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We are using an old electrical system in industry. My task is to digitalise it. As i saw, PLC is too expensive and large scale for our operation. Also, microcontroller needs programming which makes operation only depending on me. I am considering simple logic by using logic gates of TI. I would like to ask whether it is possible to use these ICs without microcontroller and also what alternatives can I use? It may be also possible with TTL, DTL logic but combinational circuit becomes too complicated with them.

System works at 24V normally but not necessarily.

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    \$\begingroup\$ PLCs are not necessarily large scale and expensive. Small all-in-one units called Logic Modules can be programmed with free software and are more rugged and maintainable/replaceable than a custom designed board. \$\endgroup\$
    – Martin
    Commented Jun 5, 2020 at 9:11
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    \$\begingroup\$ Yes, microcontroller requires programming, PLC requires ladder logic, CMOS/TTL logic gates require digital design, CPLD/FPGA require HDL design. Any of these tools will work (PLC is best for harsh industrial environment), but they all require defining the actual work that the system must perform, in detail. Same approach is needed in all cases: first define the scope of work, document the existing system requirements. Determine the available cost budget and time allowed. Determine whether additional skill sets are needed, either to learn, or to hire an engineering consultant. \$\endgroup\$
    – MarkU
    Commented Jun 5, 2020 at 9:17
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    \$\begingroup\$ PLCs were brought in as a programmable alternative to fixed simple logic gates. Nobody in their right mind would today automate something with solder-down logic, when it can be done programmably. Find the right small PLC. What happens when you want to change it? What happens when the customer wants assurance that it's suitable for the rugged factory environment? \$\endgroup\$
    – Neil_UK
    Commented Jun 5, 2020 at 9:22
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    \$\begingroup\$ For industry use equipment designed for industry. Your question doesn't say how many inputs and outputs you have. You won't find 24 V logic chips so you will need level converters. Your discrete logic approach would have been OK in the 1980s but not anymore. \$\endgroup\$
    – Transistor
    Commented Jun 5, 2020 at 9:29

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I said, 'Nothing like a PLC for industrial automation!', 35 years ago, and I would still say that.

The Programmable Logic Controller or PLC, which was specially developed to cater to the needs of the automobile manufacturing industry, is robust and flexible and has proved itself capable of withstanding the demands of harsh industrial environments.

Fifty years ago, the mainstay of automatic special purpose machine control was hard-wired relay logic which was supported by electro-pneumatic/motorised timers, electromagnetic digit-wheel counters, step selectors, and latched relays.

Special purpose machines had come into being for carrying out reliable, repetitive and sequential machining operations in the mass manufacturing of automobile components.

Then came electronic logic gates, hard-wired gate modules, timers, counters and shift registers followed by basic microprocessor control which did not gain ready acceptance (resistance to change).

The first microprocessor-based PLCs catered only to logic functions using input, output, timer, counter and latch modules. Proficiency in relay logic made 'Ladder Diagram' programming / trouble-shooting easy and that enabled the take over of automatic special purpose machine control by PLCs.

Later, introduction of arithmetic and process control functions made the PLC quite versatile. Then came servo control & computer numerical control and the rest is history!

In the test and measurement field the industrial PC rules, with the support of dedicated modules for data acquisition & control, including software, from reputed manufacturers.

Now you be the judge!

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