Mitsubishi MELSEC A Series PLC System – Modular Control with Analog and Temperature I/O

This MELSEC A series PLC system comprises five core modules: the A63P power supply, the A3ACPU CPU unit, three A616AD analog input modules, and three A68RD3 temperature input modules. Together, they form a versatile and expandable control platform for complex process and discrete automation tasks, typical of Mitsubishi Electric's legacy A series.

A63P Power Supply Module

The A63P is a dedicated power supply unit that converts 100‑240 V AC to a stabilized 5 V DC and 24 V DC output for the PLC rack. It provides 6 A of 5 V current and 0.5 A of 24 V auxiliary power, sufficient to drive multiple I/O modules. The unit features overvoltage and short-circuit protection, with LED indicators for power and error status. Its compact design fits into the A series base rack.

A3ACPU CPU Module

The A3ACPU is the central processing unit of the system, featuring a 32-bit microprocessor with a program memory capacity of 30K steps. It executes basic logic instructions in 0.2 microseconds and supports advanced functions such as floating-point arithmetic, PID control, and indirect addressing. The CPU includes a built-in RS-422 programming port and an optional memory cassette slot for data storage and program backup. The A3ACPU is compatible with MELSEC A series I/O modules and special function units.

A616AD Analog Input Modules

Three A616AD modules provide a total of 48 analog input channels (16 per module). Each channel is configurable for 4‑20 mA, 0‑20 mA, or 0‑5 V input signals. The modules feature 12‑bit resolution and 0.5% accuracy, suitable for monitoring process variables such as pressure, level, and flow. Each channel is isolated from the system bus, reducing noise interference. The conversion speed is 5 ms per channel, enabling fast response in dynamic processes.

A68RD3 Temperature Input Modules

Three A68RD3 modules are dedicated to temperature measurement using resistance temperature detectors (RTDs) or thermocouples. Each module supports 8 input channels, providing a total of 24 temperature channels. The input types include Pt100, JPt100, and thermocouple types K, J, T, and E. The modules offer 16‑bit resolution and automatic cold junction compensation for thermocouples. Linearization is performed by the module's internal processor, outputting temperature values in degrees Celsius or Fahrenheit.

System Integration and Communication

All modules are mounted on a MELSEC A series base rack with backplane bus for high-speed data transfer between the CPU and I/O. The system can be expanded with additional racks and remote I/O stations via the A series network. A programming terminal (e.g., GX Developer) is used to configure the system, including parameter settings for each analog/temperature module, scaling, and alarm thresholds.

Applications

This PLC system is ideal for process control in chemical plants, food processing, HVAC, and power generation, where multiple analog and temperature signals require centralized monitoring and control. The modular design allows easy replacement and maintenance of individual modules.

Environmental and Compliance

The modules operate within 0‑55 °C and are designed to meet CE and UL standards. They are housed in sturdy enclosures with front-accessible terminals for wiring convenience.

In essence, the Mitsubishi MELSEC A series system with A63P, A3ACPU, A616AD, and A68RD3 provides a comprehensive, reliable, and scalable solution for analog and temperature-intensive automation applications.