Tektronix TDSUSBF Instructions Manual page 12

TDSUSBF Test Fixture
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6. Set the DUT (Device Under Test) to the proper test state.
For example, in a TDR upstream test, the DUT must be configured to
SE0_NAK test mode. Otherwise, DUT transmissions can damage test
equipment connected to the fixture.
For all TDR tests, you should verify with a DMM that the DC bias level is
less than 10 mV from D+ to ground and from D- - to ground.
7. Place the switch S6 in the TEST position. Verify that the TEST status LED
is on.
8. Perform the test steps as required for your specific test.
9. After finished testing, place the switch S6 in the INIT position and discon-
nect the power supply cable from the DC Power In jack or disconnect the
USB cable providing power to the test fixture.
TDSUSBF USB 2.0 Compliance Test Fixture Instructions