Only say this as it seems I can use a 497 on the ETX70, even though the ETX70 is not in the 497 handset. The place to go search is the now old Weasner site for imformation.
However from what you have said it sonds as if the handset recognises the scope and if so then that prevents that idea.
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So even with the 506 cable kit, you will not be able to run the ETX90 from a computer.
-drl The 494 does not have the ETX 90 in it, so cannot be used, and as best I know cannot be updated.