Siemens SINUMERIK 840D SL NCU 720.3B – Advanced CNC Control with Integrated PLC

The Siemens SINUMERIK 840D SL NCU 720.3B is a high-end numerical control unit designed for complex multi-axis machining applications. With part number 6FC5372-0AA30-0AB0, this NCU integrates a powerful CNC processor and a SIMATIC S7-317-3 PN/DP PLC on a single platform, providing coordinated motion control and machine logic in a compact package.

NCU Processing and Motion Control

The NCU 720.3B features a 1.2 GHz processor capable of controlling up to 31 axes and up to 10 channels simultaneously. It supports advanced interpolation types including linear, circular, helical, spline, and NURBS, ensuring smooth and accurate tool paths for 5‑axis milling, turning, and grinding. The NCU includes 3 MB of NC program memory and a 128 MB user memory for part programs and subroutines. The system supports up to 10,000 blocks per second processing speed, ensuring high-speed machining without loss of accuracy.

Integrated PLC 317‑3 PN/DP

The integrated SIMATIC S7-317-3 PN/DP PLC provides 1 MB of program memory and 768 KB of data memory, with bit instruction execution time of 0.025 µs. It supports ladder, FBD, and STL programming languages and is fully compatible with Siemens TIA Portal and STEP 7. The PLC handles all machine-side logic, such as coolant control, tool change, safety interlocks, and I/O management. The PLC communicates with the NCU via a high-speed backplane for synchronized data exchange.

Communication Interfaces

The NCU 720.3B is equipped with two Profinet ports (PN) and one Profibus DP port for connection to servo drives, I/O devices, and HMIs. The Profinet interfaces support IRT (Isochronous Real-Time) for deterministic communication with drives, ensuring precise coordination of multiple axes. Additionally, an Ethernet port is provided for networking with higher-level MES/ERP systems and remote diagnostics.

Mechanical and Environmental Design

Housed in a durable metal enclosure, the NCU is designed for rack mounting and operates at ambient temperatures from 0 °C to 50 °C. It includes a fan unit for forced air cooling and features a front panel with LED status indicators for power, NC/PLC boot, and fault conditions. The unit accepts a 24 V DC power supply with current consumption of 5 A.

Software and Configuration

The CNC is programmed using the SINUMERIK Operate interface, which provides intuitive operation and programming capabilities including ShopMill and ShopTurn for conversational programming. The system supports high-level language (HLL) and DIN/ISO code. The integrated PLC is configured via TIA Portal, with seamless parameter sharing between NC and PLC for coordinated action.

Typical Applications

This NCU is ideal for advanced machine tools such as machining centers, lathes, grinding machines, and laser cutting systems. Its flexibility and high performance make it suitable for aerospace, automotive, and medical device manufacturing.

Diagnostics and Maintenance

The NCU includes comprehensive diagnostic functions, including system traces, error logs, and remote service capabilities. The PLC and NC programs can be backed up to an external memory card. Firmware updates are performed via the integrated USB port or network.

In summary, the SINUMERIK 840D SL NCU 720.3B with PLC 317-3 PN/DP delivers exceptional performance, integration, and reliability for the most demanding CNC applications.