Setup Button

The setup button is located next to the Ethernet port of the DS203.

The Button is used to select an operating mode of the DS203:

When the DS203 is powered up with the button pressed it enters a serial upgrade mode in which new application firmware file can be uploaded into the  DS203 through its serial port. If the DS203 is powered up with the setup button not pressed the Device proceeds to running its current application firmware. This functionality is delivered by the Monitor firmware component of the DS203.
When the application firmware is already running the setup button is used to make the DS203 enter the serial programming mode (hence, the name of a button)*.

* Strictly speaking, this is a functionality that is defined by the application firmware, not the DS203 hardware.