PLC Programming Structure

A little story about the structure of PLC programming which I think is very good reply. Any person or PLC programmer must have a way to program the PLC which according to him the most good and bad. In the sense of the word easy in the process of editing, structured, good views, and as much as possible to reduce error rates. What will I tell you a little time is actually my experience in working on some project automation system. Most of the projects that I also do more to "process plant" and uses a lot of input and output both analog and digital (> 500 IO).
Usually in programming the PLC of a plant automation, I divided into 4 levels:

1. Master Sequence Level The most highest level and is a supervisory to summon all levels below it, is a major part of the process that will be made in these plants. Master sequence is also the interface for the operator to operate plantnya, from here can see the status of what is happening in these sequences, whether the longer "run", "stopped", "stopping", "hold", "aborting", etc.. In addition it is also the place where the parameters associated with the process laid.
2. Unit Level Level where its programming logic is put, whether it is the delay time of the process, the activation of the equipment used, the interlock list for equipment, PID control, and enable / disable of the process. A master sequence must have at least 1 or more units. The division here depends on the programmer unit itself, whether to sort out the units to as small or as detailed as possible, simplify or more units in one process, except that here the consistency required of the programmer in sorting out the unit.
3. Equipment Level It is the object of objects of existing equipment. For example, such as: Valves, Motors, Analog Input, Control Valves, Digital Input, and so on. Each equipment has a way of working or a separate function that we describe in a programming language. Object object here should also provide an interface with the unit level equipment to enable those levels.
4. Field Level At this level the process of "map" of the equipment level to the field level or the address of the inputs and outputs are connected to PLC input module output. By programming 3 level, above all operate with an internal memory (in S7 I usually use a data block). It was easier when a change in the wiring, so that a simple change at this level without work especial at the level of the previous level or even to change the tag SCADA / HMI.