How can you write a basic program for a programmable logic controller?

The initial step is selecting a Programmable Logic Controller (PLC) compatible with your requirements and budget. PLCs vary in types and brands, yet share common features like inputs, outputs, memory, and CPU. Concurrently, opt for software facilitating communication with your chosen PLC, enabling program writing and downloading. Popular PLC software options include RSLogix, TIA Portal, and CODESYS.

Defining inputs and outputs is essential when writing a basic program for a Programmable Logic Controller (PLC). Inputs typically represent signals or data received by the PLC from sensors or other devices, while outputs correspond to actions or signals sent by the PLC to control actuators or machinery. By clearly defining the inputs and outputs, programmers can map the behavior of the system accurately and ensure that the PLC performs the desired functions in response to different input conditions. This involves specifying the physical addresses or digital tags associated with each input and output point in the PLC's memory or configuration settings.

Subsequently, delineate the inputs and outputs of your PLC program. Inputs encompass signals from sensors, switches, or devices conveying the process's status to the PLC. Conversely, outputs are signals sent to actuators, lights, or other devices directing the PLC's actions within the process. For instance, in a basic program, inputs could comprise two buttons, while the output could be a light.

It is always an advantage to accomplish more than one objective in a single task. Consider using a spreadsheet application to organize and track inputs and outputs. A sheet of unassigned inputs and outputs can contain columns for data such as card type, PLC address, tag name, description, signal range and units, engineering range and units, enclosure, wire number. Then sort by card type and transfer groups of I/O points to an assigned inputs and outputs sheet, grouped by physical I/O card. The resulting spreadsheet is simple to keep updated and useful for cost estimating, control panel drafting, troubleshooting, instrument programming, PLC programming, and often wiring. Don't forget to add some spare points.

Different PLC's will offer different program organization options. Planning code organization is a critical first step in writing controller logic which could be in service for decades. Troubleshooters will need a foothold to find and resolve equipment issues, and organization is major opportunity to reduce downtime over the equipment lifespan. Alarms grouped into a single routine are easier to locate. Outputs grouped into a single routine and ordered by their physical configuration in the hardware provide cross reference to the logic controlling them. In-link routines tie physical inputs to PLC registers in a single location so they can be relocated without changing code in numerous locations. Avoid obscure abbreviations wherever possible.

I have worked on microcontrollers, which are scaled down versions of PLC's eg Philips RC5, NEC RC3 etc. The memory is only 8K.The challenge is to fit the logic for say 4 TV remotes of different vendors in 1 page. This was my first attempt at writing live code, and, Philips RC5 worked. (My boss was angry when I left the job.) UV light is used to "burn" code onto the EPROM. Simulators are also used for testing. All in all a very holistic experience. This was almost 24 years ago??