advice on my first ladder program

[HornerPLC]

most humbling

I am working on adopting this to my software.
Good learning for me, can't thank you folks enough.
I had no Idea PLCs were this powerful.

 

[PLCMentor]

this is a good academic conversation given the task you have presented. Keep in mind, however, that in real world programming you will want to use a seal in circuit for the motor control with the contactor aux contact. You dont want to have a situation where the contactor drops out due to some field fault condition, and the PLC dumbly keeps the output on for the motor to run. This is an unsafe condition. The exercise that has been worked on is a great exercise to understand how some logic works in the PLC, but just dont look at this as a true real world example. Think of it as step one.

Russell

 

[HornerPLC]

Good point, this material will be used strictly for personal training and not utilized in any real-world applications. Begs the question do PLC programmers need licensing in a court of law?

 

[PLCMentor]

Ah now you hit a sore spot. In many states "integrators" are required to be professional engineering corporations and covered by the juristiction of the Engineering board of the state. That may shock many here; however, I know it to be true in North Carolina and in South Carolina. It is rarely enforced - generally only when someone turns the integrator in to the board. Or other times when the board gets a wild hair and goes after a company seemingly at random...say mine? I will add that my cease and desist letter from the state of North Carolina specifically included "programming of Programmable Logic Controllers." ...and that is when we became a professional corporation. I do know of a company that successfully fought off the SC board of engineering, but at a big legal cost and with very limiting requirements.

I can somewhat understand that their purpose is to protect the residents of the state against unqualified designers and a person can do a lot of harm with what we do. The problem is that my PE on the end of my name in no way qualifies me to program and design control systems. It does look kinda cool on my business card tho...

Russell, PE (therefore obviously capable in PLC programming)
(isn't sarcasm wonderful)

 

[Steve Bailey]

Russell, PE (therefore obviously capable in PLC programming)

[sarcasm]Not only does the PE qualify you to program PLCs, but sending the license renewal fee to the State every year automatically keeps you current with advances in the state of the art.[/sarcasm]

 

[PLCMentor]

Not to mention the fee the company has to send - now that qualifies everyone! In fact I think our new governor has a plan to make us really qualified since the corporate fee is slated to increase four fold. We must be good.

On the PE, dont forget that we are required to have 15 hours a year of professional development. If I actually put how much time we really have to spend keeping up with this field they would probably audit me in disbelief. I probably spend 15 hours a month!

Russell

 

[HornerPLC]

PEs are pretty sharp. Phds are supposed to be able to conduct statistics and be able to write. Now that I am one I see that only 10% of Phds can do either.

 

[PLCMentor]

Yeah but doesnt it look cool after your name?

Seriously, I just dont take much stock in titles, or credentials. Show me what you have done. I guess all those letters do indicate potential, but I am sure you have seen Phds, PEs, and other forms of alphabet soup that are completely useless. Likewise I have seen both that are outstanding.

Russell

 

[HornerPLC]

Right, cool until knee-deep in the ladder logic learning curve I guess.
Which is my segue to a few more questions if you allow me to indulge.

when I look at the diagrams posted I am unsure about the following:
Are the "1746-I*6" tags actually formulas attached to the contacts?
what do the EN and DN on the timers stand for?
Is there a reason the closed contact has shaded bars around it?
Are the timers latched? My program wont do that. Is there another way to depict them?

Thanks again.

 

[PLCMentor]

Sorry, didnt mean to hijack your thread... I'll try to make it up to you.

Not sure what you mean about formulas so I will let you further expand on that question.

the EN is the enable bit and the DN is the done bit. When the rung is true up to the timer (TON) the EN bit will be set. When the timer preset = (or >) accumulator, the DN bit will be set.

shaded bars? Are you referring to the xio? has |/| kinda look?

On the TON again, the enable bit will stay set until the logic goes false in front of the instruction. with the TON, the timer accum will reset with the transition from true to false in front of it. A retentive timer (RTO) will keep its accumulated value until reset with a RES instruction therefore the accum value will not change when logic leading to it goes from true to false. On true again it will simply start timing from where it left off.

 

[OkiePC]

Are the "1746-I*6" tags actually formulas attached to the contacts?

Those are references to the input and output module part numbers in an Allen Bradley PLC which I used for my test.

Your PLC timers may not have all the same bits available, but you should be able to reproduce the same logical flow, just might require more instructions.

What make and model PLC are you using?

 

[HornerPLC]

pic

Please, no need to apologize - you are helping a lot.
Here is a picture of the diagram with my questions. Hope this is better.
Thank you.

 

[HornerPLC]

Im using a Horner XLE103. Thanks for additional clarification.

 

[bkottaras]

Im using a Horner XLE103. Thanks for additional clarification.

Horner is differenjt, the screen shots were made with Allen Bradley PLC.
1746-I*16 or whatever is not formula nor an expression.
It is a generic way to display the hardware in which case it is a 1746 series (for SLC type) module I=input *16 or so is the available I/O points.
GREEN=the bit or whatever is currently evaluated by logic as being TRUE or ON.
Using Horner your bit will turn RED when True (unless you played with backgrounds and such in the options for the editor).
When using timers in the Horner PLC allocate 2 consecutive registers for one timer.
If you read the Cscape help they explain timers and counters, registers, bits within the registers of a timer which is what you should be after.
The DONE bit of the above example will be a bit of your timer register, look it up and use it when needed.

 

[bkottaras]

TT= Timing bit for the above timer
True when the timer is timing.
This is also a bit of the register allocated for your timer in the horner PLC.
DN= Timer is done timing.
EN= Timer is enabled and running
You need to read the actual help files and the "Controller resources" section so you get a rough idea on how registers, bits, internal bits & registers are to be used when using an XLe