The construction project description for the test board systems contains a first attempt deliniating the project.
The test board subsystems will each communicate with even numbers of DOMs because each cable is a quad (equivalent to two twisted pairs). The available address space for the test baords allow systems of up to eight per PC-104 CPU. Therefore, 38 functioning, communicating DOMs require ten subsystems of four test boards each, or seven subsystems of six channels each, or five subsystems of eight boards each.
Multiple subsystems could, of course, be packaged in the same chassis box for mounting in a rack at the south pole.
The subsystems will all share a common reference clock frequency delivered to them from a clock distribution system. The phase of the clock outputs will be tailored to match the phase of a house reference clock (typically a particular GPS clock 10 MHz output).
At 5 ns per meter, significant clock phase shifts can be introduced between various subsystems in the IceCube electronics house. Clock phase fed to each DOM Hub will have to be carefully controlled. The problem is not trivial, but it is by no means daunting.
The manufacture and installation of hardware at the south pole constitutes a significant part of the string 18 acquisition system, but a great deal of work is left to do. At this time the unfinished firmware in the test board, and unfinished firmware in the DOM comprise several man months of effort to achieve the level of functionality envisioned during development in October 1998, and druing 1999. The level of functionality represents a fairly high hurdle.
The exploitation of hardware coincidence circuitry in the DOMs in the
ice will allow physics to be done with string 18, as Azriel Goldschmidt
demonstrated recently with the 'demonstration DAQ' (four DOM subset), in
less time than would be needed for full functionality to be developed.