PLC ladder diagram usage rules - Database & Sql Blog Articles

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The use of plc ladder diagrams should comply with the following rules:
(1) Each ladder diagram is composed of a plurality of steps, each of which may constitute one step, and each step may be composed of a plurality of branches, each step representing a logic equation.
(2) The relays in the ladder diagram are not physical relays. Each relay and input contact is a bit in the memory. The corresponding bit is “1” state, indicating that the relay is energized or the normally open contact is closed or the normally closed contact is broken. open.
(3) The current flowing in the ladder diagram is not a physical current, but a "concept current", which is an image representation that satisfies the output execution condition in the user program solution, and the "concept current" can only flow from left to right.
(4) The relay contacts in the ladder diagram can be quoted indefinitely when programming the user program, both normally open and normally closed.
(5) In the ladder diagram, the input contact and the output coil are not physical contacts and coils. The calculation of the user program is the content of the PLC input and output status table, not the on-site switching status according to the solution.
(6) The output coil only corresponds to the corresponding bit of the output status table. The programming element cannot directly drive the field actuator. The status of this bit must pass the corresponding output transistor switch, relay or thyristor on the I/O module to drive the field. operating units.
(7) The contact cannot be connected to the right side of the output coil. The contact must be on the left side of the output coil.
(8) Two or more coils may be connected in parallel, but not in series.
(9) The left-hand busbar of the ladder diagram cannot be directly connected to the output coil and must be connected through the relay contacts.
(10) There must be an END instruction at the end of the program.