Dear Prof. Whitney,

Questionnaire to IVS-TDC coordinator,

After the intensive discussion in TDC-CRL Japan this week. 
We all know that your first VSI idea is basis to study furthur.
But it is a little bit confusing since the idea is abstract 
actual existing cables problem.
Here I exclude discussion with optical transmission in future. 
I just ask you the following question based on my action items related.

1. Will we make standard cables for the VSI or not?
   VSI will define a interface. Between the interface, we need cables in any case.
   In this frequency, the cable which maintain digital signal shape is the key 
   component for inter compatibility.
   Neither connector or pin assignment, the cable is important.  
2. Will the VSI define the cable length ?
   Without consideration, The maximum length for 64MHz clock and data will 
   keep it shapes less than 10m(33ft). 
   From my experience, we will see many troubles around 10m.
   This is not long enough, at large correlator site or 
   in sampler mounted inside telescope.    
3. Should we exclude 128MHz CLK in this discussion? 
   Since the ECL100K which handle the high frequency 
   is different technology and expensive. After my study, I know this will not be 
   unified currently used ECL10K,10KH. The clock step in the cable should be 
   maintained under 64MHz. 
4. Is the VSI define a future system compatibility or current system modification ? 
   For example, will the Mk-III or S-2 system will have a VSI interface ?
   Do we have a target correlator or target experiment to perform compatibility
   in near future ?

Junichi Nakajima, CRL-TDC VSI