Negative input logic directive???

Hi guys. I don't want to provoke a "which way do you do it and why" debate, but my boss has just told me that there's an EC directive (10+ years old), part of which states that wherever possible, inputs to (plc) control systems should be 0V = input active. I've seen a few directives by now, and I don't remember this one. Any thoughts/knowledge? Cheers.

Could this be referring to a failsafe input.

Normal condition, not activated ,normally closed contacts supply X volts to input.
Abnormal condition, activated, (or cable cut) normally closed contacts open and supply 0 volts to input.

Sliver, thanks for that, but it's not a N.C. type failsafe, although I know and agree with what you've said. This one is just about setting inputs at 0V to signal an ON condition (NPN-type switching). Thing is, he then wants the outputs to be positive; I wouldn't like to be the maintenance guy in that panel: "Hmm, I've got +24 on this contactor coil, so it's on, but 0V feedback..." It just seems muddly to me.

It's not some scheme to save power is it? Do the inputs of your system spend significantly more time in the 0V 'active' state than the other? How many inputs are there?

The Japanese use NPN inputs all the time. I have absolutely no problem with them provided the DC power supply is a switchmode to give isolation and the negative of the output side of the switchmode is not grounded. GROUND THE NEGATIVE AT YOUR OWN PERIL!!! If the negative is grounded you lose your isolation and present a ground where, if an input wire is grouded, the input may/will turn on. I really fail to understand why people insist on using a switchmode and then want to ground the negative.

Strangely enough I usually use PNP switched inputs and NPN switched outputs and have never had a problem but I have also used NPN switched inputs, and a mixture of both, without any problems. Just do not ground the negative of the siwtchmode power supply - let it float.

I am sure there will be someone here that will tell you it is not good practice to let the DC float - ask them why because there is absolutely no good reason.

Bob, I guess it's just what I'm used to; I like to see a nonzero level as "on" and a zero level as "off". What the hell, it keeps me in work. I used to always ground 0V, (a throwback from analogue days) but with optoisolated inputs on plc's and huge noise immunity there's no need, so I don't do it now.

Jeev, "power saving"... lol, but not as cool as your groovy avatar. (or were you being serious?) 1000 inputs, say, @ 8mA that's nearly 200W; point taken.

Analogues are another story. I refuse to use or accept voltage analogue inputs. Too prone to spikes from power cables etc. Should be run separately and at least 300mm from the nearest power cable - oh yeah, dream on. Very limited space to run anything in buildings these days - duct ways reduced as far as possible - non leasable space. Also have a look at the size of plant rooms - non leasable space. 4-20ma is good. Only ground the shield at one end - if that - not nearly as prone to noise and spikes as voltage inputs - they do not affect current inputs unless of a huge magnitude to blow up your inputs.

Grumpy old man these days - avoid problems if possible.

Sliver, for fail safe type inputs I would normally use PNP inputs - always held on - off if active or cut wire or loose connection. I normally use this methodology with generator syatems - if the signal goes away - start the generator. Can also be dangerous in machine control - better to have an input turn on there rather than off so no misunderstandings.

There have been several lengthy discussions on this subject. I will see if I can find some links.

http://www.plctalk.net/qanda/showthread.php?t=23957&page=2&pp=15&highlight=sink+source

http://forums.mrplc.com/index.php?showtopic=9429&hl=sink+source

http://www.plctalk.net/qanda/showthread.php?t=26075&highlight=sink+source

http://www.plctalk.net/qanda/showthread.php?t=23957&page=2&pp=15&highlight=sink+source

Paul

Yes - there have been plenty of lengthy discussions here. I stick by my guns - do not care if the inputs are NPN or PNP types - do not care if the outputs are NPN or PNP types - NEVER ground the negative output from a switch mode power supply and have NEVER had any problems.

Many seem to have this strange thing in their minds that negatives have to be grounded - 'phooey' to them.

I might add that I work mainly with battery powered systems - always ground the battery negative and then put in a switch mode and NEVER ground the negative output of the switch mode.

Cool bananas!!

An added note, I believe that grounding the negative output of a switch mode is akin to grounding the neutral side of an isolation (double wound) tranny - why put the thing in in the first place? For isolation of course.

Okie, thanks for the links; over the past few years I've followed some of them with mild amusement as the debates raged on, safe in my pnp world. until today...

...But still no comment from uk/europe members, re regulations.

DairyBoy said:

...But still no comment from uk/europe members, re regulations.

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I've been doing PLC stuff (mostly in the UK) now for 21 years and I've never heard of this one. Normally the control gear suppliers are very quick to leap on any new regulation/directive bandwaggon. Could I possibly place an order for a quarter-ounce of whatever your boss is smoking?

I've been in the machine building business for 20 years but haven't come accross this as a standard. Be carefull about how you describe your inputs, with current sourcing inpits they typically float at 24V when not connected and you connect them to 0V to pull some current and register a logical TRUE input state. The absence of a connection will not register a TRUE state.

Nick

(different pc login). Fred and Nick, Yeah, thought as much; (bovine excretia mode, methinks) or someone was winding him up and he went with it.
Nick: Yes, I am aware of the pull-ups on the inputs; it normally results in the same input current as a sinking input.
Fred: Join the queue! I'm going to need all I can get in a desperate attempt to remain oblivious to so many other (mindbogglinly irrational) edicts!

Blijj said:

Fred: Join the queue! I'm going to need all I can get in a desperate attempt to remain oblivious to so many other (mindbogglinly irrational) edicts!

Click to expand...

May I recommend:
* The Dampkring
* The Bulldog Hotel
* Homegrown Fantasy

All in Amsterdam, of course.

Re: May I recommend...

Somehow, the thought of someone so familiar with the names of these "products" coupled with the pursuits indicated by your avatar makes me just a little

BobB, What's the difference between a switcher and a linear psu with regard to isolation? I choose smpsu's for reasons of efficiency -electrical and panel area- but I've never considered these better for electrcal isolation.