How do PLCs get data

A simple question (I've had very little PLC experience, forgive my ignorance)
Say I have a device, like a thermocouple, does the PLC fetch the
current value from that device every execution interval, or does that device write its current value into the PLC?
The first option seems more realistic to me. Can some please explain.
Related to this question is that we have a smart camera device (basically a digital camera with processor and ethernet connection). If I wanted to get some numbers out of the camera and into the PLC, would a workable approach be that one tells the PLC to go fetch the value from the device? (The device has a known IP number on the network)
Thanks for any help, Kevin
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It depends upon whether the device is "passive" or "active" - passive meaning that the PLC will need to read from it to get any data; active meaning that you could go either way.

As the camera is an "active" device on the network you could go either way - but convention and available software would likely dictate that the PLC reads the data from the Camera and not the other way around.
I hope this helps, Cameron:-)
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It depends on how you scan it. Typically, for a thermocouple, you would set the input scanning to be periodic, (eg on a Selectable Time interupt on a Rockwell PLC or a Timed OB on a Siemens) and on a much slower scan that the overal PLC, since the temperatre will not change that fast. However, you can also (with typical PLC's) force an immediate input if you really need. With your camera, yes you should be able to the the PLC to go fetch the value, but you do not want the PLC to stop other things while it does that, so it's something like setting a flag to the comms card and then checking a results flag before using the new value. Francis

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Basically the PLC scans the devices instead of waiting their inputs.
If you want to know about PLC basics, visit this site:
http://www.plcs.net /
About the camera integration, generally PLC uses Modbus protocol to talk over the Ethernet. So the first thing to do is check if this camera talks Modbus. I have never seen a camera that communicates via Modbus, anyway, if this one does, you will be able to talk to most of the PLC brands in the market. Another point is to check who is going to be the master/slave of the communication.
Check out this PLC brand: https://www.smar.com/products/lc700.asp Hope this can help.
Flvio.
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you are talking from the other side of that ocean ... well ... uhmmm ... atlantic ocean ? ^^
modbus - what a laugh ^^
Mike
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Michal,
Express your opinion. Make your point, but dont put trash in this group.
You make me laugh when I think how many people waste their time in internet writing something that does not even make sense.
Flavio
Michael Marx wrote:

talk
...

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