"Old Stuff" like transducers and the like CAN have their signals converted to NMEA via a few nitch products from Tinley and Actisense; however, unless you are absolutely assured from the manufacturers above that your 'old stuff' is compatible, you assume the risk of it not working.
I would always recommend being comfortable with your purchases, if you are not sure the DST-2 will work with your old transducer then absolutely go with Pub911's advice and buy a new Active Transducer instead.
You will find a compatibility list in the manual for the DST-2, if your transducer is not in the list then care should be taken. If the transducer matches the criteria in the quote below I can see no problem using our DST-2 but as PUB911 said "you assume the risk" not us.
I would encourage anyone who has had success with a transducer not in the list to let us know so we can keep our compatibility list up to date and so less people have to decide to take the risk or shell out for a new transducer.
Depth connects to a standard piezoelectric depth sounder crystal element using a 3-wire interface.
Speed connects to a standard 'paddle-wheel' hall-effect transducer using a 3-wire interface.
Temp connects to a standard thermistor transducer using a 2-wire interface.
Adding to this, Depth wires are +5v, Ground and transducer (signal/data).
Speed wires are +5v, Ground and pulse (signal/data).
As a foot note:
AIRMAR is the OEM behind MANY brand-name products.
and Actisense are the OEM behind MOST Airmar Active Transducers (Actisense = Active Sensors) we make the boards and write the algorithms (all the good ones that is, lol). The DST-2 connected to an old analogue Airmar transducer is the same as a new Airmar Active transducer for hardware and firmware.
As for Tinley I had a quick glance and it seems they are proven with B&G so between us most the major players are covered.