TRi PLC Fx Series Tutorial: Getting Started with Ladder Logic Programming and Hardware Setup

Triangle Research International offers a unique approach to control systems with its open-board Fx series PLCs. In this hands-on tutorial, we walk through the essentials of connecting, wiring, and programming one of these controllers. From hardware setup to downloading a simple program via serial communication, this guide provides a practical starting point for engineers exploring customizable PLC platforms for OEM and embedded automation applications.

Although the exposed PCB may look unfamiliar to many engineers in the automation and control system industry, a PLC is simply a device that executes logic functions using inputs and outputs that interact with the real world. PLC designs vary widely in protection level and environmental durability, but the core function remains the same across all systems.

What is the Fx Series PLC?

The Mitsubishi Electric Fx Series PLC includes most of the functionality required for projects ranging from simple machine control to moderately complex automation systems. These controllers typically include Ethernet, RS-232, dual RS-485 communication ports, and both digital and analog I/O, depending on the model.

The number of I/O points can be expanded using Modbus networking, making the system flexible for scalable industrial automation applications.

This tutorial uses the Fx2424 model, which provides:

• 24 digital inputs
• 24 digital outputs
• 8 analog inputs
• 4 analog outputs

Important technical characteristics for programmers include:

• Digital inputs are sourcing, requiring sinking (NPN) sensors or switches connected to 0 VDC.
• Inputs receive power internally from the 24 VDC supply and do not require a separate common terminal.
• Digital outputs are sinking, meaning the load devices must be powered externally.
• Analog I/O supports 0–5 V signals, with optional resistor networks enabling 0–10 V or 4–20 mA devices.

Project Hardware Setup

This introductory tutorial uses a simple configuration consisting of one input device and one output device connected through an RS-232 communication interface. The hardware required includes:

• TRi PLC Fx Series controller
• 24 VDC industrial power supply
• USB-to-Serial converter cable
• Selector switch (input device)
• Mechanical relay (output device)
• Standard wiring

Step 1: Power the Controller

Connect the 24 VDC power supply to the power input header labeled +24 V and 0 V. Verify that the power LED indicator turns on before proceeding.

Step 2: Connect the Input Switch

Because the inputs are sourcing, connect a wire from the 0 V terminal of the power supply to the switch. Then connect the output side of the switch to terminal 1 on the input terminal block.

If the PLC and switch use different power supplies, ensure that both supplies share a common 0 V reference connection.

Step 3: Connect the Output Relay

Since the outputs are sinking, connect the relay coil to +24 VDC. Then connect the other side of the relay coil to output terminal 1 on the PLC.

The output common terminal must be connected to the 0 V terminal of the load power supply to complete the circuit.

PLC Programming Software

The Fx Series PLC uses the i-TRiLOGI programming software. Install the software according to the manufacturer’s instructions before connecting the controller.

The software provides a ladder logic programming environment used to create, edit, and download programs to the PLC.

Connecting to the PLC Using Serial Communication

This tutorial uses an RS-232 serial connection instead of Ethernet. Connect the USB-to-Serial cable between the computer and the PLC communication port.

After connecting the cable, the operating system will assign a COM port number. Verify the assigned port number using the device manager.

In the programming software:

• Open the Controller menu
• Select Connect to Server
• Choose Serial/USB Port
• Configure the correct COM port
• Click Open Port

If the connection is successful, the software will display a confirmation message indicating that communication with the PLC has been established.

Writing a Simple Ladder Logic Program

Ladder logic programming begins by creating a rung that defines the relationship between an input condition and an output action. In this example, a switch controls a relay output.

The programming steps include:

• Create a normally open contact representing the input device
• Add an output coil representing the relay
• Assign I/O addresses to both devices

The resulting ladder program performs a simple control function: when the switch is activated, the relay energizes.

Downloading the Program to the PLC

After completing the ladder logic program, transfer it to the PLC using the program download function.

The transfer procedure includes:

• Select Controller
• Choose Program Transfer to PLC
• Confirm the communication port
• Start the download process

If the transfer completes successfully, the PLC will begin executing the new program immediately.

Next Steps in PLC Programming

This example demonstrates the simplest possible ladder logic program using a serially connected PLC. In practical industrial automation systems, additional functions are typically implemented, including:

• Analog signal processing
• Timer and counter instructions
• Network communication
• Alarm and fault monitoring
• Data logging and system diagnostics

Understanding these advanced features allows engineers to build reliable and scalable automation systems for manufacturing, packaging, material handling, and process control applications.

About the Author

Daniel Chen is a senior industrial automation engineer with over 15 years of hands-on experience in PLC programming, industrial control systems, and factory automation projects. He has designed and commissioned automation solutions across manufacturing, packaging, and material handling industries.

His technical expertise includes ladder logic development, field wiring, industrial communication protocols, and troubleshooting of PLC-based control systems. He focuses on practical engineering methods that improve system reliability, simplify maintenance, and support long-term industrial operation.