1) Why are there two packages namely PADS Logic and DX Designer within the same suite when they appear to do much of the same thing ? What is the difference ?. Is the reason historical ?
Yes, the reason is historical. They arrived in PADS via different pathways. PADS Logic came from the same camp as PADS Layout, so is closer operationally/GUI, and you can share scripts & 2D lines between Logic and Layout. DxDesigner came from ViewDraw.
2) Can schematics from within PADS Logic be directly opened in DX Desiger ?.
3) Am I correct in assuming you can only build simulateable circuits from within DX Designer ?.
Here, it gets a little murkier. Spice uses Attributes and model mapping, to manage the important details for SImulation
Those will translate across from Logic, but the missing link, is a Logic-Version, of the Spice-Aware DxDesigner Libraries.
We have asked Mentor to provide a DxD to Logic translation pathway (at least for libraries, to mirror the existing Logic to DxD), and doing that would allow Logic users to create SCHs in Logic (which they are very used to), and then use DxD purely as the Spice Shell. Not sure on the status of this.