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VXI Technology, Inc.
SM8000 Series Operation 35
Starting the Device
The device resets upon application of power. The Optical Attenuators park at the minimum-loss
position.
Control Modes
The Optical Attenuators can be operated in two modes: uncalibrated and calibrated. The
uncalibrated mode is called Move-To-Absolute-Step mode. In this mode, the user sends
movement requests to the internal stepping motor through the Move-To-Absolute-Step interface.
The internal stepping motor responds by moving one step up, or one step down as requested. In
this mode, there is no conversion of step number to absolute attenuation.
Move-To-Absolute-Step is the simplest mode of operation. This method of operation is typically
used when devices are used in a feedback loop to maintain a particular attenuation regardless of
absolute position.
The calibrated, absolute conversion mode of operation sends absolute attenuation requests to the
Optical Attenuator. The circuitry then translates the commanded absolute request into a motor-
step position and rotates the motor accordingly. This method of operation is typically used when
the devices are used to calibrate other devices, or to set absolute references within a system.
Both modes of operation are described in detail in the following sections.
Uncalibrated Operation - Move-To-Absolute-Step
The motorized Optical Attenuators are all based on stepping motor technology. The easiest
method of using these devices is to simply command the motor to step in one direction, or the
other. For our purposes, stepping will increase attenuation, while stepping down will decrease
attenuation.
To utilize this mode of operation, simply command the Optical Attenuator to Move-To-Absolute-
Step. See the Attenuator Command Set in the Programming section.
Calibrated Operation
The Optical Attenuators are all based on stepping motor technology. Operating in the calibrated
mode requires use of the I
2
C interface on the optical modules.
The I
2
C interface is a linearized controller, allowing users to select for the attenuator, he absolute
attenuation in dB.
It is also possible to command uncalibrated step movements while operating in calibrated mode.
Note that following an uncalibrated step, a Query Attenuation command will return invalid data.
A subsequent calibrated movement will restore query command validity.