https://control.com/technical-articles/plc-ladder-logic-on-an-arduino-building-a-start-stop-circuit/

IMO, There is one major fault in this attempt to simulate a typical PLC.
The original PLC concept and premise was intended, not only to carry out Boolean logic control of industrial circuitry, but also to display the current status of the Boolean functions in the form of a visual representation.
This in order to rapidly trouble shoot any and all issues with the controlled industrial equipment.

 

that would require better software to go with it... it is the software (and firmware) issue.

for example, for a while now there are knockoff versions of Mitsubishi PLCs costing 10x..20x less than originals.
Never tried the clones but they seem to do a better job and support features like high speed io for position control.
the downside is that one needs Mitsubishi software and that is(was) expensive. Not sure about prices now, but GX Developer-FX should do.

The other difference is signal conditioning (level shifting, isolation, filter). With Arduino, one is forced to make additional circuits himself or use shields. From what i have seen there is not much to choose for 24V.

The Mitsubishi clones have the IO conditioning taken care of, they are ready for wiring, not breadboarding.

https://www.amazon.com/dp/B07WCLQTY...lkZ2V0TmFtZT1zcF9kZXRhaWw&smid=A2I2BTD60MRVUP

one can save few bucks if enclosure is not needed:
https://www.amazon.com/Programmable...65-af25-93fff26c0c16&pd_rd_i=B07R3X79X9&psc=1

 

When you get into high risk applications like chemicals (manufacturing or dispensing), life supporting ventilation or even lockout systems to prevent human injury, the size of the company seems to be proportional to what they are willing to pay for a PLC implementation and equipment. The big companies just don't want to risk the liability and the little companies will essentially have interns make DIY projects to "protect" employees in the manufacturing areas. I really hope the Arduino option doesn't become the new path because the software is more important than the hardware and, instead of having a management of change process, testing, sign-offs, training, ... we are going to see more and more on-site guys plugging laptops into arduinos and making changes as they see fit. Then pointing fingers when something goes wrong.

 

There is no reason to consider using an arduino toy computer for an application where an actual PLC is required.
First, the language and operating system would be inadequate, and also every aspect of the hardware is also inadequate.
As for the cost, instead of comparing with the high priced Mitsu PLC line, consider those products sold by "Automation Direct", which also include the software at no extra charge.
Reliability does matter in most applications, and assuring adequate reliability does require more effort on both design and execution of a product. That is part of the reason why some PLC systems are so very expensive, although it does not explain some of the very large price differences.

 

I have no problem with using a Picmicro etc to obtain Boolean logic control, which, BTW, it is perfectly capable of. the Issue I had was with the PLC label, which IMO should conform to the original intention and premise.
As well as replacing the old hard wired relay logic It should reproduce and display in schematic diagram format, the Boolean ladder represented in a drawings that used electromechanical relays, this was the original intention and was adopted by many manufacturers and later standardized in the IEC61131-1 control systems programming standard.
At one time I did play around with the 16 pin MC14500 that Motorola came out with some years ago as a 1 bit Boolean logic processor for simple logic control, but this does not satisfy the PLC label for e.g.

 

Executing boolean logic is one part, not so very challenging. Driving a real time graphic display is a much bigger challenge. And avoiding the occasional unfortunate lockup is rather vital. Thus my suggestions toward a manufactured PLC that has already solved those problems. A home-brew controller is fine to run lights and such, but wherever really critical tasks are involved and reliability matters, it is better to go with systems already debugged. ( Thus anything running with the windows OS is excluded.)

 

Executing boolean logic is one part, not so very challenging. Thus my suggestions toward a manufactured PLC

Basically what I said, and my point!
RE: Title of the thread.

 

It is always those details that are the pain points. Simple boolean operations can even be done using BASIC. The challenge is usually with the user interface.