schneiderM340: Minimum possible data logging rate?

Dear All,

I am new to PLC programming. I have M340 PLC intended to be used for data acquisition system from AI MODULE. The mentioned scan time of AI module is 1 ms for each channel. & the module has 8 channels.

Can some one please tell, what will be the minimum sampling time that can be acheived for data logging.

Thanks in advance.

Regards,
Rajagopal G

Welcome to the forum!

We can't really, no, based on the detail you've provided.

Some things to consider, if you have a large program, and the scan time of the PLC is 10ms, then you can only data log at 10ms.

Are you storing the results locally in the PLC like in a FIFO array? Or hoping to write it out external to the PLC?

The update rate to the processor will probably be slower than the internal scan rate of the module. This is assuming that the module is local I/O in the processor rack or on a buss extension. If it's some sort of remote I/O like on Ethernet it's even harder to predict the rate.

It seems like today's CPUs have so much to do that it's still hard to get an M340 scan time under 10 milliseconds. I agree that we need more information - are you logging to a FIFO queue in processor memory? If so, how many samples do you really need and at what interval? I haven't personally tried really large (over 65000) array sizes in the M340. You'll want to do it as some sort of pointer-based buffer and not by shifting data through the stack. Maybe there's a word-definition issue here and you want to do something outside of what we call 'logging'. If you want to catch a minimum or a maximum you don't need a big buffer.

Remember that even though these cards spit out bigger numbers the resolution of the A to D is what counts. There's been PLC cards on the market that give you counts up to 32,00 something with a 12-bit converter.

There's information on circular bufffers in the free PLC class outline download at www.corsairhmi.com. The 'asynchronous process data exchange' section around page 181 describes a dual-pointer circular buffer. The page 35 '2-5-10 rule' talks about what to consider when pushing a processor to it's limit.

I wish you well with your application.