6FC5357-0BB35-0AA0: Setup, Configuration and Replacement

What the 6FC5357-0BB35-0AA0 is

The 6FC5357-0BB35-0AA0 is a numeric control unit (NCU) for the SINUMERIK 840D sl, commonly listed as the NCU 730.3 PN. The NCU runs the CNC kernel (NCK), the integrated SIMATIC PLC and the SINAMICS drive control for the axes it manages, all in one booksize module that mounts in line with the SINAMICS S120 drive modules.

In practice, it is the brain of the machine. Part programs, axis motion, PLC logic and drive communication all pass through it. That is why an NCU fault rarely looks like a single small alarm: the whole machine tends to stop.

What lives on the unit, and what doesn't

The system software, your machine configuration and the software license sit on the CompactFlash card plugged into the NCU, not in the NCU hardware itself. This matters for replacement: if the card is healthy, moving it to the new unit brings the software and license with it.

How to read the part number

Read the full number from the rating plate on the side of the unit, not from drawings or old purchase records. Machines are often upgraded in the field, and the installed unit may not match the original documentation.

Common faults that point to the NCU

Many alarms that stop a SINUMERIK machine are not NCU failures. Rule out the cheaper causes first: 24 V supply, DRIVE-CLiQ cables, and the CompactFlash card.

A low battery is the cheapest fault on this list and one of the costliest to ignore. If buffered data is lost while the machine is powered off, you may have to restore from an archive. See our replacement controller batteries and how to replace a FANUC CNC battery without losing parameters for the habits that prevent it.

How to replace the NCU

This is the general order of work. Follow the Siemens SINUMERIK 840D sl manuals for your software version and your machine builder's instructions, and have a qualified technician do the work.

1. Back up before anything else. If the control still runs, create a series commissioning archive in SINUMERIK Operate covering NC, PLC, drive and HMI data, and save it off the machine. If it won't boot, find the most recent archive you have.
2. Photograph and label every connection. DRIVE-CLiQ cables especially. A cable moved to a different port changes the drive topology.
3. Power down and lock out. Wait the full DC link discharge time shown on the drive module warning label before touching the line-up.
4. Move the CompactFlash card. Remove the card from the old NCU and insert it in the replacement. If the card itself has failed, you will need a new card loaded with your archive, and the license will need to be transferred through Siemens.
5. Check versions. A newer hardware version can require a minimum CNC software version. Confirm the replacement unit is supported by the software on your card before power-up.
6. Mount and reconnect. Fit the unit, reconnect DRIVE-CLiQ to the same ports, then Ethernet, PROFIBUS/PROFINET and 24 V.
7. Power up and watch the boot. Follow the status display and LEDs through startup. Acknowledge any topology messages only after you've confirmed the cabling is correct.
8. Prove it out. Reference all axes, run a test program at reduced feed and check that PLC I/O responds before returning the machine to production.

New vs. IAC Certified Refurbished vs. repair

A common approach for critical machines: install a refurbished unit now to get running, then repair the failed unit and keep it as a shelf spare.

Related parts to check while you're in the cabinet

• The fan/battery module on the NCU, since a replacement is a good time to fit a fresh battery
• A spare CompactFlash card with a current archive on it
• DRIVE-CLiQ cables that have been bent or pinched during the swap
• SINAMICS and SIMODRIVE modules in the same line-up. See our guide to 6SN1145-1BA01-0DA1 faults and replacement.