how does a PLC work

i am a hvac tech, and i came under a problem regarding PLC.
can any tell me how does a plc work. or any website where i can learn more about PLC. pls friend help me

What information are you looking for that you heven't been able to find right here?

On this same website they have some info about PLC and How it works http://www.plcs.net/contents.shtml. you can also download PLC manuals from manufacture web site. WWW.automationdirect.com let you downlade manuals for their plc free in pdf format.

Steve Bailey said:

What information are you looking for that you heven't been able to find right here?

Click to expand...

i work as an hvac technician, so the hvac works on PLC, i work in a gas plant where we have A/c to maintain the temperature of a bilding, so yesterday there a gas leak in the plant so due the H2S and HC detector the hvac systen triped, now i want know how to reset the PLC systems,there are four input for the PLC, the H2S(Hydrogen sulphate), HC(hydrocarbon),Return air temprature & Supply air temprature.

Joe

FIRST are all the sensors "reset" and sending proper signals to PLC?

To get more help give us the name amd model of the PLC.

ALSO there is associated control wiring ie stop start switches, maybe a reset switch etc etc etc. The PLC may be working just fine, but is waiting for some switch to be reset or shut before it will start the outputs.

Give us a list of all the inputs and the outputs (these are the terminals on the PLC) and will be labeled
I 0, I 1, I 2, I 3 and O 0, O 1, O 2, O 3 etc.
There are other terminals on the PLC also, input power etc.

Is there a fault lite - is it lit??

There may be a set of LEDs one for inputs and one for outputs. Are any of these lit?

Dan

The information you're looking for is specific to the application running on your PLC. You will need to get in contact with the firm that created your system.

try disconnecting the power to the PLC ... wait for a minute or so ... then power the PLC back up ...

if that doesn't do it, then follow Steve's advice and get in touch with the company that built the system ...

you can basically think of the PLC as a small computer ... working on it is probably going to take special software, communications cables, a programming computer, and probably more knowledge than we're going to be able to give you over the forum ... (well, we might be able to give you the knowledge - but it's probably going to take too long to do you any good) ...

PS edit: as Dan suggested make SURE that all of the signals (sensors, etc.) leading into the PLC are OK ... there's a GOOD chance that the PLC is "just doing its job" by shutting down the system in response to a faulty input signal ...

My guess would also be that a device external to the PLC needs to be reset.

As everyone else stated, try to get hold of whoever designed the system. They'll know what's going on.

You could also try looking at any tech manuals for the system. A lot of manufacturers have free downloads in .pdf format.

PLC Reset....

I'd first take a look at the H2S panel, and make sure that it 'signal' to the PLC is back to 'normal'. We've done systems like that, so that the ventalation is cut so that the H2S won't be ventalated. Also any gas leak detection panels can also be sending input to the PLC.

Just some thought, good luck.