PLC reading PLCs without networking

TheCaptain2000

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May 2004
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Georgia
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I am on a new project involving using a PLC to monitor other PLCs in a warehouse. I figure I can use an AutomationDirect PLC I have for spare parts using only input modules since all of the PLCs we have use 24VDC inputs and read directly off the inputs of the other PLCs. However, I am concerned that I may possibly hinder the operation of the machines since some of the inputs would have to drive 2 input modules in serial. Should I be concerned? If not, great! If so, why and besides trying to network all of these different brands of PLCs together in their own network, what could I do along my same lines?
 
What brands/models are the other PLCs? What model AD? What CPU?

It's likely that wiring from one PLC input to another to another etc.. is likely to take a LOT of wires and require a LOT of install time. I would look at all the PLCs and try to find a common network protocal, like Modbus, or Devicenet etc..
 
Can be done if you use the same 24 VDC supply for ALL PLC inputs. If they have different 24 VDC supplies, most likely, DO NOT MIX THE POWER SUPPLIES!!! Only asking for trouble.

Yes, networking would be the best way to go.
 
This reminds me of an old GE Series One project I came across. There were 4 elevators that communicated back and forth with the dispatcher by using standard I/O. The setup was to use binary signals, and a psudo clock signal. Each controller had a standard AC input and AC output module dedicated for the communications scheme.

The dispatcher (aslo a Series One) would poll each of the elevators by putting out a 3 bit binary code. The controller that would match the code would then report back to the dispatcher via an 8 bit code to report it's location, direction and current call load. Then the dispatcher would assign the latest hall call to the most approperate elevator.

It actually works rather well. Additionally it wouldnt really matter what brands or models the PLCs were.
 
TheCaptain2000 said:
I am on a new project involving using a PLC to monitor other PLCs in a warehouse. I figure I can use an AutomationDirect PLC I have for spare parts using only input modules since all of the PLCs we have use 24VDC inputs and read directly off the inputs of the other PLCs.

Then you're not really 'monitoring other PLCs', you're just monitoring other PLCs' INPUTS. Regardless, as others have stated, it's not a prudent idea... (n)

Just think, once you network them, you can get the input status you're looking for, plus a WHOLE LOT more!... (y)

Give us more detail on what you're trying to accomplish, and we may have a better (and possibly simple!) solution... :nodi:

Mike, I used an almost identical method on a machine we built back in '98 that used a servo to position product at 16 different points along its travel. Back then, I wasn't as knowledgeable about serial communication between a PLC and servo drive, so I used discrete I/O to request and acknowledge moves. Eight lines (4 in and 4 out) between the PLC and servo drive. I sent a position request (from 0 to 15) to the drive, and when it finished its move, it would send the same number back to the PLC as confimation.

Definitely reliable, 'cause it's still purring along 24/7... :D

beerchug

-Eric
 
Eric:

At GE factory school, someone was always coming up with something that was un-orthodox, or didn't make sense, was too complicated, or something. Some one else would always complain or say that it wouldn't work.

If it works, specially for a long time, my old instructors words still ring true...

"You can't argue with success".

regards.....casey
 
Yes I know I can do "much more" if I just network them together. Yes, I know that I will only be reading inputs and nothing more. On these machines, all I need to know on a daily basis is just what the inputs are doing. I am really trying to avoid spending several thousand dollars in hardware and software. BTW, why does it matter that all my inputs from my three machines be running all from the same 24VDC power supply?
 
For one if they are separate power supplies with no common reference then you will have to use the + and common wire for each input. You could bond the power supply common to ground and use that for a common reference assuming all machines are bonded to earth ground..ie are bonded together. I DO NOT recommend this.

If you are monitoring the inputs for those plc's then you arent monitoring the plc's so the information will not be as effective.

Using outputs from separate plc's offers the same issues...ie common reference.

Depending on the plcs involved you could probably setup a Modbus network without spending 10K, mosts plc's can be a modbus slave so you just need to have a Master. The only other cost would be RS485 and wiring (which you would have done anyway). Think about it, you were going to run multiple wires from 3 locations to a central location, with this method you can run just 1 cable that in many cases doesnt need to be in conduit. An RS485 cable can run up to 4000 feet and more with an extender.

Its just my opinion but I think with time, wiring and power issues the use of a network would be easier and cheaper plus allow more data to be obtained and alarm messages if needed. The next option is that Master PLC could be placed on internal LAN and accessed from anywhere.
 
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From what I've read so far, I'm gonna guess is the Captain is faced with a situatation where he is dealing with more then one brand/type PLC and dosnt have the software for them all. This would mean that networking would reqiure the purchase of possably a few EXPENSIVE (like AB & GE etc) software pachages, then going through the full learning curve etc.. If that's the case, and you can actually get what you want to know from reading just the I/O then go for it. But be sure to budget a few extra $$$ for your cableing, install and TS time.
 
I would give the Captain a break here. He hasn't even described the PLC type, so elaborate networking might not even be possible (i.e. what if he has a bunch of old S5-100s)? He might not even have a budget for this.

Captain, I've had to do what you describe before, but maybe for different reasons (I used IO as handshaking between machines). I made sure that all of my power supplies were grounded and the machine grounds weren't floating, and it worked fine. I even passed analog signals this way.
 

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