limit switch wiring with shielded cable

[endgrainguy]

I just finished wiring limit switches for my new DIY router and am having problems with the system tripping intermittently. I used two conducter 22 g. shielded cable, wiring the switches in series, twisting the two conducters together for each connection, treating them as one conductor. The wire starts at pin 15, of BOB, goes to a switch, then a new piece of wire goes from that switch, finally returning to the controller box after the sixth switch where its connected to the BOB's groung lug.I twist the grounding wire together at each junction, and twisted together all the grounding wires of the limit switches and the seperately wired home switches (3 of them, and they work perfectly) in the controller box, grounding to the box itself. I get no problems when I disenable the limit switches, but when I enable them, i get periodic unexplained trips, though they work properly when any of the switches is intentionally tripped. It seems like my technique might be at fault. There was one connection where if I handled the exposed connected pair of grounding wires with my fingers I could usually cause a trip.
I'm new to all this stuff, so I'm sure I could be making some fundamental error. Thanks!

[cbcnc]

Ground loops?
Your shielding should be connected to ground in only one place. If your grounded shielding touches ground again you have just created a ground loop.

Chris

[endgrainguy]

The grounding (shielding ground that is) wire isn't attached at its origin, but when I completed the circuit the grounding wire was attached to the single shielding grounding wires of the home switches. I didn't attach this to "signal ground " of BOB -just to the controller box itself.
Here's perhaps a clue to the problem I can't interpret: On one or two of the switches, all of which are wired normally closed, when I test with a continuity tester, the tester lights brightly, and when I hit the switch, it only fades about 80% -it doesn't go off completely the way it does on other switches. I actually don't see where this should create a problem. This circuit should only be tripped by a switch losing its continuity, and I have the reverse problem, right? But when I check with continuity tester, all switches seem closed at rest. As you can see, I don't know a lot about these things. Mayble I should try rewiring them normally open, though I see where that's less safe, because one isn't notified of a wire torn lose.
I have set up Mach3 soft limits and they work great, but I have so many hours into setting up the six limit switches it seems a shame to not get them working because of what's probably some minor wiring problem I can't identify. thanks!

[cbcnc]

Which Break Out Board are you using?
This is just a stab at the problem but it sounds like you have a floating ground. Where it's not 0 volts and is floating too close to the trip point.
Here is a link to some grounding info:
pminmo.com

Chris

[endgrainguy]

Well, maybe my two problems, inability to start spindle with relay board, and constant false limit switch trips, are related. I know very little about grounding. Should I somehow be grounding the "signal ground" lug of my BOB to my shop's AC power supply? Is this what is shown here?:

http://pminmo.com/PMinMOwiki/index.p...=Image:Gnd.png

Does this mean running a wire from signal ground of BOB to connect to a bare "green" wire in the AC line supplying Ac power to the system? And then, how should I be grounding the bare shield grounding wires for the shielded cable used for my steppers and the bare wires in the shielded cable used for my home and limit switches? Right now I simply have them tied to the metal controller box which simply sits on a wooden shelf. Thanks, and please excuse my ignorance on these issues, but I've got too much invested in this router to abandon the project!

[cbcnc]

The parallel port coming from your computer has several lines that are grounded. This is assuming you are using the proper power cables to your computer. And also that your power system that you are tying into has a proper ground. Old house wiring didn't include a ground at one time. Have you checked your wiring? I'm not an electrician, but I have one of those testers that has 3 lights and tells you if your wall outlet has the correct wiring between hot , neutral and ground.
Take it one step at a time and test each component in the chain. Does your computer case have continuity of the ground to your controller/BOB case?
I'm hoping that pminmo or someone else will chime in here because I am wondering myself if the shielding of the cable should be tied to the case ground or the power supply ground. I've had grounding issues with my machine too.

Chris

[mcyr]

You might want to try using a small capacitor between the ground and the other input lug you use on your BOB. This will filter out the noise spikes that are causing the trip to occur. I had a similar problem and a 0.01uF cap soldered across these two leads solved the problem. Also, you can try playing with the debounce in Mach3 (setting it higher) although I wouldn't go crazy with stepping it up. BTW, I didn't use shielded wires for my limits/home switches all wired on one circuit in series.

Best of luck,
Mike

[cbcnc]

Alright, after thinking about this a little more and looking at the pictures of your BOB, here's my guess.
The end of your limit switches should be connected to the signal ground since that completes the circuit to the negative side of your power supply. In other words, you want to go from the + side of the power supply thru the switches to the - side.
The shielding of the cable should go the the case ground.

Chris

[Al_The_Man]

Originally Posted by cbcnc The parallel port coming from your computer has several lines that are grounded. This is assuming you are using the proper power cables to your computer.

Unfortunately that is not always the case, the three cord supply cable coming into your computer is 120v line, 120v neutral, and service ground to the case.
The output of the PC switching supply is isolated and is not normally referenced (connected) to ground at the P.S. output.
It depends on the PC Manufacturer if any of the MB ground plane mounting holes are connected to chassis or just mounted on insulated stand-offs.
I use industrial style PC style units and I ensure they are mounted on metal stand offs so that the ground plane is at ground in all the mounting points.
So your Parallel port common is not always at ground potential.
It is your decision if you ground this at the port or at mounting or both.
In any case all grounding should be bonded to the service ground, shields and power suplly commons , if you wish.
Al.

[cbcnc]

I'm so confused.
Not really, it just takes a lot of mental sorting out of the different parts of the problem. Since I'm not a electronics engineer I haven't been through it a bunch of times and therefore it takes me longer.
I think the main point is that the limit switches are part of a circuit where the + side of the supply should terminate at the - side of the power supply.
The AC ground to the case is different or maybe the same.

Chris

[Al_The_Man]

To start there should be a common ground point that has as a minimum, the service ground, cable shields and ground from any motors or servo's and spindle motors.
Whether you take any power supply commons to this point, depends on whether you are adopting complete ground bonding of all systems and power supplies, or are using the total isolation technique, where all interfacing uses opto isolation or equivalent.
Either method should be done methodically, any haphazard method usually results in spurious tripping of inputs or noise etc.
If using common ground bonding for all systems, the ground cannot be used as a conductor.
Where the supply neutral is grounded at the panel, you cannot ground the neutral at any other point.
Personally, if I found my PC's power supply was at ground potential, I would also ground the PC common where it leaves the parallel port.
As I have mentioned on many occasions before, I prefer the common grounding technique, even If I happen to use a isolated module, I will have the input and output commons, bonded to ground.
Al.

[endgrainguy]

Well I'm afraid much of this is over my head. I did connect the shielding wires to 110 bare ground wire, with no discerniable improvement. Then I found and experimented with "debounce". That sure changed things! I had debounce interval at 0. I changed it to 500, and got no false trips, but one switch no longer trips the system at all , and another one or two only after a second or two. The others continue to work as before. I can set this number lower, and get the reluctant switches to work, but set low enough for all to work (20) I get some false trippings again. I left index debounce at 0 the whole time. How should "debounce interval" and "index debounce" be related?
But clearly there's some very relevant information obtained here, and that the switches behave differently also suggests problems that some of you electronic geniuses out there should be able to identiy! The most reluctant switch of all had three longish wires near it (two go to it), and I wired switches not to limit total distance of wire, but to simplify wire protection due to gantry movement. I made what is probably a mistake of putting my X limits at the ends of the axis rather than on the traveling head, which requires more wire. Total wire length is an issue, even with shielded cable, right? Best to minimize? Help! Thanks!