Mitsubishi AX71 16-Point DC Input Module for Melsec A Series

The Mitsubishi AX71 is a digital input module designed for the Melsec A series programmable logic controllers (PLC). It provides 16 input points for 24V DC signals, utilizing positive logic (sink-type input). This module is widely used for connecting push buttons, limit switches, proximity sensors, and other discrete field devices in industrial control systems.

Input Specifications

Each input channel has a rated voltage of 24V DC (range 20.4V to 28.8V). Input current is 7 mA typical at 24V. OFF voltage is 5V max, OFF current 1.5 mA. The module accepts signals from both 2-wire and 3-wire sensors. A built-in surge absorber protects against voltage spikes.

Response Time

The hardware response time is adjustable via the PLC’s digital filter setting. Without filtering, OFF-to-ON and ON-to-OFF delays are 1.5 ms typical. A user-programmable filter (1 ms to 50 ms) can be applied to debounce noisy inputs. This makes the AX71 suitable for high-speed applications such as counting and position sensing.

Isolation and Protection

All input channels are grouped into one common (COM) terminal per 16 points. Each group is optically isolated from the PLC’s internal logic (1,500V AC, 1 minute). Reverse polarity protection is provided on the input circuit, preventing damage from miswiring. An LED indicator next to each terminal shows the ON/OFF status of each point.

Integration with Melsec A

The AX71 module mounts directly on the Melsec A series base rack (A32B, A35B, etc.) and connects via the backplane bus. It is assigned an I/O address based on the slot position. No external power is required for the logic side; the module draws 5V DC (70 mA) from the backplane. The field-side 24V supply must be provided externally (typical 0.2 A at 24V).

Mechanical Specifications

Dimensions: 88 x 140 x 50 mm (W x H x D). Weight 0.2 kg. The module uses a 24-pin spring clamp terminal block or a screw terminal adapter (sold separately). Operating temperature: 0°C to 55°C; humidity: 10% to 90% non-condensing. It is rated IP20 and must be installed in a protective enclosure.

Diagnostic Indicators

A front-panel “PWR” LED lights when the backplane power is present. Individual “IN” LEDs for each of the 16 points show the input state. There is no onboard diagnostic for field wiring; however, the PLC can detect missing external 24V supply via input monitoring.

Applications

Commonly used in legacy Melsec A-based control systems for packaging machines, conveyor controls, plastic injection molding, and industrial ovens. It is also a drop-in replacement for older AX modules.

Conclusion

The Mitsubishi AX71 input module offers 16 reliable 24V DC inputs for Melsec A series PLCs. Its fast response, optical isolation, and simple addressing make it a standard choice for discrete input expansion.