Sizing PLC

Hello guys,

I´m new in this forum and id really appreciate your help.
I have to do the stimation of a biogas plant automation system.
The control system doesnt seem so difficult, 30 DO, 15DI, 10 AI from some field sensors and 8 AI from the biogas generator control cabinet.
The analogic signal will be just monitored (using the necesary elements in the program to transform and storage them).
For the program Clock timer, about 28 timers, signals trnsducters, comparators... (not so difficult).
The point is that I dont know how to size the PLC memory COULD YOU PLEASE HELP ME WITH THAT?.
Monitoring and storing trends and historical data will have to be done.
The point is, Should I take into account the number of words stored every five secs in one month (as a example) for sizing the plc memory or the data is automaticatly stored in the server from the memory places I configurate in the PLC (no data storage in the plc memory)?

Id really appreciate your suggestions coss i dont have too much experience about this.

I was thinking about using SIemens, Omron or Wago.


Thank you a lot.

Hi josealjim.

Storing data and displaying it in trends is typically done in the SCADA software.
Do you have a special requirement that data must be stored in the PLC ?

Hi Jesper,

Thank you a lot for your answer. I have been reading some projects about industrial automation and in one of them the memory of the PLC was sized for storing 10000 events (state changes ON-OFF with its comentary and date) in that project there were 200 digital inputs and the plc memory was sized for 10000 (the memory required was about 70 Kbytes) it made me confuse about sizing the PLC memory, so some doubts came to me.
I just want to storage the data and trends though the scada, also alarms. So i guess if i have 20 AI, and about 50 D I/O, i just have to take into account one place in the memory of the plc for each signal i want to monitoring. Would this stimation be correct?

lets say that i want to storage 40 events (20 analogic+20digital) every 5 secs:

- Analogic:
16 bits for the signal
16 bits for time stamp
5 bits for 20 events codification
8bits for backup

= 45 bits/ event * 20 = 900 bits

- Digital:
1 bit for the signal
16 bits for time stamp
5 bits for 20 events codification
8 bits for backup

= 30 bits/event * 20 = 600 bits

Data memory= 900+600=1500 bits = 187.5 bytes
Data memory plus internal work (no the program) + future ampliation= lets say 1 kbyte?

Could be this correct?

Working memory:
lets say 150 lines of code * 3bytes =450 bytes
20 AI*200 bits= 4000 bits= 500 bytes
40 D I/O*100= 4000 bits= 500 bytes

Working memory= 1.5 kbytes+ 100%(future ampliation) = 3 kbytes?
Loading memory= Working memory + Data memory= 4 Kbytes?


What do you guys think about this stimation? I´m a bit lost and I dont want to kill a fly with a cannon so I really apreciate your help.

Thank you and Best Regards

Since you use the SCADA to store the data (which is also the normal thing to do), the PLC will not need to reserve any special memory for storing data.

Do you have some special requirements ? Such as a precise timestamp (*) for each 'event' ?
I think for a biogas application there will not be any particular requirements like that.

* I mean precise to the millisecond.

I dont need that accuracy, just to store it and generate trends for values taken every 5 secs.

Thank you again.

The doubt about data logging has already become clrear, but could please anyone help me to make a rough stimation about my plc working and load memory?

Thank you in advance

Any PLC, except the extremely small, and 'programmable relays', can handle 30 DI, 15 DO and 10 AI.

The point is that I dont know how to size the PLC memory. Could you PLEASE help me with that?

Click to expand...

In addition to space for saving the value in PLC memory of each digital and analog input and output, your PLC also needs space to store the user program, all the timers and counters used, symbols, description text, and about 20% extra for additions, changes, and future needs.

There is no precise way to completely anticipate all the possible memory requirements because they are somewhat arbitrary and unpredictable. Usually, past experiences with similar-sized projects is good indicator. If you have no experience, then you must start at 0 and make a prediction based on what you do know. The need for future expansion is a key decision factor. If there is no need to allow for future expansion, then the memory sizing becomes easier, and much smaller.

PLC memory size is not anywhere near as critical as it once was. Memory has become smaller and cheaper, so it is hard to find a PLC with inadequate memory storage space, or too-small program storage space. Once or twice I have run out of program space in the PLC CPU, due to a program size much larger than anticipated or predicted. That meant buying the next-larger size of CPU and putting the original CPU unit on the shelf as a spare for future projects.

Lancie1 said:

In addition to space for saving the value in PLC memory of each digital and analog input and output, your PLC also needs space to store the user program, all the timers and counters used, symbols, description text, and about 20% extra for additions, changes, and future needs.

There is no precise way to completely anticipate all the possible memory requirements because they are somewhat arbitrary and unpredictable. Usually, past experiences with similar-sized projects is good indicator. If you have no experience, then you must start at 0 and make a prediction based on what you do know. The need for future expansion is a key decision factor. If there is no need to allow for future expansion, then the memory sizing becomes easier, and much smaller.

PLC memory size is not anywhere near as critical as it once was. Memory has become smaller and cheaper, so it is hard to find a PLC with inadequate memory storage space, or too-small program storage space. Once or twice I have run out of program space in the PLC CPU, due to a program size much larger than anticipated or predicted. That meant buying the next-larger size of CPU and putting the original CPU unit on the shelf as a spare for future projects.

Click to expand...

Thank you a lot for your advice, I see it is not easy to get an occurate stimation, anyway I found something interesting at Siemens online support forum

https://www.automation.siemens.com/WW/forum/guests/PostShow.aspx?PostID=5170&language=en

Thank you all for your help