The digital optical link uses the same components as the analog link for the transmitter section located in the radiative environment of the experiment. Since two-way communication is required on the control part, pin diodes and a radiation tolerant receiver ASIC [56] will be included on the digital optohybrids as well.
On the control room side however, commercially available optical transceivers will be used. Suitable components have been selected and tested at HEPHY and a prototype was built and presented in early 2001 [57]. Finally, these parts will be included in the FEC.
The best digital signal transmission on fiber optics is obtained with DC balanced data containing the same amount of low and high level bits on average, as it is implicitly the case with the clock. For the data line, this is achieved with a special four to five bit encoding and the NRZI (Non Return to Zero with Invert 1 on change) scheme. Basically, this scheme uses a line transition to represent a ``1'' and no transition to represent a ``0''. The idle symbol, which is sent when no other data are pending, is ``11111'', resulting in a square wave output of half the clock frequency.