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from the Automation List department...
non-isolated analog inputs
PLCs and related questions. topic
Posted by Gantt Robert on 20 February, 2001 - 1:23 pm
I'm going to use 1746-NI4 analog input modules with an AB SLC-5/04 to monitor 0-10 VDC signals from a few drives and transducers. No problem up to here, but the manual mentions that the module's 4 channels are not individually isolated, and that analog commons are internally interconnected (each channel has IN+, IN- and ANL COM terminals); however, it does mention that double-ended differential inputs can use the IN+ and IN- terminals, as long as they remain within +/- 20 volts of COMMON to insure proper operation. My analog signals will not necesarily share or have the same common or negative reference, so I'm a little confused and concerned on the "not individually isolated" spec on the module's manual. Does it mean that I can safely connect differential signals from dissimilar equipment into the module, as long as I ensure that the +/- 20 volt restriction is met? Or does it mean that a spike or short on one of the inputs can damage equipment connected to the other channels? Or, both of the above? Also, if all inputs are differential and use the IN+ and IN- terminals, can I leave the ANL COM terminals unwired? Any light shed on these matter will be greatly appreciated. Regards, Bob Gantt


Posted by Mark Erdle on 20 February, 2001 - 4:06 pm
Bob, For differential inputs you will leave analog common unwired. Each input channel is connected to the analog common by a 500k resistor. Let's assume that one input is 20V above ground (+) and another is 20V below ground (-). There will be a 0.04ma current flowing between these inputs. This may or may not cause an accuracy problem depending on the equipment involved. If all of your drives and transducers outputs float with respect to ground or are at the same potential to ground, you shouldn't have a problem. When in doubt use an isolator. =================================== Mark Erdle Alpha Geek The Boolean Embassy http://www.booleanembassy.org


Posted by Curt Wuollet on 20 February, 2001 - 4:08 pm
Hi Robert I don't have access to the info for the module you're using but it sounds like what you have is differential inputs with a common-mode rating of 20 volts to the analog common. What that means is that all individual + or - inputs must stay within 20 volts of the ANL common. If that common is floating, which you could tell by a voltage rating to PLC ground, you would want to strap it to a point that all signal currents return to. It may well be tied to PLC ground and provided as a shield ground that carries only signal or induced noise current. The object is to prevent this ground from carrying current due to differences in ground potential between machines, etc. A common application would be for all the cable grounds to be connected here at one end only. Fault currents should flow to this ground or the power supplies, not between inputs. I'm not sure if that's any clearer but I hope it helps. Regards cww


Posted by Bill Sturm on 21 February, 2001 - 11:24 am
When I have used these cards, I have always very carefully tied each devices common together to a single point in a star fashion. You must be very careful that each devices power supply common only has one path to the common point, or you will have a ground loop. If you cannot meet this requirement, due to distance... then you should individually isolate the signals. I actually used unshielded 16 ga. MTW wire to tie my commons. This creates a low resistance path. Noise should not be a problem because the power supply is a low impedance circuit. Then I needed to only bring one conductor from each device to the PLC input card. Do not bring two wires back or there is your ground loop. Also, I tied the common of each input card to the star point. There should only be one common wire attached to the input card. If you, and the panel shop, faithfully follow this advice, then you shouldn't have noise problems. Please email me if you do not understand what I am explaining. Bill Sturm


Posted by Jim Pinto on 21 February, 2001 - 2:53 pm
Regarding non-isolated inputs to PLCs - remember the old adage : When in doubt isolate - always be in doubt! There is an excellent article on analog input isolation on the Action Instruments website - "Using Signal-conditioners - Myths & Misconceptions" : http://www.actionio.com/appnotes/sigconqa/index.html Also : "PLC/DCS Loop Isolation" at : http://www.actionio.com/appnotes/appnote5/index.html BTW, Action Instruments will soon be my "former" company - I'll be officially retiring in a couple weeks. Cheers: jim ----------/ Jim Pinto email : jim@jimpinto.com web: www.JimPinto.com San Diego, CA., USA ----------/

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