how fast a PLC can count?

If I want to know how fast my Micrologix 1100 can count, would I look up counter frequency? It seems like that the default frequency is 63 Hz but can be set at 40,000 Hz. Really? Can Micrologix 1100 can count up to 40000 times a second? Hard to believe.

I got the info from the page 160 of following source. http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1763-um001_-en-p.pdf

thanks.

Yes, you have read correctly, the High Speed Counter DC inputs on this unit can actually count very quickly. But, depending on how your program is designed you may not be able to take advantage of this resolution. You can set actions to happen when the count reaches a specific terminal count but the actual reaction to this may take (relatively) an appreciable amount of time. This is really application dependent.

That table is specific to the first four inputs, which can be used with the High Speed Counter function files in the controller.

The MicroLogix HSC is an onboard high-speed counter feature that's made to run independently of the standard inputs and the program scan. That's why it can run up to 40 kHz with the filters set to minimum.

The next page has the filter settings for the standard inputs on the controller; a maximum of 1000 Hz.

The MicroLogix family does differ in the construction of the HSC; the MicroLogix 1000 can only go to 6.6 kHz, while the MicroLogix 1400 can go to 100 kHz.

The actual speed of a signal you can count with an ordinary ladder logic counter instruction depends not only on the input filters, but also on the ladder logic scan time.

Let's say a PB triggers a timer with a base of 0.001 second and preset 1, and every DN bit of the timer increases a counter accumulate value. The timer recycles every 0.001 second with its DN XIO in front of. If the PB is held for 3 seconds, would the CTU acc value be 3000?

I'm attempting to insert a picture of the program.


failed. all it shows is a image icon. Can someone tell me how I can add a photo to this forum? I used the "insert image" and added the URL but didn't work.

In the 'Additional Options' area use the 'Attach Files' button to upload the picture.

In your timer example the count only occurs if on the previous scan the count input was false then on this one it is true. The timer, given the very low preset, will probably time out one one scan and reset on the next. So the counter would be counting up once every 2 scans. A scan is typically on the order of at least a couple of milliseconds.

Can someone tell me how I can add a photo to this forum?

Click to expand...

Yes, it is not as simple as it seems it should be. See this old post for the 9 steps to attach files (including picture files) to your post:

http://www.plctalk.net/qanda/showthread.php?t=78821&highlight=attach+files

unsaint32 said:

Let's say a PB triggers a timer with a base of 0.001 second and preset 1, and every DN bit of the timer increases a counter accumulate value. The timer recycles every 0.001 second with its DN XIO in front of. If the PB is held for 3 seconds, would the CTU acc value be 3000?

Click to expand...

I seriously doubt it, unless your scan time is well under 1 millisecond.

The timer instruction doesn't trigger .DN when the .ACC = .PRE immediately, it only triggers .DN if .ACC >= .PRE when it is scanned. With your 1ms .PRE example, if the scan time of the program is 25 ms, the timer will only trigger 25ms after you start holding your PB. Then it would need another scan to reset, adding another 25ms before it would start timing again giving a minimum of 50ms for your 1ms timer.

yes and yes.
a high speed counter uses its own register to collect the pulses, meanwhile the program is just running.
when you use a simple input, you are bound by the looptime of the complete program, plus i/o and comm time.
generally it is about 100 Hz max. However you could loose pulses due to interupts from communication etc.
or if program is complex by stacking, timing programs it will be worse.

Thank you so much for the explanation. I didn't know about scan time, I/O time, and comm time. Makes sense though. I didn't know about scan time, I/O time, and comm time. Makes sense though.

When I held the PB down for 3 seconds to keep activate the timer of preset 1 & base 1ms, I got 30 counts. So, from the default setting of my Micrologix 1100, the counter seems to be counting as fast as every 100ms.

I will explore more to figure out a way to adjust the filter setting to make the count faster. I'm sure I will come back here to ask more question about this.
Thanks.