To write a program for a Siemens S7-300 PLC using the STEP 7 programming software (now known as TIA Portal), follow these general steps:

1. Open TIA Portal:

   • Launch the TIA Portal software on your computer.

2. Create a New Project:

   • Create a new project by selecting "Create new project" or "New project" from the menu.
   • Specify the project name, location, and any other relevant information.

3. Select the Hardware Configuration:

   • Choose the hardware configuration for your project, including the PLC model (S7-300), communication modules, I/O modules, and other components.
   • Add the hardware components to your project and configure the hardware layout as per your system requirements.

4. Create PLC Program:

   • Once the hardware configuration is complete, navigate to the PLC section in the project view.
   • Right-click on the PLC and select "Insert New Object" or similar option to add a new PLC program.
   • Choose the programming language you want to use (e.g., ladder logic, function block diagram, structured text) and create a new program.

5. Write the Program Logic:

   • Write the program logic using the selected programming language. This involves creating rungs, adding instructions, and configuring the logic to control the desired process or machinery.
   • Use the available programming blocks, functions, and libraries provided by TIA Portal to implement the required functionality.

6. Configure I/O Tags:

   • Define the input and output tags used in your program to interface with the physical I/O modules connected to the PLC.
   • Assign symbolic names to I/O addresses to improve program readability and maintainability.

7. Compile the Program:

   • Once the program logic is written, compile the program to check for syntax errors, logical errors, and other issues.
   • Fix any errors or warnings identified during the compilation process.

8. Download the Program to PLC:

   • Connect the programming cable between your computer and the PLC.
   • Establish a connection between TIA Portal and the PLC by selecting the appropriate communication interface (e.g., Ethernet, PROFIBUS).
   • Download the compiled program to the PLC by clicking the "Download" or "Transfer" button in TIA Portal.
   • Follow the prompts to transfer the program to the PLC memory.

9. Monitor and Test the Program:

   • Once the program is downloaded, switch the PLC to Run mode.
   • Monitor the PLC's operation using the TIA Portal simulation tools or by observing the physical outputs connected to the PLC.
   • Test the program under various conditions to ensure that it behaves as expected and meets the system requirements.

10. Save and Backup the Project:

    • Save the project in TIA Portal to preserve the program logic, configuration settings, and other project data.
    • Create backups of the project files regularly to prevent data loss and facilitate recovery in case of system failures or hardware issues.

By following these steps, you can write a program for a Siemens S7-300 PLC using the STEP 7 programming software (TIA Portal) and implement control logic for your industrial automation application.