HighSecLabs SC21H-3 User Manual page 12

Products support video resolutions of up to 4K-2K Ultra HD (3840 X
2160 pixels).
User Audio Devices
Product is compatible with the following types of user audio
devices:

Stereo headphones;

Amplified stereo speakers.
Note: In any case do not connect a microphone to product audio
output port including headsets.
fUSB Port (applicable models)
The product operates with authorized USB devices plugged into the
console fUSB Port, such as USB smart-card reader or Common Access
Card (CAC) reader.
By default, authentication devices such as smart card readers and
CACs are authorized for use.
For authorizing additional USB devices to work with product,
Configurable Device Filtering (CDF) mechanism is used with
configuration permissions limited to authenticated administrators.
For more details please refer to the "fUSB Configuration Manual".
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Section 4 - Operation
2/4 Ports Secure KVM Combiner User Manual