Memory sizing of PLC

Hi all,



I am new in this forum. I want to know how to calculate the memory size of PLC if we have following data;

(1) Digital inputs = 31

(2) Analogue inputs 0-10V = 02

(3) Digital outputs = 22



Can anybody help me?

memory size and number of I/O is two different things. For a 'normal' program, memory size is not an issue, but if your program is very complex then you might need more memory.

/Mats

Generally speaking, program size is not directly dependent on I/O quantity.
I can easily imagine real-life applications with little signals but lots of logics/calculations and vise versa.
The I/O list provided is within lower size expandable PLC, which may feature memory from about 2Kwords (very old models) to over 16Kwords.

Add up the I/O points and multiply by 10, in your case 550 words. This will give you an estimate of the memory required for a low complexity program. Almost any small PLC will have 2 to 4 times this amount of memeory.

As the other replies have stated the amount of memory is really dependent on the size and complexity of the program.

I did a job just recently where there were few I/O but a lot of stuff coming in on Modbsu RTU. There were 49 shutdowns and 179 alarms coming into 1 channel as a binary number. These had to be decoded and turned into digital bits and then inro shutdown and alarm routines. I used up a processor memory of 10k instructions very wuickly and required a 20k instruction CPU. This finished up using 18k anyway.

The control opf the 2 generator sets consisted of 5 rungs of logic. The rest was alarm routines - and a few pushbuttons, digital inputs and driveing LED indicators.

The memory size not only depends upon the number of Analog/Digital I/Os but also it depends on the complexicity of the logic you are going to build using the Is to generate the Os. The number of logicgates (i.e) 'AND', 'OR', 'NOT' , Memory, Timers etc. mainly decide the size of the memory.

R.Thiyagarajan.

Okay. Now how can we determine the complexity of program and required memory at the earliest stage? This question often confuses to me as I have worked at a project in which the automation of a Down Stacker of a Corrugator machine was decided. That was my first experience. We decided to use an Allen Bradley SLC 5/03 with 8K memory. But when we started the job, we found that 8K memory wasn’t enough. We had to replace the SLC with a 16K memory. So, that experience forced me to thought about a roll of thumb for calculating the size of memory.

Thanks for any input.

Write your program before buying a processor, then you will know more or less what your requirements are

go for STL programming

hi there

Also if you program in STL then you can save a little more memory.