How to Troubleshoot the "bb" (Baseblock) Error in Yaskawa VFDs

Seeing the "bb" code on your Yaskawa drive? Learn what Baseblock means, common causes like external interlocks or safety circuits, and how to clear this status to get your motor running.

 

If you are working with a Yaskawa V1000, A1000, or GA700 drive and the display suddenly shows a flashing "bb", you might think the drive has failed. However, "bb" is unique because it is often a status indicator rather than a permanent hardware fault.

In the world of Yaskawa, "bb" stands for Baseblock. When this appears, the drive has cut off the power to the motor by "blocking" the firing of the internal transistors (IGBTs). The motor will coast to a stop, and the drive will refuse to start until the "bb" condition is cleared. Here is how to find out what is triggering it.

What is a Baseblock (bb) Signal?

Think of the Baseblock as a "Software E-Stop." It is an instruction given to the drive to immediately stop sending current to the motor without necessarily triggering a "Fault" that requires a manual reset. It is commonly used for safety interlocks, sequencing, or protecting the drive during specific machine states.

Common Causes of the "bb" Status

1. External Baseblock via Digital Input

The most common cause is a physical wire connected to one of the drive's digital inputs (S1 through S8). If an input is programmed as an "External Baseblock" (H1-xx = 8 or 9), and that circuit opens, the drive will show "bb."

• Normally Open (Value 8): The drive blocks when the contact closes.
• Normally Closed (Value 9): The drive blocks when the contact opens (this is the most common for safety).

2. Safe Torque Off (STO) Activation

On modern Yaskawa drives, the Safe Torque Off inputs (H1 and H2 terminals) are designed to provide a hardware-level safety stop. If the safety relay or E-stop button connected to these terminals is tripped, the drive will enter a Baseblock state to ensure the motor cannot generate torque.

3. Direct-On-Line (DOL) Transition

In some complex panel designs, a contactor is placed between the drive and the motor. If that contactor opens while the drive is running, an auxiliary contact often sends a "bb" signal to the drive to prevent it from trying to push power into an open circuit, which could damage the IGBTs.

4. Controller Interlocks (PLC)

If your drive is controlled by a PLC, the programmer may have logic that triggers a Baseblock during a specific process step (like a tool change or a hopper jam) to ensure the motor cannot move even if a "Run" command is present.

Step-by-Step Troubleshooting Guide

Step 1: Check the Digital Inputs

Navigate to the Monitor Menu (U1-10) to see the status of the digital inputs. Look for any input programmed to function 8 or 9. If that input is active, you have found your source. Trace the wire back to the physical switch or relay to see why it is open/closed.

Step 2: Inspect the Safety Circuit (STO)

Check the H1 and H2 terminals on the control terminal block. There should be a jumper between HC, H1, and H2 if you aren't using an external safety relay. If the jumpers are loose or the safety relay is faulted, the "bb" signal will persist.

Step 3: Verify Command Sequencing

Does the "bb" appear only when you try to start the motor? If so, check if your PLC is sending the "Run" command and the "Baseblock" command simultaneously. The Baseblock will always take priority and prevent the motor from turning.

How to Clear the "bb" Display

Unlike a "Fault" (like OC or OV), you usually do not need to press the [RESET] button to clear a Baseblock. To clear it:

1. Restore the signal: Close the safety gate, pull out the E-stop, or clear the PLC interlock.
2. Remove the Run command: Most Yaskawa drives require you to cycle the "Run" command (turn it OFF and then back ON) after the Baseblock signal is restored before the motor will start again.

Summary Checklist

Checkpoint	What to Look For
Terminals S1-S8	Check for wires labeled "Baseblock" or "Interlock."
Terminals H1/H2	Ensure safety inputs have 24V DC or are properly jumpered.
Parameter H1-xx	Identify which input is set to "8" or "9."
External Relays	Check if a cabinet relay has failed or a fuse is blown in the 24V circuit.