"Looking for direction"

Icekold

Member
Join Date
Sep 2012
Location
Indiana
Posts
16
Hello, I may be over my head coming here but I was referred. I'm am looking for help understanding the PLC process. I have a program I created but only with conventional hardware, eg. timers, relays, thermostats and such. My goal is recreate the method with digital hardware and software for remote supervision and management. I am paused with lack of knowledge.
 
To me a PLC is just a electronic version of all the old Relay/Timer logic. You have basically inputs and outputs with the PLC in the middle deciding how to react to them.

To begin you need to decide how many inputs and outputs you need to accomplish your goals. You will find that most PLC system will have many more inputs than outputs. Once you know how many I/O you need then you need to decide on the type of inputs and outputs. Relay, NPN sink, PNP source, triac, 4-20mA and 1-5V.

Once you know all that then you can go shopping for the PLC that fits your needs. What brand you go with will depend on several things. Price, availability, support, ease of use and expandability will all figure into your decision.

As for programming, sounds like you are familiar with Electrical Ladder Logic. With that experience you should be able to pick up PLC Ladder Logic pretty quick. Sounds like your application is a perfect application for a small PLC. Also a good project to cut your teeth on.

Best way to learn is to by a PLC and the Software and start trying different things. In no time at all you will get the idea.

If you want to give us more details on what you need to do I am sure that everyone will offer up opinion on the brand and model to go with. I am partial to AB Micrologix products. The PLC isn't that expensive IMHO but they do get a good price for the RSLogix 500 Software. There is a free version that will let you program ML1000 and I think ML1100 PLC's. I have never used the free version so I cant tell you what it can or cant do for sure.

Hope this helps you get going.
 
Two major differences between actual relay - controller wiring and PLCs.

First is the concept of the scan. In wired logic, all the logic is being 'solved' at the same time.

In PLCs the logic is being 'solved' one contact at a time. This is probably the primary difference.

Secondly are analog signals. In wired logic these are 'wires' with a varying voltage or current into a device then either varying voltage, current or On/Off contacts timing as outputs.

In PLCs signals are NUMBERS. They are processed as numerical data. The results are NUMBERS. These are then translated back into varying voltage, current or On/Off timings.

If these two major differences are a problem then the jump from standard wiring to PLCs is a major problem. But, if these concepts are grasped then PLCs are a vastly liberating tool.
 
Thank you for your advice and response. And I agree with buying one and experimenting. I am stopped with, and might sound stupid, simple basics I can't grab. Ill try one example to begin. If I want to turn on a simple light does an external switch, relay, input the PLC and the output turns the light on directly or with a control relay through a set of dry contacts?
I don't know how to configure the input or output hardware. If I am explaining this correctly.
 
I do understand how the PLC is different than electrical schematics in the way the program functions. That is the part that makes the PLC valuable and versatile. How do I switch different multiple devices with different control voltages or can it?
 
Sorry, I did read the first link at this site. I have watched all of Ron Beauforts you tubes. I just cannot wrap my head around this totally. I'm having writers block with controls, so to speak. I can describe what I need to control and when, I can understand what the PLC function needs to achieve to become true or false..and there is the my stopping point.
 
Maybe if I ask specific questions I could grasp this ?
Does the PLC run the same voltage from inputs to outputs on every device?
Is the PLC capable of energizing the output device directly or through a control relay?
 
In standard control wiring the arrangement of contacts and wires produced the 'logic' which ties together 'conditions' to 'results'. The 'conditions now become 'inputs' to the PLC. The 'results' are 'outputs' from the PLC. Everything in-between is the logic program which you write.

If you write programs you will transition from 'how' is this wired?' to 'what do we need to accomplish'. How do the 'inputs' (those items which report on the state of items in the circuit) through the 'logic' affect the 'outputs' (those item which cause an actual change in the system.)

You will start to look at the old standard wiring diagrams in a new way. You will come to recognise the power and organization of a PLC. Good luck.
 
Does the PLC run the same voltage from inputs to outputs on every device?

No, input voltage can be ( the most common) 24vdc ,120vac and others depends on the specs of the input module (type)
Outputs can be relays, or solid state devices or analog ( voltage, current)

Is the PLC capable of energizing the output device directly or through a control relay?

It can do it directly or you can use a slave relay.
 
Inputs, once they reach terminals, cease as 'open' 'closed' 'on' or 'off' and become simply ON (the input circuitry is energised) or OFF (the input circuitry is not energised.)

Look on Youtube for 'PLC Training Videos'. Watch all of them in order. They are somewhat specific in naming items with the Allen Bradley PLCs but they give, in a very concise format, the essence of understanding of inputs, outputs and PLC scanning.
 
Where are you at in Indiana? If you are down south I might be able to help you get some hands on experience.
 
Ok, I will try again. Maybe it will become clear. Thank you for your help and patience.
I think I know where your question is coming from.

The inputs are grouped together and isolated from the outputs in the physical wiring.

Often, this is done with separate modules with signal counts of 4, 8, 16 and 32 most commonly. There are specialty modules with both inputs and outputs for some models. This is especially true with analog.

You can get modules for nearly any control application voltage and a wide range of current carrying capacity.



In addition to the comments above (especially understanding the scan cycle), think of relay ladder logic in a PLC, just like you would if you had relays with a virtually infinite number of "contacts". These contacts are not really contacts, they are references toa "bit box", they are best read as questions..."If" and "If Not", the bit instructions that pass logic are simply saying if the address holds a 1 or not.

Am I warm?

And going further anyway, when designing your first program, start with a written plan, and start writing the program starting with the outputs and work your way back, by inserting things that must be "true" or "on" or contain a "1" before that output device is energized.
 
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