You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old April 6th, 2006, 08:32 AM   #1
Steve Kemp
Member
Wales

Steve Kemp is offline
 
Join Date: Nov 2003
Posts: 122
Light guards: PNP vs NPN & safety

Hi all. It would appear that the japanese "standard" is to use npn devices. I've always used pnp and grounded the 0V of the DC psu at source. However, if these guys are used to everything being this way up, is there a problem with my tying the +24VDC rail to the machine's chassis? I can't think of one and I'd sooner do that than leave it all floating. Any thoughts? (now that's a silly question).
__________________
For the Present is the point at which Time touches Eternity. C.S. Lewis.
  Reply With Quote
Old April 6th, 2006, 09:38 AM   #2
CaseyK
Member
United States

CaseyK is offline
 
CaseyK's Avatar
 
Join Date: Feb 2004
Location: In the cornfields, on the prarie, outside Chi-Town, a few miles beyond the boondocks.
Posts: 1,731
Lightbulb

I like to ground the "0" line myself.

Now, if you are building something for "them", then you may want to used "their" standard.

If it is something for you, then that is another choice.

I have seen a couple of machines that had both positive and negative logic being used, so no grounding was used. The grounding became an issue with an local building inspector. He could not provide a solution, so it went ungrounded. City officials later asked why he was poking ariund in a machine, anyway.

If you do ground the positive, you will not be xast out of the plcs.net family.

I did a poll on this about a year ago. "POLL"

best regards.....kc

Last edited by CaseyK; April 6th, 2006 at 10:09 AM.
  Reply With Quote
Old April 6th, 2006, 09:54 AM   #3
bob1371
Member
United States

bob1371 is offline
 
bob1371's Avatar
 
Join Date: Mar 2003
Location: Southern Indiana
Posts: 223
Steve,
http://www.mrplc.com/kb/index.php?pa..._v2&id=44&c=38
Here is a great link that was posted in this thread.
http://www.plctalk.net/qanda/showthread.php?t=20812


Bob
  Reply With Quote
Old April 6th, 2006, 10:01 AM   #4
Steve Kemp
Member
Wales

Steve Kemp is offline
 
Join Date: Nov 2003
Posts: 122
Bob, I've had a print of Jim Rowell's article in my desk for some time now; yes, it's a good reminder.
__________________
For the Present is the point at which Time touches Eternity. C.S. Lewis.
  Reply With Quote
Old April 6th, 2006, 05:21 PM   #5
panic mode
Member
Canada

panic mode is offline
 
panic mode's Avatar
 
Join Date: May 2003
Location: Toronto, Canada
Posts: 1,962
we do a lot of work with npn guys (japanese car makers).
positive side of 24V supply for I/O is grounded. safety circuit is on separate supply and that supply has negative side grounded. i've never seen it but i was told that in japan one can buy safety products like light curtains with pair of npn outputs.
  Reply With Quote
Old April 7th, 2006, 02:14 AM   #6
Steve Kemp
Member
Wales

Steve Kemp is offline
 
Join Date: Nov 2003
Posts: 122
Mr. Mode, you're right. We're using the SL-C range of light guards by Keyence. Both o/p types are present at the receiver connector, making it universal. Buying the appropriate cable (n or p type) connects the same colour cores to the npn or pnp ossd's -neat.

...And, reading the Standards and Regulations section in the SL-C glossy, I note that this model -even when used in conjuction with the dedicated controller- has not received certification in accordance with Article 44-2 of the Japanese H & S Law, pertaining to its use with presses and shearing machines. I find this a curious irony, given that our customer imports almost all of its test/assy equipment from Japan (CE marked), using light guards to inhibit a minimum of outputs -in one case, just the downstroke of a pressing cylinder- via the (non-safety) plc. There are no pneumatic dump valves, no isolator taps, and gaps in the "guarding" through which various limbs or small humanoids can easily pass. Funny ole world.
__________________
For the Present is the point at which Time touches Eternity. C.S. Lewis.

Last edited by Steve Kemp; April 7th, 2006 at 02:28 AM.
  Reply With Quote
Old April 7th, 2006, 02:56 AM   #7
ushidayo
Member
Japan

ushidayo is offline
 
ushidayo's Avatar
 
Join Date: Jan 2004
Location: Not far from Tokyo
Posts: 200
All the systems I've seen in Japan use NPN sensors and the DC24V is left floating. Machine safety doesn't seem to be such a high priority here as it is in the west. No lockable isolators and e-stops connected directly to PLC inputs. Maybe the operators are a bit more careful here, or perhaps they just keep quiet when they lose a finger!
  Reply With Quote
Old May 1st, 2006, 10:17 PM   #8
panic mode
Member
Canada

panic mode is offline
 
panic mode's Avatar
 
Join Date: May 2003
Location: Toronto, Canada
Posts: 1,962
Hi Steve,

You are right. It's quite strange but that's how it is. At least we can joke about it. I checked SUNX catalogue (customer wants it) and they also have light curtains with NPN outputs. And there was same note about such products being inadequate for press applications in Japan. Also there is statement saying that they never asked for evaluation by 3rd party / independent recognized institution, they just did their own test and found that is is ok. I called supplier and asked about it, since customer wants to use PNP version of same product on a press.... They did send me somehting that looks like proper certificate (this was probably done after catalogue was printed), but I would rather see it in the catalogue or datasheet of the product than on a piece of paper comming out of fax machine...

Last edited by panic mode; May 1st, 2006 at 10:24 PM.
  Reply With Quote
Old May 1st, 2006, 10:34 PM   #9
OkiePC
Lifetime Supporting Member
United States

OkiePC is offline
 
OkiePC's Avatar
 
Join Date: Mar 2005
Location: ENE of Nowhere Oklahoma
Posts: 7,969
Am I missing something?

The important part is not so much the use of NPN outputs as which side of the PS is grounded. You don't want a short to ground to ever be able to cause inputs to go true. Especially inputs to a saefty relay! They are grounding the positive right? So, the negative still has potential to ground and will (hopefully) trigger a short circuit protection device if it is shorted to ground, and even if it doesn't, can't cause the relay to pick up.


Or, am I missing something? I have been confused on the subject before...

I had a bunch of machines with sourcing inputs from photoeyes and a negative (or 0vdc) ground. Any time one or more of the inputs were "stuck on" we knew there was likely a short somewhere. It wasn't dangerous, but could cause waste and confusion. When we upgraded from a mechanical drum stepper to SLCs, I switched them to sinking inputs and it was much better, now the machine just stops, creates no waste (now a short to ground does not trigger a change of states and advance the cycle), and is easy to troubleshoot.

Quote:
...Imagine my surprise when I noticed the inputs to the PLC were all from the grounded side of the power supply....
by Jim Rowell


http://www.mrplc.com/kb/index.php?pa..._v2&id=44&c=38

Okie

EDIT:
OH, AND MY HAT'S OFF TO THE AUTHOR! THIS IS THE BEST INDIVIDUAL COMMENTARY ON WHAT NOT TO DO WITH CONTROL SYSTEMS I'VE EVER READ. I FORWARDED A LINK TO IT TO OUR ENG. DEPT. HOPEFULLY THEY WILL READ THE WHOLE THING AND TAKE IT TO HEART.

MANY THANKS
PAUL

Last edited by OkiePC; May 1st, 2006 at 10:52 PM.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Sinking Input cards Lead Maintenance LIVE PLC Questions And Answers 3 March 8th, 2006 03:49 AM
Can You Help? Buddha LIVE PLC Questions And Answers 6 May 31st, 2005 07:10 AM
NPN and PNP Sensors Don_Dubé LIVE PLC Questions And Answers 15 July 10th, 2004 06:55 PM
Need simple circuit to convert NPN output to 2 PNP output yctiew LIVE PLC Questions And Answers 5 September 30th, 2003 06:27 AM
Converting PNP output for device Expecting NPN output ndzied1 LIVE PLC Questions And Answers 3 December 12th, 2002 12:39 PM


All times are GMT -5. The time now is 05:48 AM.


.