The green slow and tidal servos have been engaged. This is done through the arm guardian which in turn select a corresponding state from the ALS auto-locker. These states are "End Locked", "Slow Engaged", "Transition" and "Red Locked". With "End Locked" neither the slow nor tidal servo are engaged. With "Slow Engaged" they are both on, tidal feedback only in "Transition" and tidal and common mode feedback in "Red Locked". These three new feedback paths use the new integrator filter module. When the green PDH is not locked, the integrators are bled off.

The feedback topology is as follows:

• The PDH slow signal is fed into an ARM_ERR standard filter module followed by the ARM_CTRL integrator. This signal is added to the output of the common path, send through the ARM_DRIVE filter and then to the suspension controller, where it is added to the L1 stage.
• If the slow path is engaged and if "Frequency Servo" is enabled in the PDH auto-locker, the VCO servo is engaged. This should keep the VCO frequency near nominal.
• The control signal of the L1 suspension stage constitutes the input of the TIDAL_ERR filter. Its output is routed through the TIDAL_CTRL integrator to the HEPI length input.
• The common path consists of a COMM_ERR filter and a COMM_CTRL integrator which is added to the arm drive. Currently, the input to the common path is just an EPICS value, but will be replaced with the IMC-MC_F signal later.

The "Red Locked" state triggeres on the transmitted red power. Once in this state, it no longer matters whether green is locked. It can only transition back, when the red lock is lost.