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Discussion Starter · #1 ·
Ive wired them but never got into the programming of Allen bradley PLCs taking a test that has me stumped! appreciate any feedback.

In Allen bradley ladder logic draw an XON logic statement (ON) ?
XOF " " (OFF) ?
TON " " (TIME)?

IN a ladder logic program how do you know a statement is true ? ( truth tables)?

What is a FORCE
 

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Ive wired them but never got into the programming of Allen bradley PLCs taking a test that has me stumped! appreciate any feedback.

In Allen bradley ladder logic draw an XON logic statement (ON) ?
XOF " " (OFF) ?
TON " " (TIME)?

IN a ladder logic program how do you know a statement is true ? ( truth tables)?

What is a FORCE
I love when people write tests about something they know little about.

1. In RSLogix there is no instructions XON. I believe there might be with RSLinx, but thats dealing with communication and handshaking.....They might be referring to XIO, which means "examine if open." All it is is a normally open contact.

2. To know if a statement is true, I just hook up to the PLC and monitor the program through RSLogix. The person that wrote this test may be talking about truth tables though?????

3. A force is where you "force" a bit through the software(0 or 1, true or false, on or off, etc.)

I'd be able to help more if the questions were not so vague.
 

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Discussion Starter · #3 ·
This place is a state of the art recycling plant they crush cars and make steel baseballs that they send overseas,havent been there yet but the noise is supposed to be in the stupid zone. I only have an Allen bradley cataloge,and I beleive the question on FORCE is the module for hydraulic control.thanks though !wont be the first or last time I did no...t get a Gold star...In control ,where in NY are you, I Got thrown out of Cobleskill and graduated DELHI or" I GO DELHI!" thanks Again
 

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Ive wired them but never got into the programming of Allen bradley PLCs taking a test that has me stumped! appreciate any feedback.

In Allen bradley ladder logic draw an XON logic statement (ON) ?
XOF " " (OFF) ?
TON " " (TIME)?

IN a ladder logic program how do you know a statement is true ? ( truth tables)?

What is a FORCE
Ditto what InControl said about XON/XOFF. The only XON/XOF I've run across is not an Allen Bradley mnemonic, but is software flow control protocall to handle the flow of data between a computer or PLC and a modem.

XIO means examine if open and is true (logical state of 1) if the bit is energized. XIC means examine if closed and is true if the bit is denergized.

TON is a non retentive on delay timer. It consists of an input, an enable bit (EN), a timing bit (TT), a done bit (DN), and a preset value. When the input becomes true EN is true and the timer value starts from zero and begins accumulating until it reaches the preset value. While the timer is accumulating, TT is true. If the input stays true long enough for the accumulated value reach the preset value, DN becomes true and stays true untill the input goes false again and the timer resets itself to zero. If the input goes false (logical state of 0) before the accumulated value reaches the preset value, the accumulated value will reset to 0 untill the next time the input goes true. A retentive timer would keep the value and start timing from that point the next time the input went true.

I'm not sure what they mean by how to tell when a statement is true, but true equals a logical value of 1, or high, or on, or +5v. Truth tables are used in boolean algebra and logic gates, which are the building blocks used to make digital circuits, and also show the way a plc executes its program. Lets look at the 2 most basic logic gates.

AND gates are series logic: If Input A and Input B are true, then output Q is true. If either input is false then the output is false. If both inputs are false, the output is false.

AND Truth Table:

A B Q
------
0 0 0
1 0 0
0 1 0
1 1 1

----------[A]-------------------------(Q)


OR gates are parallell logic. If either input A or input B is true, than output Q is true. If both inputs are false the output is false. If both inputs are true, the output is true.

OR Truth Table:

A B Q
------
0 0 0
1 0 1
0 1 1
1 1 1

-----------[A]---|------------------------(Q)

--------------|


There are 5 more basic logic gates; XOR, NOT, NAND, NOR, and XNOR. I wont get into the truth tables for them, but you can get a lot of valuable information online. When I learned about Boolean algebra and logic gates, it made understanding PLC programming much easier. A great book is PLC Practices and Concepts, by Doug Lewellyen. I've lost it and tried to find it over the years, but still haven't had any luck. I may have the name or author wrong, its been 12 years. But it was by far the most helpful PLC book I've ever read.

A force is a PLC programming software function that allows you to turn a bit on or off independent of the program logic. Forcing in often used for program debugging at startup, or in maintenance situations where real world devices may need to be removed for short periods of time for repair or replacement. Forcing is dangerous, and should only be done by qualified personnell. Forcing overrides the PLC program execution, and can affect real world devices.

Good luck with your test!
 

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Forcing an I/O is what factory maintenance men who have no troubleshooting skills do to get the machine running, then dump the whole mess on the next shift. :censored:
 

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Forcing is strictly prohibited where I work, just for the fact that years ago we had a guy that would do this.
I went to work for a saw mill that had no control technician working for them, and for years has done all their troubleshooting and maint. over a modem with the OEM, and half of the programming was done in Quebec so the program descriptions were in French! Talk about a mess. There were troubleshooting "traps" everywhere with no explainations, and forces installed for no apparant reason. Some of the programs are rather involved, and I was constantly finding stuff. It can be a real headache when I end up wasting time chasing something that doesn't have anything to do with the current problem. And of course the customer doesn't want to spend a bunch of time "fixing something thats not broke". I worked for ther mill itself for about 18 months before I went out on my own and it didn't bother me too much then, but now I'm worried about liability issues. I'm real careful to document everything I do aside from just program notes. I wont be responsible for someone elses mistakes.
 

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This place is a state of the art recycling plant they crush cars and make steel baseballs that they send overseas,havent been there yet but the noise is supposed to be in the stupid zone. I only have an Allen bradley cataloge,and I beleive the question on FORCE is the module for hydraulic control.thanks though !wont be the first or last time I did no...t get a Gold star...In control ,where in NY are you, I Got thrown out of Cobleskill and graduated DELHI or" I GO DELHI!" thanks Again
Oh, an employment test. My favorite!!!! I still shake my head about some of the interviews i've had. Off the top of my head heres one....interviewer shows me a picture of a 3 phase motor starter circuit. Interviewer: "the motor is single phasing, what would you do?" Me: "first thing i would do is check the voltage at both sides of the contactor." Interviewer: "how would you check for voltage?" Me: "with my meter." Inerviewer: "how would you check with your meter?" Right then and there the interview was over in my mind.

I've had alot of interviews, even though i've been at the same place for 10 years. Mainly because I just like to see what else is out there and I get tours of other plants. I can tell how the interview is going to go as soon as I see the maintenance manager. If he's got a white collared shirt on, I know i'm going to get HR type questions like "what are your strengths and weakneses?" :censored: The best interviews i've had are when they send me to the plant floor and the maintenance manager is dirty.

BTW, im in Saratoga Louie.
 

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What do you do now? Do you still do controls? If so, how hard is to find that type of work on your own?
Now I have a one man shop in a remote area of the state. I mostly do resi, but I still do controls for the mill I worked at before I went out on my own. There is one other industrial facility (a biomass plant) in my small town, and I'm trying to get them as a customer as well. Other than that, to do controls I'll have to travel, and right now that defeats the purpose of working for myself. I read a lot of posts by guys who go out on their own and have problems trying to break into markets that already have plenty of ECs, but my problem is different. I'm the only EC in my "four seasons resort" area, so there isnt as much comp, but I hate residential work. After doing resi and commercial for the first 5 years of my career, I went into industrial construction and maintenance for 17 years, and now going back to resi is driving me nuts. The work just isn't anywhere near as fun, and I'm not even that good at it. At 40, I'm a whole lot slower than I was at 20, and if I had to compete with a bunch of other ECs, I'd probably be screwed. Its only been a year, so I'm going to give it more time, but I'm thinking about looking for more industrial customers outside of my area. Although even when you dont like the type of work, its kind of foolish to turn good money away from your front door to try to compete in an established market away from home.
 

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Discussion Starter · #12 ·
Wow you guys are awesome coming out with all that technology without cheating...The only controls I do now are for HVAC systems and not really all that complicated,I havent done a system in years where they give you a box full of probes,VVt boxes,etc and say make it work ,ohyea the Plc.I am also a one man band ,and from time to time get recruited to go with bigger companies for different projects,kinda like a paid vacation without the alchol or wife.

The Industrial control work in this area Now is more conveyors than proccesses....The insurance costs are HUGE when you get into specifics with your carrier on what type of installations you are actually working on ,when dealing with petrochemical compared to residential,just like when you decide to hire staff and go into the world of Workmans comp and payroll taxes.Thats why allot of huge plant type work goes to the Intergalatic contractors like Brown n Scoot.Allot of the plants I worked in this area for other contractors are gone,Dupont,Anchor glass.Again thanks!
 

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The contractor I am working for has a great deal doing control work for Proctor and Gamble. The owner was with another contractor several years ago doing work for them. They especially liked the work he himself was doing and offered him the opportunity to break away, start his own business and continue doing the same work. He took up on the offer and now that is 90% of his work! Bidding work for P&G doesn't need to happen, he just automatically gets it. The company stays small employing myself and only 5 other electricians most of the time. P&G has endless pockets and I'm sure my boss is making a killin'!! ;)
 

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Ditto what InControl said about XON/XOFF. The only XON/XOF I've run across is not an Allen Bradley mnemonic, but is software flow control protocall to handle the flow of data between a computer or PLC and a modem.

XIO means examine if open and is true (logical state of 1) if the bit is energized. XIC means examine if closed and is true if the bit is denergized.

TON is a non retentive on delay timer. It consists of an input, an enable bit (EN), a timing bit (TT), a done bit (DN), and a preset value. When the input becomes true EN is true and the timer value starts from zero and begins accumulating until it reaches the preset value. While the timer is accumulating, TT is true. If the input stays true long enough for the accumulated value reach the preset value, DN becomes true and stays true untill the input goes false again and the timer resets itself to zero. If the input goes false (logical state of 0) before the accumulated value reaches the preset value, the accumulated value will reset to 0 untill the next time the input goes true. A retentive timer would keep the value and start timing from that point the next time the input went true.

I'm not sure what they mean by how to tell when a statement is true, but true equals a logical value of 1, or high, or on, or +5v. Truth tables are used in boolean algebra and logic gates, which are the building blocks used to make digital circuits, and also show the way a plc executes its program. Lets look at the 2 most basic logic gates.

AND gates are series logic: If Input A and Input B are true, then output Q is true. If either input is false then the output is false. If both inputs are false, the output is false.

AND Truth Table:

A B Q
------
0 0 0
1 0 0
0 1 0
1 1 1

----------[A]-------------------------(Q)


OR gates are parallell logic. If either input A or input B is true, than output Q is true. If both inputs are false the output is false. If both inputs are true, the output is true.

OR Truth Table:

A B Q
------
0 0 0
1 0 1
0 1 1
1 1 1

-----------[A]---|------------------------(Q)

--------------|


There are 5 more basic logic gates; XOR, NOT, NAND, NOR, and XNOR. I wont get into the truth tables for them, but you can get a lot of valuable information online. When I learned about Boolean algebra and logic gates, it made understanding PLC programming much easier. A great book is PLC Practices and Concepts, by Doug Lewellyen. I've lost it and tried to find it over the years, but still haven't had any luck. I may have the name or author wrong, its been 12 years. But it was by far the most helpful PLC book I've ever read.

A force is a PLC programming software function that allows you to turn a bit on or off independent of the program logic. Forcing in often used for program debugging at startup, or in maintenance situations where real world devices may need to be removed for short periods of time for repair or replacement. Forcing is dangerous, and should only be done by qualified personnell. Forcing overrides the PLC program execution, and can affect real world devices.

Good luck with your test!



I don't know a damn thing about PLC's. But after looking at all that my heads about to explode.
 

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I don't know a damn thing about PLC's. But after looking at all that my heads about to explode.
It's not all that high-tech, really. As a computer savvy electrician, you could get your head around it faster than you might think. Consider it as a series of relays (outputs) that are controlled by a computer program you write that can take data from switches and sensors (inputs). Just a bunch of relays with a brain.
 

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Go to your local community college and sign up for a PLC class. This way you will get the basics. Another option is to go to your supplier and see if they offer classes on PLCs. Most AB distributors do.
 

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Its all cause and effect. You flip the switch and the light comes on. You push the button and the motor starts. The flow rate raises 5 GPM, and the control valve closes or the VFD slows the pump motor down to bring it back to set point. None of these things are conceptually different from each other. A PLC is nothing more than a way to tie many of these small control schemes together without a bunch of hardwiring. I started by borrowing an old PLC from the spare parts pile of a refinery I worked at, strapping it to a piece of plywood, hooking switches and sensors to the inputs, lights and relays to the outputs, and taking the users manual and going through it untill I could make the lights go on and off when I pushed the buttons. It took quite some time as you entered every instruction one at a time through a handheld programmer, but when I was done, I had a pretty good basic understanding of how that PLC worked. It was fun, and I learned a lot. I encourage anyone with any interest to explore PLCs. they arent that hard to learn, at least from a installation and maintenance standpoint, and you'll always have a job. These days, you find programmable controllers everywhere.
 
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