When programming or implementing a control system, organization is key. A larger project will require more organization, especially if there are multiple people involved. A successful control solution is not only about organization. It also requires the ability to implement it.
Programmers must be able to understand the PLC control task, the controlled devices and the equipment required for the job. After these details have been understood, the programmer is able to begin drawing the control program solution.
Figure 1: Electromechanical relay circuit diagram.
This time is crucial for project documentation and the system. It can be difficult to document a system after it has been installed and is in use, especially if it doesn’t work the first time. Documenting the system from its inception will make it easier to understand the final result.
1. How to Create Flowcharts and Output Sequences
2. How to Configure the PLC System
During logic sequencing and flowcharting, it is important to consider the configuration of the PLC. Configuration of the PLC determines which I/O modules are used with what types I/O signals. It also specifies where the modules will reside in the rack enclosures. The module locations will determine which I/O addresses will be used by the control program.
When configuring a system, it is important to consider future expansions, special I/O modules such as wire fault inputs or fast-response I/O, and where interfaces will be placed within the rack (all AC/DC I/O together, all low-level analog and DC I/O together). These details should be considered along with the system configuration documentation to ensure a better design.
3. Internal and Real I/O Assignment
4. Assignment of Register Addresses
Another important aspect of PLC organization is the assignment of addresses to registers used for control programs. It is easiest to list all available PLC registers in order to assign them. Next, you will need to describe the contents, function, and description of each register in a register assignment table. The register assignment table shown in Table 6 for the 15 first registers of a PLC system ranges from address 20008 up to address 20168.
Register assignment table
5. Elements Not to Be Wired to the Controller
6. Special Input Device Programming
7. Program Coding/Translation
Program Coding is the process of converting a logic diagram or relay diagram into a PLC ladder program form. The ladder program is what will control the machine or process. It is stored in the application RAM. The order of the previous stages (control task description, I/O assignment etc.) directly affects how easy it is to code programs. a program code has been completed. Figure 21 shows an example program code that was generated from electromechanical relay diagrams and logic gates (internal coil 1,000 replaces the control relay). The coding represents the logic in a PLC format. It doesn’t matter if it’s a new application, or modernization. This section will provide several programming examples and a closer look at the coding process.
Figure 21: Translation from (a) logic gates and (b) an electromechanical relay diagram.