Are modems, made for a more recent DSL standard, downwards compatible with older DSL technology?

I understand that older DSL modems aren't compatible with newer technology, but is the case same the other way around? I.e.:

• Is VDSL2 modem compatible with a VDSL connection? What about with ADSL or ADSL2 line?
• Is VDSL modem compatible with an ADSL/2 connection?
• Is ADSL2 modem compatible with an ADSL connection?
• Or could it be so that *2 modems are downwards compatible only with *2 connections (this is just guessing...)

Or is there other logic? Or are they all just incompatible?

VDSL is not compatible in any way with ADSL.

VDSL2 capable line cards (the ISP end) are compatible with ADSL2+ ADSL2 and ADSL - so you can use these modems on a VSDL2+ connection - Its easier to find on this juniper networks document, as well as this broadband-forum presentation - I am unclear if its possible to use a VDSL2 modem with an ADSL connection

ADSL 2+ should work with ADSL2 and ADSL in both situations

Actually there are modems that are both ADSL2+ and VDSL2+ capable, such as the Actiontec Q1000 or the Actiontec C1000A. The Actiontec Pk5000 originally was only ADSL2+ capable, but the current Actiontec PK5000 modems are now both ADSL2+ and VDSL2+ capable and Actiontec is working on an update to make their older P5000 modems also VDSL2+ capable.

ADSL2+ technology generally has speed capabilities of up to 20M downstream/896k upstream. VDSL2+ is newer technology and has speed capabilities far greater, such as 40M downstream and upstream speeds of 5M, 10M or faster, instead of the standard 896k speed. In some areas - generally major cities - there are now VDSL2+ speeds of up to 100M downstream/40M upstream available.