analog wiring to plc pratice

irondesk40

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Jan 2008
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nc
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Have a question about what others on this site think is the best pratice when wiring analog inputs and outputs. Have a project where I will be using a allen bradley micrologix 1100 with a 1762-of4 analog module. At the bottom of the control panel we always have a terminal strip where all field wiring coming into the panel is terminated at. I plan to desinate each analog output as AO 0, AO 1, etc.. I plan on running a two conductor shielded cable from each analog output to the terminal strip. For example, AO 1 will have a two conductor shielded cable, one of the wires will go to AO 1, the other to COM on the analog module. The two wire cable will go down to the terminal strip, which will have a terminal labeled AO 1, A0 1 Common. At the terminal strip I plan on terminating the shield to a terminal labeled PE, which is Ground. Question is , what would be the best pratice for the termination of the shield at the end of the cable that connects to the analog card. And also, when the field wiring is connected to the terminal strip at the bottom of the panel, the shield in field device cable will also be connected to the terminal labeled PE, which is frame ground.
Is this the best approach?
Thanks for any input.
 
What you describe is what most would strive for.

My best practice is use paired wires so the resultant current flow in that pair is zero,this reduces noise pickup, better still is twisted pair cable, if a screen is involved earth the screens at one point only, sleeving the other ends so they can't touch earth to create circulating earth currents.
Any unused wires in the cables need to be earthed also and again only at one point.
These would be my top 3 or 4 in that order.
 
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Irondesk40,
You didn't mention it & I've seen it done other ways, the shielded cable from the I/O module terminals to the lower terminal strip should be ran as far away from high current/high voltage wiring as possible. Ideally in it's own panduit wireway. I have heard of panduit wireways with seperate compartments but I have never used one. I'm not certian it is necessary, but we are talking best practices here. I prefer the red/black wiring colors over the white/black @ 18awg. The foil shield is a welcome feature along with the bare shield.

I never did like the "Braided" screen because it always pokes my fingers & makes me cuss.

BD
 
We specify 3 different colors of wire ducts. White (below 50 volts), grey (120 VAC), and black (480VAC). This tends to keep low level 4-20 mA away form motor lead wiring. The white duct is always on one side of the cabinet and black on the other. The grey duct catches a little of everything and is a good place to avoid sensitive signals.
 
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Sounds like you have a good plan. For DC circuits we use blue (+) and white (-) wire.

Only question now is how to terminate. Do you use crimp-on terminal ends or bare wire straight into the terminal lug?

My advice, just strip the wire and put it in. Don't twist the wire though. Some people like to twist the end to keep stray wires from poking out, but this can cause problems.

First, it adds stress to the individual wire strands. Second, it reduces the contact area betwen the wire and the terminal lug. Twisting makes the wire round, then the lug tries to smash it between two flat plates. It can break the wire and cause intermittent connections years down the road.

The crimp-on connectors can break the wire inside the insultion and you will never see it. Very hard to troubleshoot.

And as Gil47 said, only ground the shield at one end and make sure it goes to the cabinet ground, not the power supply ground. If you ground both ends you have completed the circuit. Any stray currrent flowing through the shield will actually add noise to the system.
 
thanks for advice. I have attached a quick drawing of what I was thinking would be best pratice. Main question is, the end of the shield cable that connects to the analog input and output cards, I plan on cutting flush with end of cable and wrapping with heat shrink tubing, is this a good practice or should that end be connected to panel ground as well.
 
The crimp-on connectors can break the wire inside the insultion and you will never see it. Very hard to troubleshoot.

I've never had that problem with crimp on connectors. I don't even see how you can break the wire using the proper tools and crimp connector. Maybe if you were using 26 gage solid wire and some improper tools.

Using a crimp connector eliminates the problem of "hairs" and also makes it very easy to land the connection under the actual terminal without crimping onto the insulation by pushing it too far in (because of the plastic hood on the connector).

I would recommend using the crimp on connectors for how easy and clean they make the cabinet. I have 8 year old machines with them and have never had issues with wires coming loose or breaking inside the cabinet.

On the other hand, I've had dozens of issues with wires between crimped on insulation rather than wire, wires popping out of terminals, etc. when no crimp connector is used.
 
From my persoal experience it is better to ground all shields in the control cabinet vs the field.

Sometimes grounds get removed during maintenance of equipment and do not get reconnected correctly or sometimes not at all.

Very dangerous i know . but that does not change the fact that it does happen
 
I have seen problems with the crimp connectors in "high-traffic" areas. You know that part of the panel that people are always digging in when the machine won't run.

For example, a terminal strip mounted at the bottom of the panel without much clearance to the case.

I think the crimp connectors concentrate all the bending into a specific area of the wire. Crimps work great inside plugs and jacks where they have a strain relief, like an AMP plug.

Just out in the open I think they may be more suseptible to mis-handling.
 
Your circuit is OK for 4 wire instruments but your 2 wire instruments need +/- 24VDC also. If you looping wires down the terminal strip for example -24VDC I like to loop back to the point of origin in case a jumper falls out. You will also need short circuit protection, I prefer minature circuit breakers over fuses.
Roy
 
Hello every one

I have a serious problem here with two PLCs , S7-400 and S7-200. The problem is, I have a single sensor which I need to connect to both 400 and 200 CPUs. Unfortunately I have no possibility to make a network because 400 DP and MPI ports are already reserved for communicating with other PLCs and the port on the 200 is reserved for a HMI. I thought I can import the analog signal to s200 and send it to the s400 using the analog output card (232-0Hd22). But I don’t know if it’s possible to use an output module on the s200 as an input for an input module on a s400. Please help…
By the way, the connection on the S200 output includes 3 terminals (M0 V0 I0) but the analog input card on the s400 includes only 2 wires (A+ A-)
 
Hello syavash_rus,

You should start a new topic not a old one

You can add a profibus module to the 200 (depending on the 200) ??

Welcome to the site....
 
oh sorry. thank you anyway. I can't add a new profibus module because I have no access to one in job site. I was wondering if some one could say how I can use the analog output and inputs to transfer the signal.
 
Hi
I always wire in 2 core screened cable direcly to the plc and always connect the sceen at the plc end only. cable segregation is always good too. In the field we aim for 300mm minimum segegation between power and control cables , which typically means seperate trays. In the panel this segregation is almost impossible so we use the split duct tyoe and ALWAYS route as far as possible from inverter cables etc
 
Attached is a drawing we put together to refer to when connecting analog devices.

NOTE: We didn't go to the trouble of showing the wiring as shielded-twisted-pair. That should be understood.
 

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