PLC Sequence Control Course
PLC Sequence Control Course 
Elementary Level PLC Wiring Methods
This page explains PLC wiring methods. There are three types of wiring: power supply input, signal input from sensors (X terminals), and output to lamps or actuators (Y terminals).
Key Point: Using the Mitsubishi FX Series PLC as an example, we explain the three types of wiring — power, input, and output — in order. This is fundamental knowledge for PLC wiring.
There are many types of PLCs available. This course primarily uses Mitsubishi PLCs (sequencers). Mitsubishi PLCs are often referred to as "sequencers," and this site uses that term as well. Since there are many models even within Mitsubishi's lineup, we will use the F Series for this elementary-level section.
Created: January 10, 2018
Updated: May 13, 2026
- Video Tutorial
- What is the F-Series PLC?
- Power Supply Wiring (L/N Terminals)
- Sensor Input Wiring (X Terminals)
- Output Wiring (Y Terminals)
- Frequently Asked Questions (FAQ)
What is the F-Series PLC?
Simply put, the F Series is Mitsubishi Electric's entry-class (economical) sequencer line. However, today's entry-level sequencers offer more than enough performance. Beginners will find them fully capable. The photo below shows the FX1N sequencer. The FX1N Series (excluding FX1NC) comes standard with a terminal block and the minimum required I/O, making it suitable for simple equipment. In contrast, higher-end models (such as the Q Series) require you to individually select I/O units and build your own PLC configuration with the functions you need.
On the left side it reads "MELSEC." Below that is a lid-like cover, behind which is the port (RS-422 communication) for connecting to a PC. This round connector is used to connect to a computer and write programs to the PLC.
There is a switch on the side of the opened cover. Flipping it up puts it into "RUN" mode; flipping it down sets it to "STOP". Programs execute in the RUN state. In the STOP state, the program does not run. In other words, to execute a written program, you must set the switch to "RUN."
Power Supply Wiring (L/N Terminals)
Connect the power supply to the "L" and "N" terminal blocks. The standard specification is AC100V, but DC-powered models also exist depending on the specification, so be careful. Supplying 100V to the L and N terminals will power the unit. When connecting to a wall outlet, the wires go here. Note: when adding an emergency stop function, insert the b-contact (NC contact) of the emergency stop switch in series in the power line. Depending on safety standards, it is more reliable to cut the power to the CPU itself on emergency stop, rather than just sending a signal to the PLC.
Caution: The DC 24V output terminal on the sequencer has limited capacity — use it exclusively for sensor power. Driving loads larger than its capacity may prevent the unit from starting up.
Wiring is easy if done as shown in the diagram below. Although the sequencer can output DC 24V, its capacity is limited, so when driving larger loads, install a separate DC power supply (power supply unit).
The DC power supply refers to the power supply unit. Wire two wires to each power terminal on the sequencer — one side to 100V, the other to the DC power supply terminal. Connecting the same voltage in this shared-wiring manner is called "looping". Next is the DC section wiring.
When using the DC power supply as a sensor power source, connect the negative terminal of the DC power supply to the COM terminal of the sequencer. Unless the ground side is shared, input signals from sensor-driven outputs will not reach the sequencer. Connect them unless there is a specific reason not to.
Conversely, do not connect the 24+ terminal of the sequencer to the positive terminal of the DC power supply. Even though both are 24V, they are not exactly the same voltage, which will shorten the lifespan of the power supplies.
It is fine to connect the ground side of DC power supplies with different voltages.
To branch wiring in the way described above, having a separate terminal block is convenient. These can be easily purchased from RS Components.
Sensor Input Wiring (X Terminals)
Now let's input signals into the PLC. Signal input is straightforward. Connect as shown in the diagram.
"X" is the input address (symbol) of the sequencer. Terminal blocks labeled "X0" and onwards are lined up in order. Signal wires are connected here, which means the number of inputs is limited to the number of terminal blocks. When building actual equipment, you must carefully check the number of I/Os (inputs and outputs). In relay control, a sensor operated a relay; in a sequencer, the sensor operates this "X" contact. Short-circuiting (connecting) the COM terminal to any "X" terminal activates the input. Shorting X0 and COM will light up the "X0" indicator on the sequencer face, confirming signal input.
So how do you connect sensors that require power, such as through-beam or reflective sensors? Connect them as shown in the diagram below.
It looks complex, but it's simple. Connect the brown wire to DC 24V (+), the blue wire to the COM terminal, and the black wire to the "X" input terminal. The only addition is the 24V wiring. Be careful about polarity. Note: for 2-wire cylinder sensors, only brown and blue wires are present. In this case, connect brown to the "X" input terminal and blue to COM. Rather than relying solely on wire color, always identify the sensor type before wiring.
Wire color correspondence (for older equipment):
· Red → Brown (DC+)
· Black → Blue (DC−)
· White → Black (signal wire)
Output Wiring (Y Terminals)
Next is the output. Connecting inputs alone will run the program inside the sequencer, but the equipment will not actually operate. Connect as shown in the diagram below.
"Y" is the output address (symbol). Note that "COM0" here is different from the COM described in the input section. To light up a lamp, connect the power positive (+) to the lamp's positive side and connect COM0 to the power negative (−). Then connect "Y0" to the lamp's negative side. When the sequencer outputs Y0, the lamp will turn on.
The internal structure that causes this is shown in the diagram below.
The output terminal of the sequencer simply contains a relay contact. The type described here is the relay output type. When "Y0" is output, the relay contact for "Y0" activates, and the load is switched ON/OFF via that contact. For this reason, the output is sometimes referred to as a coil within the program.
Current flows when the contact closes, causing the lamp to light up. Relay outputs support both AC and DC loads, but attaching a load that exceeds the contact capacity will damage the contact. Note: transistor output types have polarity — connecting with the wrong polarity will prevent operation. Since transistor outputs are now the standard in modern PLCs, always confirm this at the time of purchase.
In the FX Series, as shown in the illustration, outputs are independent per COM group. This means you can mix AC and DC loads in the same wiring.
"Y2" to "Y5" are DC loads and "COM1" and "COM3" are connected (looped). "Y6" to "Y10" are AC loads. This is just an example — you are not required to wire exactly this way.
This concludes the basic wiring explanation. In the FX Series, I/O addresses start from 0 and increment in octal (the digit after "X7" is "X10"). In the Q Series, X0 to XF consists of 16 points. Note that addresses change depending on the unit number.
For purchasing connectors, RS Components is a convenient option.
Next Page: How to Read Ladder Diagrams
Frequently Asked Questions (FAQ)
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