(a) Determine the actions and sequence required by the system.
When using a programmable logic controller (PLC), one of the most important steps is determining the system's required inputs and outputs. Input and output requirements:
(1) The first step is to set the number of system inputs and outputs.
(2) The second step is to decide the order of control, the corresponding relationship between the devices, and what kind of response to take.
(ii) Numbering of input and output devices
Each input and output, including timers, counters, and built-in registers, has a unique corresponding number and cannot be mixed.
(iii) Draw the trapezoid diagram.
Draw a ladder diagram based on the action requirements of the control system.
Trapezoid Diagram Design Rules
(1) The contacts should be drawn on a horizontal line, and drawn according to the principle of left to right and top to bottom and the control path of the output coil.
(2) Branches that do not contain contacts should be placed in the vertical direction to facilitate identification of contact combinations and control paths for the output coil.
(3) When several series circuits are connected in parallel, the series circuit with the most contacts should be placed at the top of the ladder diagram. When several parallel circuits are connected in series, the parallel circuit with the most contacts should be placed at the leftmost side of the ladder diagram. This arrangement results in a concise and clear program with fewer statements.
(4) The contacts cannot be drawn to the right of the coil.
(iv) Convert the ladder diagram into a program
The next step after converting the relay ladder diagram into programmable controller (PLC) code is to compile the code into a programmable controller.
This programming language consists of serial numbers (i.e., addresses), instructions (control statements), and device numbers (i.e., data). The address is the location where the control statements and data are stored or placed, and the instructions tell the programmable controller how to use the device to perform the corresponding action.
(v) Input the program using the keyboard in programming mode.
(vi) Programming and designing control programs.
(vii) Test and correct errors in the control program.
(viii) Keep the complete control program.
