Mitsubishi Melsec PM-120M Compact PLC for Small Automation Systems

The Mitsubishi Melsec PM-120M is a compact programmable logic controller designed for small to medium industrial control tasks. It features 24 integrated I/O points (14 inputs, 10 outputs) and can be expanded with external modules. The PM-120M is part of the Melsec PM series, known for its ease of programming and high reliability. Typical applications include packaging machines, conveyors, and simple process control.

CPU and Memory

CPU type: 8-bit microprocessor with dedicated PLC logic. Program capacity: 1200 steps (EEPROM). Data memory: 128 words (DM). Execution speed: 5 µs per basic instruction (LD/OUT). The PLC uses relay ladder logic programming with a maximum of 120 timers and 120 counters. The CPU includes a real-time clock (battery backup) and a watchdog timer. Program retention: EEPROM stores program and parameters without battery. The unit has battery backup for the clock only; battery life is approximately 5 years.

Built-in I/O

Digital inputs: 14 points (24V DC, sink/source selectable). Input current: 7mA typical at 24V. Input ON voltage: >15V, OFF voltage:<5V. Input response time adjustable from 0.5ms to 10ms. Filter protection against noise. Digital outputs: 10 points (transistor sinking, 24V DC, 0.5A per point). Outputs are short-circuit protected and have built-in freewheeling diodes for inductive loads. LED indicators for each I/O point on the front panel. The unit can directly drive relays, solenoids, and indicator lights.

Expansion Capabilities

The PM-120M supports expansion via the MELSEC PM extension bus. Up to two expansion units can be connected, adding a maximum of 24 additional I/O points (e.g., PM-8EX (8 inputs), PM-8EYR (8 relay outputs)). The expansion units are powered from the main unit’s 5V bus, so no separate power supply is needed for small expansions. The unit also supports analog I/O expansion modules (PM-AD (analog input), PM-DA (analog output)). The total I/O count after expansion can reach 48 points.

Communication Ports

The PM-120M includes one RS-422 programming port for connection to a PC with MELSEC programming software (GX Developer or GX Works2 via serial adapter). An optional RS-232C adapter (PM-232) can be installed for connection to an HMI or third-party device. The protocol is proprietary MELSEC over serial, supporting baud rates up to 115200. The unit can also be configured as a Modbus RTU slave with a special driver. No Ethernet port is available on the base model.

Programming Software

Programming is performed using Mitsubishi GX Developer (version 8 or later) or GX Works2. Languages supported: Ladder diagram (LD) only. The software includes online editing, monitoring, and forced I/O. The program can be uploaded/downloaded via the RS-422 port using a USB-to-RS422 converter (Mitsubishi SC-09 cable). A simple hand-held programmer (FX-20P) is also compatible. The PM-120M uses a subset of the standard MELSEC instruction set: LD, OUT, AND, OR, ANB, ORB, TIM, CNT, PLS, etc.

Timers and Counters

Timers: 120 on-board timers (T0 to T119). Timer types: ON-delay, OFF-delay, retentive. Time range: 0.01s to 999s depending on the timer resolution (10ms or 100ms). Counters: 120 16-bit up-counters (C0 to C119). Count range: 0-9999. High-speed counters: 2 channels (single-phase 5kHz, or differential phase 2.5kHz) are available via special inputs X0 and X1 with dedicated counter instructions. These high-speed counters can be used for encoder feedback.

Physical and Power

Power supply: 100-240V AC (built-in switching power supply). Consumption: 30VA max. Dimensions: 130mm (W) x 90mm (H) x 75mm (D). Weight: 0.7 kg. Mounting: 35mm DIN rail or screw panel. Protection: IP20. Operating temperature: 0-55°C. Storage: -20-75°C. Humidity: 10-95% non-condensing. The unit includes a removable terminal block for easy wiring replacement. All I/O terminals are labeled and accept 0.2-2.5mm² wire.

Built-in Functions

The PM-120M includes several useful functions: 1) Data registers (D0-D127) for storing analog values or counts. 2) Comparator instructions for conditional branching. 3) Math instructions (ADD, SUB, MUL, DIV). 4) Shift registers and drum sequencer functions. 5) Interrupt inputs (X0-X3) with maskable interrupts. 6) Pulse output (one channel, open collector, 1 kHz max) for simple stepper motor positioning without an external pulse generator.

Diagnostic Features

The PLC performs a self-test at power-up, checking memory and I/O bus. Error LED indicates CPU error, program error, or watchdog timeout. Error codes can be read via the programming software. A built-in real-time clock maintains event logs. The unit also has a forced input function for debugging. Battery low detection: an LED and diagnostic bit indicate when the clock battery needs replacement.

Applications

The PM-120M is suitable for small machines: automatic door control, conveyor belt logic, drill presses, packaging sealers, small pumping stations, and lighting control panels. It is also used in educational environments for PLC training due to its simple instruction set and low cost. The PLC is a legacy product but remains in service in many installations; Mitsubishi recommends FX series for new designs.

Conclusion

The Mitsubishi Melsec PM-120M provides a compact, cost-effective solution for small-scale automation. With 24 built-in I/O and expansion options, it handles a wide range of relay-replacement and simple control tasks efficiently.