-based" PLCs and programming softwares, you'll find clearly how many steps your instruction set occupies (much more difficult in IEC61131 environments, since there is a compiler with its compiling rules between your design and the final code). Only notice that each instruction (or ladder symbol!) uses from 1 to xx steps (xx = see your instruction set. I'm currently programming a PLC with up to 17 steps function blocks).

Many softwares (IEC61131...) allow you to build your own function blocks, and to use them in programs. But, depending on your programming method / compiler / and... many other factors, your function code can be written once (and re-used as a subroutine) or added in final code each time your function is called (occurence)!

Summarized : as for PCs, big amounts of "k" are vendor's or manufacturer's marketing arguments (and what means 1k ? I know PLCs where "10k-steps" means... 9727 program steps ! ). You can't easily deduce from this value, what can be performed with products (so, avoid to use it for PLC comparisons). Today's PLCs -as for PCs- are usually... powerful enough! (and you could meet your own limits before reaching their memory's ones).

In some cases, you'll perhaps have to optimize your code for scan time decrease. But by the way, what about a powerful "79380k" CPU if your program is scanned in... 8 seconds (at 0,1ms/k-step!) ?

As stated earlier, it would help if you had a specific manufacturer. Everyone states their parameters differently.

Not being familiar with other manufacturer's programming software, I don't know if they provide this information when you compile. Other manufacturers like Mitsubishi tell you the number of steps available, and still others tell you the number of k words available. As mentioned earlier, AD gives you the total memory and then gives you an allocation.

For example, Entertron's software compiler indicates the size of your program in a compiled hex format. The program memory that we state for our controllers is what is available for the compiled hex.

You also have to determine if 1K ladder logic is compiled or non-compiled code

Rung length and program size will not be equal all the time in that complex rungs will require more memory than simple rungs. We have a 300 rung test program that compiles at 2.8K. A customer program that is 100 rungs (one third the number of rungs) compiles at only 600 bytes. (not even 1/4 the compiled size).

God Bless,

Stephen Luft