Relay Logic vs Ladder Logic - instruction order - Electrician Talk - Professional Electrical Contractors Forum
CLICK HERE AND JOIN OUR COMMUNITY TODAY, IT'S FREE!
Go Back   Electrician Talk - Professional Electrical Contractors Forum > Electrical Trade Topics > PLCs, VFDs, Motors and Controls


Like Tree3Likes
  • 1 Post By dronai
  • 1 Post By gpop
  • 1 Post By bill39
Reply
 
Thread Tools Display Modes
Old 11-26-2019, 04:57 PM   #1
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default Relay Logic vs Ladder Logic - instruction order

Looking for a simple way (preferably a graphic or flow chart) to explain why in relay logic we show (wire) the (N/C) stop button ahead of the(N/O) start button, and in ladder logic we show the (XIC) stop button after the (XIC) start button.

I have tried to explain the difference regarding the scan cycle on some PLCs as well as the use of structured text as a programming language to explain the difference (notwithstanding electrically it works exactly the same). Just to clarify, this is not a discussion about using an XIO as a stop button; that is very easily demonstrated.

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Join Contractor Talk

Join the #1 Electrician Forum Today - It's Totally Free!

ElectricianTalk.com - Are you a Professional Electrical Contractor? If so we invite you to join our community and see what it has to offer. Our site is specifically designed for you and it's the leading place for electricians to meet online. No homeowners asking DIY questions. Just fellow tradesmen who enjoy talking about their business, their trade, and anything else that comes up. No matter what your specialty is you'll find that ElectricianTalk.com is a great community to join. Best of all it's totally free!

Join ElectricianTalk.com - Click Here JOIN FOR FREE


Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ElectricianTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

Sponsored Links
Advertisement
 
Old 11-26-2019, 06:07 PM   #2
Senior Member
 
Join Date: May 2018
Location: florida
Posts: 2,396
Rewards Points: 3,014
Default

In relay its easier to chase the hot wire with a voltmeter if the stop is before the start.

In logic we tend to look at argument, permissive, result.

Most of the time we still use a fail safe NC/XIO as the stop

Last edited by gpop; 11-26-2019 at 06:11 PM.
gpop is offline   Reply With Quote
Old 11-26-2019, 08:52 PM   #3
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default

PLCS.NET is the best place to ask the guys that do this for a living.
I asked our Engineer this same question, because he drew it after the start, and he said it doesn't matter.
Navyguy likes this.
dronai is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 11-26-2019, 08:57 PM   #4
Senior Member
 
Join Date: May 2018
Location: florida
Posts: 2,396
Rewards Points: 3,014
Default

Quote:
Originally Posted by Navyguy View Post
Looking for a simple way (preferably a graphic or flow chart) to explain why in relay logic we show (wire) the (N/C) stop button ahead of the(N/O) start button, and in ladder logic we show the (XIC) stop button after the (XIC) start button.

I have tried to explain the difference regarding the scan cycle on some PLCs as well as the use of structured text as a programming language to explain the difference (notwithstanding electrically it works exactly the same). Just to clarify, this is not a discussion about using an XIO as a stop button; that is very easily demonstrated.

Cheers
John
Not everything written in ladder logic will work in relay logic. Ladder only has 2 states which is on or off.
A relay (non solid state) technically has 3 states as there is a point in time when the common on the relay is between N/O and N/C.
gpop is offline   Reply With Quote
Old 11-26-2019, 09:00 PM   #5
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default

Quote:
Originally Posted by gpop View Post
In relay its easier to chase the hot wire with a voltmeter if the stop is before the start.

In logic we tend to look at argument, permissive, result.

Most of the time we still use a fail safe NC/XIO as the stop
You must have a mechanical E stop (NC), and I think the PLC uses a NO, which will be aTrue condition
dronai is offline   Reply With Quote
Old 11-26-2019, 09:46 PM   #6
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default

Quote:
Originally Posted by dronai View Post
You must have a mechanical E stop (NC), and I think the PLC uses a NO, which will be aTrue condition
It is pretty rare that you would use and XIO as a stop condition. All emergency stops are outside the PLC program using an MCR and are mostly hard wired to the power supply of the outputs of the PLC. In some cases, such as a complex shutdown system you may be permitted to use an MCR PLC for shut down, but even that works on the outside of the PLC that is running the process.

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Old 11-26-2019, 09:49 PM   #7
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default

Quote:
Originally Posted by Navyguy View Post
It is pretty rare that you would use and XIO as a stop condition. All emergency stops are outside the PLC program using an MCR and are mostly hard wired to the power supply of the outputs of the PLC. In some cases, such as a complex shutdown system you may be permitted to use an MCR PLC for shut down, but even that works on the outside of the PLC that is running the process.

Cheers
John

I think we are saying the same thing ? What I wrote above, is how I remember being taught in a class.

Last edited by dronai; 11-26-2019 at 09:51 PM.
dronai is offline   Reply With Quote
Old 11-26-2019, 09:49 PM   #8
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default

Quote:
Originally Posted by gpop View Post
Not everything written in ladder logic will work in relay logic. Ladder only has 2 states which is on or off.
A relay (non solid state) technically has 3 states as there is a point in time when the common on the relay is between N/O and N/C.
That is true, but a similar time exists during the scan cycle to; while not a physical change in state, there are times when instructions don't match with their bits that they are associated to.

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Old 11-26-2019, 09:55 PM   #9
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default

Quote:
Originally Posted by dronai View Post
I think we are saying the same thing ? What I wrote above, is how I remember being taught in a class.
I understood that you wire the e-stop or standard stop to the NO contacts and use and XIO in the PLC. That is generally considered a potentially dangerous situation.

Did I misread / misunderstand your post?

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Old 11-26-2019, 10:19 PM   #10
Senior Member
 
Join Date: Sep 2009
Location: Midwest
Posts: 999
Rewards Points: 1,758
Default

Wow, XIO & XIC are possibly the worst abbreviations ever. I’ve been working with Allen-Bradley PLC’s since 1983 and I still can’t remember the difference.

They used to refer to them as Examine On and Examine Off.
bill39 is online now   Reply With Quote
Old 11-26-2019, 10:31 PM   #11
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default

Quote:
Originally Posted by Navyguy View Post
I understood that you wire the e-stop or standard stop to the NO contacts and use and XIO in the PLC. That is generally considered a potentially dangerous situation.

Did I misread / misunderstand your post?

Cheers
John
Opposite, N/C E-Stop, N/O PLC
dronai is offline   Reply With Quote
Old 11-26-2019, 11:02 PM   #12
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default

Quote:
Originally Posted by dronai View Post
Opposite, N/C E-Stop, N/O PLC
Are you calling an XIC a PLC N/O? Maybe that is where I am losing you...

You are not actually wiring the physical N/O contact on the e-stop?

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Old 11-26-2019, 11:05 PM   #13
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default

Quote:
Originally Posted by bill39 View Post
Wow, XIO & XIC are possibly the worst abbreviations ever. I’ve been working with Allen-Bradley PLC’s since 1983 and I still can’t remember the difference.

They used to refer to them as Examine On and Examine Off.
I agree to some extent. I seem to go through cycles of using and not using PLCs, it takes me a bit to get "reoriented" as it were.

I like to use the language "go look for 1 or 0" and "go write a 1 or 0"; I find it makes it easier to understand what the instruction is meant to do.

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Old 11-26-2019, 11:08 PM   #14
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default

Yep, Field wiring E-stop N/C, PLC stop - N/O There's nothing complicated about this. The N/O is just an input from the field wiring. It's a true condition, because the Field E-stop is N/C
dronai is offline   Reply With Quote
Old 11-26-2019, 11:14 PM   #15
Senior Member
 
Navyguy's Avatar
 
Join Date: Mar 2010
Location: Welland, Ontario Canada
Posts: 2,878
Rewards Points: 5,950
Default

Quote:
Originally Posted by dronai View Post
Yep, Field wiring E-stop N/C, PLC stop - N/O There's nothing complicated about this. The N/O is just an input from the field wiring. It's a true condition, because the Field E-stop is N/C
OK, I am just not used to referring to the PLC instructions as N/O and N/C; I tend to only use those terms in relay logic. I find using those terms in PLC programming only confuses the situation because they actually have nothing to do with a "change in state" of the physical contact in the real-world.

Cheers
John
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Navyguy is offline   Reply With Quote
Old 11-26-2019, 11:20 PM   #16
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default

Quote:
Originally Posted by Navyguy View Post
OK, I am just not used to referring to the PLC instructions as N/O and N/C; I tend to only use those terms in relay logic. I find using those terms in PLC programming only confuses the situation because they actually have nothing to do with a "change in state" of the physical contact in the real-world.

Cheers
John
I work around our Engineer/ programmer, and learned a lot from him. I think of everything as relay ladder, PLC or Relay doesn't matter with the simple stuff, contacts, and coils. When you get into the advanced functions, it gets complicated, but that's what Engineers are for.
dronai is offline   Reply With Quote
Old 11-27-2019, 12:18 AM   #17
Premium Member
 
dronai's Avatar
 
Join Date: Apr 2011
Location: California
Posts: 7,419
Rewards Points: 439
Default



There you go
dronai is offline   Reply With Quote
Old 11-27-2019, 05:57 AM   #18
Senior Member
 
Join Date: May 2018
Location: florida
Posts: 2,396
Rewards Points: 3,014
Default

Quote:
Originally Posted by dronai View Post
You must have a mechanical E stop (NC), and I think the PLC uses a NO, which will be aTrue condition
My bad. Thankfully ladder logic has pictures which avoids having to use the term xio and xic.
dronai likes this.
gpop is offline   Reply With Quote
Old 11-27-2019, 08:42 AM   #19
Senior Member
 
Join Date: Jan 2011
Location: canada
Posts: 1,505
Rewards Points: 2,124
Default

i always use nc contact on the stop even if it goes into plc inputs, it is easier to understand that every stop, e-stops, safety limit switchs are all nc contacts
oliquir is offline   Reply With Quote
Old 11-27-2019, 02:20 PM   #20
Senior Member
 
Join Date: Sep 2009
Location: Midwest
Posts: 999
Rewards Points: 1,758
Default

I believe the terms XIO & XIC are unique to Allen-Bradley PLC’s. While AB has the major share in the USA, workers here who have only used Omron, Automation Direct, Siemens and others may not be familiar with XIO & XIC. This is why I continue to use NO & NC terminology.

I understand the confusion w/using NO & NC but it is what it is. This is why newbies need to understand the concept of failsafe and actual wiring. There is more to it than just programming.

Next comes the importance of creating the proper PLC descriptors. ALWAYS make the description be as if it is a 1 (HI or ON). Don’t describe an input simply as “Ram extended LS”. Describe it as “Ram is extended” or “Ram is retracted”.
dronai likes this.

Last edited by bill39; 11-27-2019 at 02:29 PM.
bill39 is online now   Reply With Quote
Reply

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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -4. The time now is 12:52 PM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.1
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Our Pro Sites Network
ContractorTalk.com | DrywallTalk.com | HVACSite.com | PaintTalk.com | PlumbingZone.com | RoofingTalk.com