We decided to take on the deceptively easy-sounding task of checking that the LSC whitening switching was happening as anticipated. We hoped to discover that when we clicked the "unwhitening" switches in FM1 of the LSC PDs, we would see the analog whitening turn on and off for the matching channel. That is what is supposed to happen.
Tragically, it is instead one big giant crazy disaster of a mess.
What we did:
Made a 24tapus (octopus like last time, except more...), with a 50kOhm resistor as our white noise source (instead of using a DAC channel and AWG).
We plugged our 24tapus into the 3 of 4 whitening boards on the LSC rack that are currently in use. One of the boards just has 8 terminators on the input, so we left that one alone for now.
We put the whitening gains to 0dB so that all the channels looked the same.
We looked at the PD _IN1 channels in DTT, and monitored which signals had whitening switching when we clicked the "unwhitening" buttons on the PD filter banks.
So far, we can find no rhyme or reason as to why some of the channels work (click unwhite on that PD, see that signal have whitening switching), and others don't. Some channels we just can't get to switch no matter what, others are just mis-mapped. There is no discernible pattern.
What we think (so far) is going on:
All of the cables from the PD demod boards are going to the Whitening board inputs, exactly as in Suresh's Diagram. The only difference is that Refl33, AS165 and Refl165 demod boards don't exist in the rack at this time.
The Whitening and AA boards in Suresh's Diagram labeled 0-7 are connected to Binary Output channels 0-7. This is a good thing.
The Whitening and AA boards in the diagram labeled 8-15 are connected to Binary Output channels 24-31. This is not so awesome.
This is all we are confident about at this time.
We are hoping that Ben has a secret stash (or can tell us who would) of LSC rack wiring diagrams. We would like to find out, without the pain of tracing wires and cables by hand, how the Binary I/O information gets through the cross-connect on the LSC rack up to the whitening boards.
We are leaving the 24tapus in place for now, so that we can carry on tomorrow, either with a wiring diagram in hand, or carefully tracing cables.