The RxThresholdTest scans and then sets the threshold of the opto receiver in the BOC. It's done in clock/2 mode, so the output should be 50% 1's and 50% 0's
As the threshold increases from 0 towards 255 one typically sees the transition from all 1's (black) to 50/50 (tartan!). Occasionally the upper transition to all 0's (white) is also seen at very high threshold.
The modules are configured to return clock/2. This produces the tartan pattern when summed on subsequent captures where the capture could be either 0101 or 1010.
Example at: http://www-pnp.physics.ox.ac.uk/~daquser/ExamplePlots/RxThresholdExample.eps
See also RxDelayTest, BocSetupTests