Can't find a Prior Post

[WierdScience]

Help! I'm invloved in building a machine that may change the world!! "so we all think when we start a new fan-tabulistic project. Truth is that I need to find an article that was posted by one of your plc brainies earlyer this year. (pardon please my spelling, my first language to speak was english but my first to read and write was spanish. Just laugh and read on. Thx.) This article discused the poor wisdom of using a grounded source for signals in and out of a plc. This author cited several posible cenerios like a fork-lift running into a conduit shorting the "start" signal causing havoc to an operator. etc., etc., etc. Anybody remember reading this article and know where I can retrieve it???

[Bob O]

Is this it? http://www.mrplc.com/kb/index.php?page=ind...x_v2&id=44&c=38

[Guest Guest]

That's it!! Thanks WierdS.

[gravitar]

Ok don't keep us in suspense, let us know how you plan on changing the world. Hopefully you haven't figured out how to make an atomic bomb

[Guest Guest]

I would like some comments on the idea of not grounding the 24V from the DC power supply. Jim Rowell's article is full of good information, however, we use alot of NPN inputs. It seems to me if the 24V neg is not grounded the situation his article refers to would not happen.

[GerryM]

I would think that would be fine to isolate the 24V P.S. for NPN. Also, if you just have a small self enclosed machine that doesn't use long runs of conduit, etc. then i don't see much diff. between NPN and PNP. Because in that case there really isn't much opportunity for a faulty input wire.

[gravitar]

Don't let size be your guide.. I would look at how many sensors/cords are exposed to harm/motion/etc. Some sensors/cords are just more likely to get damaged because of how or where they're applied. If you have an operator-accessable fixture with sensors in it, or somewhere else that is "close to the action", plan on replacing at least a cord or two over time. Often you find out they need to be replaced because you see bare copper :) There will be plenty of opportunity for these cords to false-trigger the inputs before you get around to changing it. OTOH, some machines have sensors tucked away from the action, and cords thoughtfully routed where they'll never get clipped/pulled/burned. I wouldn't be too afraid of using NPN switches there. Want to hear something REALLY bone-headed? We have a number of machines that a large, "seemingly-reputable" panel shop decided to build with *AC* sensors all over the operator load station. So.. now you have easily-damaged prox cords in arm's reach, carrying 120VAC. Got a new-hire weld tech pretty good last week, he went to swap a prox and in the process bent one of the pins. Apparently, when screwed in that way, the shell of the prox was at line potential. I can just imagine how SHOCKED he was (pun intended) when he held the prox in one hand and touched the frame of the machine with the other! (Yes, I know, he should have shut off the disconnect first..) Still though, 120V in reach of the operator is a really screwed up design, one that could have been easliy avoided!