Is it correct to take LD or LDI instruction as the beginning of a logical line in PLC ladder diagram? Tell me your understanding.

Not necessarily. I don't understand. I can make it up as I want. I compiled the program. As long as the functional requirements are correct, who cares if I start with normally open and normally closed contacts or something else?

For example, if you need to use an interrupt, I like to put the EI instruction first, and the interrupt will be started when the program is running, but there is no control in front of the EI instruction, and the left bus is directly connected to the right bus.

It is purely to see whether the program can meet the functional requirements and the programmer's personal habits.

Hope to adopt. . . . . .